Bulletin of the Geological Survey of Japan，vo1．43（10），p．603－657，1992

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Hamo YAMAzAKI＊

YAMAZAKI Haruo（1992） Tectonics of a plate collision along the northem margin of Izu Peninsula，central Japan．Bul1．Geol．Surv．Japan，vo1． 43（10），p．603－657，24fig．，6tab．

Aわstr＆ct二The northem tip of the oceanic Philippine Sea plate （PHS）is generally thought to have been colliding with the continental Eurasian plate （EUR）at the northem margin ofIzu Peninsula（lzu Borderland），central Japan． Intense Quatemary crustal movement，such as active faulting with high slip－ rate and rapid uplifting or subsiding，occurs along this inland plate boundary． Although this crustal movement seems to have close relationship with the convergence of the plates，the regional diversity of the tectonics along the boundary has not yet been studied so well as to reveal their characteristics in the framework of plate tectonics． To clarify the significance of these regional Quatemary tectonics，the author described the faulting and related geological history in the following three subdivisions ofthe Izu Borderland：1）the areato tゑenorth ofSuruga Bay， 2）the area along the southem margin of Tanzawa Mts．and3）Ashigara Plain－ Oiso Hills areas。Withrespectto evaluating ofthe tectonics in eacharea，much attention was paid not only to the relative slip－rate of each active fault used in the previous works，but also to the absolute movement of each block divided by active faults． Many features associated with Quatemary tectonics in and around the Izu Borderland were revealed．The Quatemary system in the studied area is classified into basin－fill type deposits （1a and lb） and non－basin－fill type （2a and2b）according to their thickness，1ithofacies and tectonic features．The study of the tectonic evolution in the Izu Borderland revealed two common features，i．e．1）subsiding basins gradually turned into uplifting areas，2）active faulting and subsiding migrated systematically towards Izu Peninsula through theQuatemaryperiod．AcircularevolutionofthetectonicsintheIzu Borderland was concluded from these geological and tectonic features．The process of this tectonic evolution is very similar to the growth of accretionary prism which is formed along the trench axis at the foot of continental slope associated with the plate subduction． The Quatemary tectonics in most part of the Izu Borderland has been controlled not by a uniform plate collision but by buoyant subduction of the P：HS under the EUR resulting in the development of the accretionary prism on land，On the other hand，intense regional uplift occurred along the southwestem margin of Tanzawa Mts．since the Middle Pleistocene with no conspicuous

＊Seismotectonic Research Sect圭on，Dept． Environmenta1 Keywor（is：seismotectonics，plate collision，plate bound－ Geology，Geological Survey of Japan。 ary，Philippine Sea Plate，Quatemary tectonics，active fault，South Fossa Magna，accretionary prism，imbricated thrust．

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migration of faults and subsiding area．This feature is interpreted as a transformation of the formerly existing subduction to present－day collision． Many active faults with short length and high slip－rate around the Izu Borderland are interpreted as imbricated thrusts in the accretionary prism． Hence，the active faults inthe plateboundary zone cannot be placed on the same level with the active faults in other inland regions．

1．INTR（）DUCT亘ON faults suggest that the fault movements have close relation to the plate convergence 1．l Prob亙emset撫gsa翻聖腿r脚sesO量 betweenthe PHS andtheEUR plates（Yama－ t恥量sst磁y zaki，1984）。However，nothing is definite on The oceanic Philippine Sea plate（PHS）is the implication and role of each active fault underthrusting northwestwar（i beneath the in the setting of plate tectonics．Therefore， Eurasian plate（EUR）along Nankai trough this paper first describes the history of fault－ off Southwest Japan．The direction of this ing and of related crustal movement in the plate boun（lary tums toward north on the following three areas along the subaerial west of Izu Peninsula and extends from Sum・ Plate convergence boundary：1） the area to ga trough through the subaerial region on the north of Sumga Bay inclu（1ing the south－ the northern bor（1er of Izu Peninsula （Izu westem foot of Mt．Fuji，2）the southem end Borderland）to Sagami trough（Sugimura， of the Tanzawa Mts．，3）Oiso Hills and 19721Fig．1）．These areas constitute a part Ashigara Plain area．As Izu block is migrat－ of the major tectonic province calle（i the ing to the northwest，each area seems to show South Fossa Magna that comprises thick the various tectonic histories to correspond accumulation of Neogene sediments and vo1－ to the （iirection of convergence boundary． canics．These rocks are strongly shortened Then，based on these data，the author dis－ and a northward curving fo1（i zone is formed cusses the relationship between each active along the northem margin of Izu Peninsula faults and plate tectonics through the com－ （Matsuda，1962）．This structure has been parison of the local tectonic evolution． caused by repeated subduction of the buoyant Figure2is the index map of locality name Izu－Ogasawara arc including Izu Peninsula， used in this paper． the volcanic arc in the eastem margin of the PHS，beneath the EUR （Sugimura，19721 1．2 Sig聾量f量cance ＆聾雌 ｛e我t甑re o£ 意擬宜s Matsuda，1978）． s重掘y On the other hand，many active faults with The stu（1y areas include the inferred high slip－rate occur in the Izu Borderland as hypocentral region of Sumga Bay for the a result of strong shortening （Yamazaki， future great Tokai earthquake （lshibashi， 1979）。Th“term active fault denotes a fault 1981），and Sagami Bay for the west Kanag－ having evidences of repeated fault move－ awa earthquake（lshibashi，1988a，b）．Study ments in the present tectonic stress臼：field an（1 on the implication of active fault in the set－ 1iable to be activated in the future．The ting of plate tectonics will contribute to the author thinks that the active faults constitute progress of the effective earthquake predic－ a major basic element of the Quatemary tion that was proposed by Ishibashi（1978）． tectonics in Japan， and configurations of The investigated area has a unique tectonic active faults，their activity an（i history of feature：the convergence boun（iary at the faulting represent the actual con（1ition an（i northem margin ofthe PHS crosses an island evolution of tectonic setting in this region． arc system continuing from Northeast Japan The intense activity an（i the strikes of these arc to Izu－Ogasawara arc，and the intersec一

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Fig．1 Plate tectonic setting around Izu Peninsula，central Japan．Arrows show the direction of motion of the Pacific and Philippine Sea plate relative to the Eurasian plate．Filled circles indicate the Quaternary volcanoes．Bold lines represent the plate convergence boun（iary． PAC：Pacific plate，PHS：Philippine Sea plate，EVR：Eurasian plate，MTL：Median Tectonic Line，

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Fig．2 Index map showing the geographical names in and around the study area．Bold－solid， bold－broken and fine．solid lines show the active faults，major geological faults and outline of the Quatemary volcanoes．Ha：Habuna Hi11s，Ho：Hoshiyama Hills． tion part emerges in the subaerial area to the north of Izu Peninsula．As most of the con－ vergence boundaries exist in the deep sea 1 Fault scarp 2 3 bottom such as trenches or troughs，the geo－ 10gical information is less reliable than that of in the subaerial area． Therefore the detaile（i （1ata on geological history and tectonics of study area will provide the world’s best analogue for the tectonic evolution of the submarine plate boundary．

In order to obtain the detailed information Fig．3 Relationship between the formation of the on the Quaternary geology and faulting in the fault scarp or tectonic landform and absolute study area， this paper introduces the crustal movement of each block divided by a following new i（iea． Previous stu（iies on fault．Arrows indicate the direction and rate of tectonic block．Dotted parts represent the active faults have used the long－term slip－rate fill sediments． to evaluate their activity．This rate means BLE：Base level of erosion the average rate of the relative slip of two blocks bounded by ：fault and indicates the

一606一 7セo渉o％乞os αlo％9 オh6 多zoπh6ηz 解zごz響1π （ゾZ2z6P62zづ％s％1ごz侭． hz盟z召zσ為の rate of crustal strain accumulated around the survey for a tunnel construction before1930， fault．The author，however，argues that the investigations to trace the earth（luake faults long－term slip－rate is not always adequate to of1930by Ihara＆Ishii（1932），Otuka（1933） interpret the evolution of tectonic landforms． and others revealed the cumulative tectonic For instance，as shown in Fig．3，the tectonic features along the fault system．Particularly， lan（iform caused by dip－slip faulting greatly Kuno（1936）pointed out that the Tanna fault varies with the motion of individual block had l km of total left－1αteral displacement bounded by fault toward the base level of based on the length of valley offset and the erosion。 If a faulting occurs in the subsiding distance of separated same lava flow． area，most of the fault．topography wou1（1 In the late 19307s to early 1940’s， the soon be buried by basin fill se（1iments．The tectonic histories of the Iriyama thrust，the absolute rate of uplifting or subsiding on each faults in southwestem foot of Mt．Fuji and faulted block seems to be a more important the Kannawa fault at so“themmargine ofthe factor in the formation of tectonic landforms Tanzawa Mts．were described by Otuka than the long－term slip－rate of the fault． （1984），Tsuya（1940）an（i Tsuya（1942）respec－ Furthermore，the absolute crustal movement tively based on the progress of strati－ is fmdamentally significant for the study of graphical and chronological studies on the tectonic evolution in the plate convergence Quaternary formation． region where the behavior of cmst changes In the late1960’s，the active fault study has from subsiding to uplifting in a short period． been introduce（1 in the program of earth－ Therefore，to recoustruct the fault evolution， quakepredictionasoneofthenationalpro－ the author examines not only the relative jects．By the end of1970’s，most of the major displacement of fault，but also the absolute active faults in Japan have been discovered crustal movement（1e（1uced fromthe elevation through fault survey using aerial photo－ of faulted surfaces， the thickness of the interpretation．In the study areas，the quanti－ Quatemary sediments，their ages and tative information on active faults，such as paleoenvironment and so on． long－term slip－rate，vertical and latera1（iis－ placements of the fault，was accumulate（1by 2．PREVIOUS WORKS means of progress of tephrochronology an（1 radiometric dating method． Machida＆ 2．1 St聡岨y o箪act置ve f我眠亙ts亘聾a盟〔丑aro魍nd Moriyama（1968）reported geomorphic his－ IZ聰Pe】臣且恥S聰且a， tory around the Kozu－Matsuda fault。Matsu－ In1920’s and1930’s，several investigations shima＆Imanaga（1968），Machidaαα1。 carried out in South Kanto an（l Izu Peninsula （1975） and Kano 6♂ α1．（1979） revealed the on crustal deformations associated with the fault movement on the Kamawa fault in the great Kanto earthquake of 1923 and the mid（11e to late Quatemary．Yamazaki（1979） Kita－lzu e＆rthquake of1930．These surveys reveaIed the fault behavior an（i the tectonic depicted the outline of major active faults in significance on the active faults in southwest－ these regions such astheKozu－Matsuda fault em foot of Mt．Fuji．All these data obtained and the Kita－lzu fault system． Yamasaki by1980were compiled in “Active faults in （1926）and Otuka（1929，1930）estimated the Japan”which is an inventory book on active existence of Kozu－Matsuda fault from a faults in Japan by Research Group for Active prominent straight scarp between the Oiso Fault Japan（R．G．A．F．」．）（1980）． Hills and Ashigara Plain，and（iiscussed the Since then，the active fault stu（1y in Japan Quaternary tectonics in these areas． branched off into two fields．One is the study Although existence of the Tanna fault，a part on the（1etailed history of each fault move－ of northern portion of the Kita－Izu faults ment during Holocene time．Many trenches system，had been known by the geologica1 were excavated across various faults to

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know the age of recent faulting an（i the recur－ active fault along the new subduction zone rence interval that provide us useful informa－ cause（i perio（iically large earthquakes and tion about the long－term earthquake pre（iic・ moved coseismically at the event of1923 tion． Archaeological records an（1ancient great Kanto earthquake． manuscripts were also used to specify the age Contrary to this hypothesis， Somerville and locality of faulting that caused historical （1978） discusse（i that the crustal strain earthquakes．Another study is on the clarifi－ caused by plate collision was accommo（iated cation of the implication o：f each active fault by the slip of intraplate faulting in Izu block． in the regional tectonics．With increase of He argued that the subduction zone of：f the the information on plate collision at Izu east coast of Izu Peninsula was not needed Borderland，Yamazaki（1984）and Itoαα1． for the explanation of the decrease of inter－ （1989）discussed the meaning of active faults plate convergence rate at Izu Borderland． in the framework of plate tectonics．This Although the above mentione（1hypothesis paper lies on the field of the latter． postulated that the active faulting in Izu Bor（1erland directly represente（i the plate 2．2Re且aせio蒐s恥恥めe意wee恥襯童ve蝕磁s motion along the convergence bomdary， 蹴齢late恥o撒ぬry Nakamura6渉α1．（1984）proposed a new i（1ea Sugimura（1972）first delineate（1that the to classify the plate boun（iary into mechani－ plate boundary in the South Fossa Magna cal b6undary zone and material bomdary． region extended from Suruga trough through They showed that the inland plate bom（iary the inland area to Sagami trough．He also in Izu Borderlan（1，pointed out by Sugimura thought that the buoyant Izu block on the （1972），was the material boundary，an（i the PHS collided with the EUR at the northem mechanical boundary zone correspon（1e（1to margin of Izu Peninsula an（1northward the active faults zone to the north ofmaterial migration of Izu block bent the plate bound－ boundary． ary in an arc．The：Kozu－Matsuda and Kan－ Yamazaki（1984）investigated the history nawa faults to the north of Izu Peninsulawere of fault movement and its tectonic feature thought to be the plate－boundary faults along the mechanical boundary zone． He where the PHS directly contacted with the showed that the crustal evolution of this EUR． region was caused by the formation of ac－ However，as these faults had extraordinar－ cretionary prism associated with the plate ily smaller slip－rate than the relative plate subduction． He also pointed out that the motion between the PHS an（1the EUR，it was active faults in this region corresponded to unnatural to think that these faults represent the imbricated thrusts in the accretionary

directly the plate boundary．Ishibashi（1976） prlsm。 thought that the commencement of new sub－ Recently，Ishibashi（1988a，b）reconstructe（i duction zone off the east coast of Izu Penin． his original hypothesis and showed that the sula diminished the inter－plate convergence west Sagami bay fault was not a interplate rate in Izu Borderland． According to this fault at the subduction boundary but a kind thought，some transform faults across Izu of intraplate hinge fault boun（1ing the buoy－ Peninsula are needed to connect Suruga ant inner arc from the subducting outer arc in trough with a new subduction zone．There－ the Izu－Ogasawara arc． He renamed the fore，he argued that the numerous NW－SE fracture zone as the west Sagami fracture trending faults in the peninsula acted as the zone（WSF）．He wamedthe reactivation of transform fault comecting the both subduc－ WSFinthenearfuturebasedonthefactthat tion zones，an（1named the group of these the periodical movement of the WSF had faults as Izu transform belt．Ishibashi（1977） repeatedly damaged Odawara area every70 further suggested the possibility that the years through the historical time．

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2．3 Re互寵童ve mot亙o聾o£騒e PHS toward sive，recentlyYamazaki（1988）andMaritime t恥e EUR Safety Agency（1988）proposed a（10ubt for Many earth scientists in Japan supported the existence of this new plate boundary the NW direction for the relative motion of along the eastem margin of Japan sea on the PHS toward the EUR along Nankai which the both hypothesis are based． trough．However，two estimations：1）N to Consequently，the author thinks that the NNW direction and2）NW to WNW direc－ interpretation of tectonic evolution in the Izu tion，were proposed for the motion of the Borderland is indispensable to solve t五e prob－ PHIS in Izu Bor（ierlan（i （lshibashi，1984）． 1em of different direction of plate conver－ The former was derived from the studies on gence between Nankai an（1Sagami troughs． the geological history and landform evolution in the South Fossa Magna region（Kaizuka， 3．OUTLINE（）F G・E（）LOGY 19751Matsuda， 1978）． The later was obtained from the plate motion mo（1el on the 3．1Setti醜gi恥蝕e幽tetecto漉s basis of the focal mechanism of great earth－ From the viewpoint of island arc system， quakes along the plate boun（iary（Seno，19771 Izu Borderland is a intersection between the Minster＆Jordan，1978，1979）．To solve this East Japan island arc system（Sugimura＆ inconsistency，Nakamura ＆ Shimazaki Uyeda，1973）exten（1ing from Kuril Islands （1981）conclude（i that the PHS had moved through Northeast Japan to Izu－Ogasawara toward north before l Ma，and then changed islands and the plate convergence zone at the its direction to the northwest． Kaizuka northern end of the PHS（Kaizuka，1975）．Izu・ （1984）showed many geological data to sup． Ogasawara arc is divided into the non－vol－ port this idea． canic outer arc and the volcanic．inner arc by Seno （1985）tried to interpret the above the volcanic front． mentioned inconsistency using the new In and around Izu Peninsula，the N－S trend－ hypothesis proposed by Nakamura（1983）and ing volcanic front runs in Sagami Bay．Izu－ Kobayashi（1983）that northern Honshu is a Ogasawara imer arc to the west of the portion of the North American plate（：NAM） volcanic front contains Mt．Fuji，Hakone， and migrating to the west．He considered Amagi and some other Quatemary volcanoes that the migrating direction of the PHS in Izu Peninsula and Izu Borderland（Fig．1）． changedfromNNWtoWNWatabout1．5Ma， Because of this imer arc is made ofrelatively andthenat O．5Ma theboundarybetweenthe low density volcanic rocks，the crust is thick－ EUR and NAM which originally rm through er than that o：f non－volcanic outer arc． the central Hokkaido，jumped to the eastem Conse（1uently，the subduction of the inner arc margin of Japan Sea and the central Japan． beneath the EUR becomes difficult and the He thought that this jumping of plate boun（1－ convergence boundary is pushed northward． ary caused the westward migration of North－ Therefore，Izu Bor（1erland is a place subject east Japan as a portion of NAM．Onthe other to extreme crustal deformation． hand，Ishibashi（1984）who shared the view point that Northeast Japan was a indePen－ 3．2 To夢ogr我夢恥亘cal a醜d geo亙09童e我且str聡c－ dent microPlate，conclude（1that many phe－ t櫨eo貴藍eS側騒FossaM＆9黙 nomena on the tectonic evolution in late The Izu Borderland constitutes a part of Neogene Japan，such as the different conver－ the Fossa Magna which is a huge depressed gence direction between Nankai and Sagami zone（1ividing the Japanese Islands into South－ trough，are interpreted consistently with the west Japan and Northeast Japan．The Fossa idea that the Southwest Japan commenced its Magna region is estimated to initiate its eastward migration at about l to2Ma． deformation in the middle Early Miocene to Although this controversy remains inconclu一 early Middle Miocene（Kato，1992）． The

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thick accumulation of Neogene Tertiary and and their thickness exceeds1，000m．The Quatemary volcanics fill the depression of latter is sedimentary rocks composed of the pre－Neogene rocks．The Fossa Magna is thick mudstones and conglomerates．Their divided into two sedimentary basins，i．e．the sedimentary facies and structure indicate North and South Fossa Magna，by the rise of that some of them were the subsiding trough the Central Upheaval Zone in the central fill se（iiments（lto，19851Amano6砲1．，1986）． part．Izu Peninsula and Izu Borderlan（i con－ These rocks experienced the most extreme stitute some portion of the South Fossa shortening of more than20km in Tertiary Magna．In and around Izu Peninsula，the formations ofthe JapaneseIslands（Matsuda， South Fossa Magna is divided mainly into 1962，1984）． The folded structure extends two tectonic provinces l1）the stable Izu around the Izu block with a strong convexity Peninsula（lzu block）consisting of the Terti－ towards the north．In the folded belt，the ary an（i Quaternary volcanics and2） the lower to middle Miocene crops out in the extra peninsula fo1（ied belt（the South Fossa portion of anticlinal axis and the upPer Magna foldedbelt）consisting ofsedimentary Miocene to Pliocene fillsthesynclinalportion rocks and volcanics since the Miocene time （Matsuda，1962）． （Matsuda，1962）． These geological structure in（iicates that The Izu block consists mainly of the Yuga－ the South Fossa Magna folded belt is the shima Group（Lowerto Middle Miocene）and Iocus of collision an（1accretion of the buoy－ the Shirahama Group（Upper Miocene to ant crust，such as Izu block，towards the Lower Pliocene）underlying Quatemary vo1－ EUR。 The formation of sub（1uctive trough canics．Shirahama Gfoup（11to8Ma inage） between the PHS and the EUR plates has yielded the Foraminiferum L吻40の61乞鰯 been repeate（1through the Neogene perio（i （〈吻hzolの観％）ノのo勉oσof which has not （Matsuda，19781Amano6齢乙，1986）。The been fomd in the contemporaneous Tertiary active faults in this region extend along the rocks o：f Japanese islands except for Izu southem margin of the South Fossa Magna Peninsula （Saito，1963）． 五のりづ40のノolズ按z has folded belt to surround the northem margin lived in tropical shallow waters indicating of Izu Peninsula． that the Izu block comprise（1a number of volcanic islands situate（i to the south of 4．ACTIVE EAULITS AND CRUSTAL Japanese Islands at the late Miocene（lbara． DEFORMATIONS IN THE AREA TO ki，1981）．The Miocene sediments in this THE N（》RTH OF SURUGA BAY Peninsula do not show any large scale defor－ mations cause（1by faulting，fo1（iing or uplift－ 4．1 0utl亙聾e ing．The topographic relief exceeding1，000m The geological an（l topographical structure in Izu Peninsula has made by the growth of in the area to the north of Suruga Bay is Quaternary volcanoes． characterize（1by a N－S trending zonal struc－ On the other hand，many steep momtains ture as shown in Fig．4．Two N－S trending such as the Misaka，Tenshu and Tanzawa fault systems with W－upthrow divide the Mts．1ie on the South Fossa Magna folded region into three blocks． The east fault belt in consequence of vigorous uplifting． system is composed of the Iriyamase，Omiya This folded zone consists of the Tanzawa， and Agoyama faults．They consist the geo－ Koma and Nishi－Yatsushiro Groups of the 10gical and topographical boundary between lower to middle Miocene and the Fujigawa， alluvial lowlan（1s of Fuji an（1Fujinomiya in Nishi－Katsura，Aikawa and Ashigara Groups the east， an（1 the Kambara， Habuna an（i of the upper Miocene to Pleistocene（Ma－ Hoshiyama Hills in the west．The west one tsuda，1958）．The former is mainly marine consists of the：N－S trending Iriyama and deposits interbe（1ded v守ith volcanic materials Shibakawa thmsts and divides the area into

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Fig．4 Geologic sketch map of the inland area to the north of Suruga Bay．Geological sections are shown in Fig．8．Bold and bold－broken lines indicate the active faults． Fault name l IJriyamase fault，2．Omiya fault，3．Agoyama fault，4．lriyama fault， 5．Shibakawa fault，6．Matsuno fault，7．Noshita fault．Legend：A．AIluvial fan and backmarsh（paralle口ines）．B．Volcanic slope of Mt．Fuji．C．Lava flows from Mt．Fuji． D、Older Fuji mudflow deposits．E。Saginota gravelsinthe Middle Pleistocene．F．Bessho gravels in the lower Pleistocene．G．Iwabuchi andesite．H．Kambara conglomerate in the early Pleistocene．1．Tertiary rocks．

一611一 βz6116！乞％ （～プ オh6 （360109づoαl Sπ7∂6ッ （～プノi6z1）召％， Viol，43，ノ〉o，ヱ0 hills to the east and Hamaishi－dake and in altitude without flat surfaces on it．The Tenshu Mts．to the west． geology of these hills consists mostly of the lower Pleistocene Kambara conglomerate， 4．2 Tee亡o醜童e evo亘腿髄o恥 o£ eac恥 £a翻te《丑 Iwabuchi andesite an（i mi（idle Pleistocene 腿o磁 Saginota gravels which uncomfortably rest （且）H：蹴ais恥亘・ぬ鼠e蹴西丁錨曲髄Mo翻t廊S on the former tWo fQrmations（Table1，Fig． The Hamaishi－dake and Tenshu Mts．situ－ 4）．These Quatemary units have been verti－ ated on the westem margin of study area are cally displaced by the reverse faulting at both steep and dissected mountains ranging in margins of the．hills and also have undergone altitude from500to1β00meters．These two small scale deformation caused by faulting momtains are composed of the upper Fuji－ and folding in thehills（Sugiyama＆Shimo－ gawa Group of the Pliocene，which consists kawa，1982）．Duetotherecentuplift，anarrow mainly of conglomerate and sandstone with a Holocene terrace㌦has been formed on the maximum thickness of more than4，000m． eastem margin of the hills． Fujigawa Group is the thick fi11（1eposits of The Kambara conglomerate （Konno＆ the ancient trough located at the Pliocene Otuka，1933），extensively distributed in the Plate boundary to the south of the Kanto and southem Kambara Hills，consists mostly of Misaka Mts． This subsided trough has 600mthickpebblygravelswithsand－en－ changed to uplifting area since the beginning riched matrix．The relatively monotonous of the Pleistocene（Matsuda，1984）． facies despite the large thickness of this for－ （2）臨麟ara，H曲撒a蹉d Hosh量y＆m我 mation indicatesthat the Kambara conglom－ Hi且亘s erate is fluvial and shallow water deposits The Kambara，Habuna and Hoshiyama originally accumulated in the subsi（iing Hi11s，10cated to the east of the mountains， depressi・n畦suc血astheplace・nthen・rthem are bounded on west and east by the Iriyama bor（1er of present Suruga trough． －Shibakawa thrust system and the Iriyamase The Iwabuchi．andesite （Otuka，1938）， －Omiya－Agoyama fault system respectively． remains of the early Pleistocene volcano，is a．Kambam H量亘亙s：The Kambara Hills， composed mainly of tuff breccia and lavas 10cated in the southwestem side of River and encloses a series of sedimentary beds Fuji，is a（1issected hilly area400－500meters correlated with the Bessho gravels in the

Table1 Quaternary stratigraphy of the area to the north of Suruga bay．

KambaraHills Habuna－Hoshiyama GeologicalAge （Sugiyama＆Sh㎞lokawa，1982） H且ls

Anuvhm ＿ Hol㏄ene 一 一 一 一 一 一 一 一 一 ＿ A皿uvium Iwabuchitenlacedeposit

一 一 一 一 一 一 一 ＿ ＿ ＿ ＿ 一 一 一 一 一 一 一 一 一 F司ilava且OWS YoungerF㎎曾i’lavaflows Late 一 一 一 一 一 一 噸 一 ＿ ＿ ＿ ＿ 一 一 一 一 一 一 一 一 一 〇lder珂imudflows 一 一 陶 一 一 一 一 一 一 一 一 一 一 一 一 一 一 Pleistocene Middle Saginotagravels Saginotagravels 一 一 一 藺 隔 一 一 一 一 ＿ 一 一 一 一 一 一 一 一 一 Iwabuchi andesite Iwabuc1丘andesite

Sed㎞ents Besshogravels

肋ly Iwabuchi andesite 隔 一 一 Kambaraconglomerate 一 一 一 一 一 『 噌 一 ＿ ＿ ＿ ＿ 一 一 一 一 ｝ 一 嘲 一 ＿ P互iocene 珂ikawaGroup F雨ikawaGroup

一一一一Unconfbmlity一Con蚕01血ty

一612一 T66孟o競sαlo％9云h6％oπh6吻耀顧％げ伽P6痂sぬ但．y碗α2剃

Habma and Hoshiyama Hills． This present Kambara Hills an（i its adjacent area formation overlies the Kambara conglomer－ as fluvial deposits．Then，subsiding of these ate conformably and mderlies the middle areas gradually changed into uplifting．As a Pleistocene Saginota gravels unconformably． result o：f the change，the Saginota gravels The age of this volcano is estimate（i to be the emerged and the erosional process began in Matuyama reversed magnetic epoch in the the Kambara Hills area． early Pleistocene from the reversed magneti－ め． H＆恥聡髄a－Hos血量yam我 H亘亙亙s：The zation of lava魅blocks obtained from the Habuna and Hoshiyama Hills ranging in upPer part of the formation． altitude from100to300m，in the northeast The Saginota gravels．（Konno＆Otuka， bank of River Fuji，are bounded by the 1933）arefluvialfan（1epositsof200min Iriyamase，Omiya，Agoyama and Shibakawa thickness distribute（i in the westem half of faults．The gentle inclination of depositionaI Kambara Hills an（i in the southem Habuna surfaces preserved well on both hills in（1icate Hills．The age is estimated to be O．4to O．5 that the Habma an（1Hoshiyama Hills were Ma on the basis of yielding of S惚040η tilted to the west and to the north，respective－ 0吻磁擁s and the occurrence of widespread 1y． The geology of these hills consists of vitric ash which is potentially correlated to Iwabuchi andesite intercalating with the Ks－10ash in the mi（1dle Pleistocene Kazusa thick Bessho gravels，th6 1ate Pleistocene Group，Boso Peninsula（Machida6孟α1．， Older Fuji mudflow units and several sheets 1980）． of Younger Fuji Iavas in the late Pleistocene Thelwabuchiterraceof30to40min to Holocene． altitude，on the eastem margin of Kambara Hitherto，the Bessho gravels extensively Hills，is an uplifted Holocene terrace com－ distributed in both hills were assume（1to be posed of coarse fluvial and fine brackish equivalent to the Sagi耳ota gravels in the water sediments．Akahoya ash（Ah）empte（1 Kambara Hills because the Bessho gravels in Southem Kyushu at6，300years ago was seemed to overlie the agglomerate of Iwabu－ found in the brackish water sediment at13．7 chi andesite mconformably at several out－ m in height（Yamazakiαα1．，1981）．These crops along the River Fuji（Tsuya，19401 facts indicate that1）Iwabuchi terrace is a 0tuka，1938）．But this study has confirmed filltop terrace created by the postglacial that the Bessho gravels can be classified into transgression，and2）assuming the paleo－sea two formations l upper formation which is level of6，000years ago was about2m above relative to the Saginota gravels in the Kam－ the present one，the eastem margin of Kam－ bara Hills an（110wer one enclosed in Iwabu－ baraHillswasuplifted11，7matarateof chi an（iesite． 1．9m／103years since after the fall of Ah ash． The former，gravels of200m in thickness， The following geological evolution of the occupies the southem Habuna Hills with N－S Kambara Hills is obtained from the above or NE－SW strike and E dip．The mud layer mentioned stratigraphical data． bearing abun（iant plant fossils at the base of In the early Pleistocene，the Kambara con－ upper formation intercalates with a thin glomerate accumulated thickly in the subsid－ glassy marker tephra identical with the ash ed depression similar to present bottom of layer in Saginota gravels． The Saginota Suruga trough． The activity of Iwabuchi gravels in the Habuna Hills have undergone volcano has also occurred in the（iepression more intense deformation than those in the following the deposition of Kambara ：Kambar皐 Hills． This fact implies the conglomerate and a thick series of andesitic existence of a fault（the Matsuno fault）mn－ lavas and agglomerate has consequently ning through between two hills along the accumulated． In the early Middle Pleis－ River Fuji． tocene，the Saginota gravels overspread the The latter formation （Bessho gravels）

一613一 B％Jl6ガ銘 （ゾ 云hθ 060109乞oαl S％7∂βッ （ゾノ砿）ごz％， Vbl．43メ〈b．10

extensively distribute（i in the Habuna an（1 other．As there has not been observed any Hoshiyama H皿s is composed of totally more erosional surface on the contact between the than1000m thick and well stratified cobble－ Iwabuchi andesite and the un（ierlying mud pebble－sized layers intercalated with thin layer top of Kambara conglomerate sand beds．The extremely large thickness （Sugiyama＆Shimokawa，1982），the Bessho and monotonous facies of this formation gravels and the Kambara conglomerate are suggest that the Bessho gravels are resulted assumed to have accumulated in a series of by the accumulation of coarse sediments in a subsiding basin． The thickness of Bessho contimous depression．They were transpoted gravels is increasing toward the north in－ from Misaka Mts．by the Paleo－River Fuji． dicating that the center of subsi（ience has The geological structure of the Bessho migrated to the north during early gravels is different largely between that of Pleistocene． the Habuna Hills with NW－SE strike，SW The Older Fuji mudflows in the late Pleis－ （1ip of40to60degrees and of the Hoshiyama tocene （Tsuya，19681Agglomeratic mud－ Hills with：NW－SEstrike，NE dip of30to50 flows of Tsuya，1940）are composed mainly degrees． This fact also indicates that the of original mudflow（1eposits derived from Agoyama fault has commenced its activity Older Fuji volcano，their secon（iary（1eposits after the accumulation of Bessho gravels and alluvial fan gravels at volcanic foot．As traversing its sedimentary basin，and conse－ the depositional surfaces Qf these mudflows quently the blocks on either si（ie of the fault are well preserved on both hills，they can have（1eveloped（1ifferent tilting from each subdivide into several units through the

Key Age Mudflow Fuji Iavas 亡ephra x104y．B－」》． surfaces Om OS＿伽 0．3 KgP SSW4，SW6 MS一噸 羅 ＿＿＿Mf－WSW1，2，5，SSW1，2 一Mf一皿 2．15 AT一｛ o o o o o o拗

scoriaゆ ▲ ▲▲ ▲ A ▲ ％ 1一…

5m Pum臨 4．9 nOW 一Mf－1

1－MH・

Fig．5 Tephrochronology of lava andmudflow depositsinthe area ofsouthemfoot ofMt．Fuji． OS：Osawa scoria，KgP：Kawagodaira pumice，MS：Murayama scoria，AT：Aira－Tn ash，Pumice flow：Hakone younger pyroclastic flow，Mf－r to Mf－IV represent the depositional surfaces of the Older Fuji mudflow．Filled triangles，open circles and oblique lines in the columnar section indicate the sCoria layer，pumice or vitric ash layer and humic soil respectively．

一614一 Too云onぢos ごzlo多zg 渉h6 ％oπh67％ ％zα響づ％ （ゾノ2z6P6多z1％s％♂α 侭． Kzηz召2召たの chronological survey of the surfaces．Ma－ faces on the mudflow deposits（Figs．5，6）． chi（1a（1977）divides the mudflows into older These surfaces contain severa1（1epositional an（i younger units on the basis of thickness of surfaces of original mudflows from the older the covering tephra．Yamazaki（1979）and Fuji volcano and fluvial surfaces of tributary Yamazakiαごzl．，（1981）also studie（i the strati－ flow of the River Fuji． graphy of tephra and distinguished five sur一 These numerous surfaces have been form一

Younger Iavas 》 0．3 Om ，1輌導 er lavas Mudfbw IV 0S ゑ纏？ 甥 獺i難 □ 。慨’三羅一 ▲A▲▲▲ MS 1 ｝ 望彰 錫 Alluvial fan ／／ Mudflow U ATn Mudflow III 2」 2 欝撚 翻 ▲ A 曙≡三 ※／ 啄 Sbpes ow I’ 3 げロおえ Mudflow I’ Mudflow l 5．O 婁騒き 》4》4 TP 4 flo 5 Xlx104yr． i 號現 6

％夢 錫 i難 監．纐‘孟 ＼ 雛’、ノー甲 霧欝 ＃堀 縮 瑛：i d’

’彰き一，．響．躁 旨 3 鮎 ，羅集． ．霧 d〆

．．：：：欝 4 ・難μ、糀 Fujinomiya 黛 e l～ ・鵜：： 灘 騒灘 ・一． 2c 露 羅瀦 ……u藤：灘 i難 諺 f ，難 “ a ＼ 護 澱 鰍う1

唖’z摺 e 賦 River Fuji 1．lriyamase fault ざ 2．Omiyafault 3．Agoyama fault b 灘 4．Shibakawafault a ヂ 1

Fig．6 Geomorphological map of Hoshiyama and Habuna Hills．Older Fuji mudflow deposits can be divided into five units based on the tephrochronology． Columnar section correspon（is to that in Fig．5

一615一 醸灘．．，

β％ll6だ％ （Zプ 渉h6 G60109づo召l S勿7z7¢y ρプノ41）ごzフz， VbJ．43，〈石o．ヱ0

ed by the intermittent uplifting of two hills， swamp onthe southemflank of Mt．Ashitaka cause（1by faulting along the eastern margin and Fujinomiya basin。 of the hills system． But the （1istribution ＆． E臆」亘g我w＆＆亘旦朋v量a旦ね醜：Thick fluvial pattem of mu（iflow surfaces shows the gravels transported from the Misaka and regional characteristics，that is，the surfaces Akaishi Mts．by the River Fuji are the con－ older than50，000years ago（Mf－I and I’） stituent material of this fan．These gravels （1evelop well on the Habuna Hills，while intercalate the SSWl lava of Fuji volcano younger surfaces，especially the surface of with the radiocarbon age of14，000yr B．P． 20，000years ago（Mf－III）extend over the （Yamazaki，αα1．，1981）． The maximum Hoshiyama Hills．This contrast is due to a depthofthislavareachesupto－140matthe relatively higher uplifting rate and earlier mouth of River Fuji（Murashita，1977）． emergence of the Habma Hills than those of b． Uki曲imaga恥ara：Ukishimagahara is the Hoshiyama Hills．Each of surfaces onthe a lagoon formed by the continual sub（1uction Hoshiyama Hills extends from north to south and the formation of barrier alon墓the south－ in direction with narrow width．The younger em flank of Mt．Ashitaka． The marker Fuji lavas（SSW2，SW5）eruptedabout10，000 tephra of KgP（2，900yr B．P．）and K－Ah ash years ago flew over the hi11s an（1reached （6，300yr B．P．）were found in this lagoon River Fuji．Although Tsuya（1940）assumed sedimentsatdepthof－6mand－17．8m that these lavas have not been（1isplace（1by respectively（Yamazakiαα1．，1981）．The Omiya fault at the eastern margin of the hills， depth of these tephras approximately indi－ the large gap of lava height，exceeding50m， cates the subsidence of Ukishimagahara between Fujinomiya Basin and Hoshiyama since after the deposition of these tephras Hills indicates that the Omiya fault has verti－ because the sea level has been relatively cally displace（i these lavas． stable during last6，000years． The mean Above data lead to the following conclu－ subsiding rate is estimated to be2m／103 sion on geological evolution of the Habuna years since the KgP age and to be3m／103 and Hoshiyama Hills． The Habuna an（1 years since the Ah ash age．Figure7shows Hoshiyama Hills were a place intensely sub－ the relationship between the altitude of sam－ si（1ing where the thick Bessho gravels had pled peats in the lagoon sediments and their accumulated during early Pleistocene time． radiocarbon age． In the Middle Pleistocene，this area was still c． F細亘聾om量ya ba，s沁：This basin is a subsi（1ing contemporaneously with the accu－ fault angle depression formed on t五e south－ mulation of Saginota gravels．But around westemfootofFujivolcano．A300mdri1－ 80，000years ago when older Fuji volcano 1ing revealed that this basin was composed of commenced its activity，the crustal thick Holocene Fuji lavas an（101der Fuji movement in this area had gradually changed mudflows younger than80，000years ago． from subsidence into uplift．Since50，000yr The bottom of drilled core did not reach the B．P．，though both h皿s have undergone the Bessho gravels uhderlying Older Fuji mu（1－ same significant uplifting，the difference of flows（Yamazaki6渉σ1．，1981）． This fact distribution pattem of mudflow surfaces indicates that Fujinomiya basin has under－ became prominent between two hills（1ue to gone intense subsiding at a rate of2to3m／ the difference of tilting direction． 103years since the late Pleistocene． （3）Fuj旦一Fujinomiyalowla織 Fuji－Fujinomiya lowland，10cate（i in the 4．3 Moveme恥t of aetive f＆賜且ts northern extension of axis o：f Suruga trough （1） hiyamase－Om量ya－Agoy＆ma鉛腿亘t sys・ and extending over the east si（1e of Habuna tem an（i Hoshiyama Hills，is composed of the This fault system divides the region into Fujigawa alluvial fan， Ukishimagahara subsiding lowland to the east and uplifting

一616一 T66孟o吻sαlo％9オh6％oπh6解耀智初げ伽P6痂s吻硯。y4〃zα2剃

RADlOCARBON AGE ago and Ah ash fall respectively（Fig．8）。The 10日） J 5 long－term slip－rate of the fault（ierived from I I 『 コ I I I l 30 ？？幹 these data is estimated to be constant with a value of7to5m／103years during Holocene … time．This is the largest value of vertical slip ， ’ ’！ノ ， ノ ω rate of the active faults in the lan（i of Japan． ＞ ノ ζ 一〇 At the event of the Ansei－Tokai O m r earthquake that occurred in Suruga trough in ノ 磯 ／ ！！ m＞ ，轡 l854，a small mound called“Jishinyama” ／i 31m 5 （earthquake mou項（1） apPeared on the floo（玉 ／嬢 ε ♂ノ plain near the mouth of River Fuji（Omori， 46 需 1919）．Judging from the fact that a vast area of cultivated land was（1eveloped to the west iノ O I陥BuCHl 一30 of the mound after the earthquake，it is con－ clu（1ed that the formation of“Jishinyama”is 毒’’ Ah’’ i 欝UKISHIMA GAHARA not a small local event but a tectonic land－ □（Hatan。αai．， 1979） form accompanied with extensive uplifting of 一50 the westem side of the Iriyamase fault．This means that the southem segment of Fig．7 Comparison of the sample heights and their Iriyamase fault has moved at the Ansei－ ages between two drillings on the both sides Tokai earthquake of1854． of the Iriyamase fault． SSWl lava of14，000yr B．P．outcrops to the west of the Iriyamase fault．This lava hilly land to the west．This system exten（is seems to have（1eposited on the river floor from the mouth of River Fuji to the north and because it rests directly on the flood loam． disappears into the younger lava field。The Assuming that the sea level of14，000yr B。P。 Iriyamase and Agoyama faults inthis system was50to60mlowerthanpresentleveland are reverse faults showing a sinistral en eche－ the height of the outcrop above sea level at 10n configuration． The Omiya fault is a that time was the same as the present value normal fault comecting the Iriyamase an（1 （十15m），the original altitude of this lava Agoyama faults（Fig．4）． might be－35to－45m。Consequently，the a． 敢亘yam＆se飴翻t（Tsuya，1940）：This westem block of the Iriyamase fault has been fault，bounding the eastem margin of Kam－ uplifted50to60matarateof3．5to4．4m／ bara and Hoshiyama Hills，is a thmst having 103years since the deposition of SSWl lava， theWpartupthrown，andextendsfromthe and the eastern block has subsi（1ed55to45m mouth of Riyer Fuji to the north。Southern with the rate of3．3to3．9m／103years． extension of the fault is also assume（i to Table．2shows the relative slip rate of the continue to the submarine active fault along faults and the absolute rates of block move－ the base of westem slope of Suruga trough． ment which were obtained from the heights Although no tectonic landform is found on of several DRH and the heights of sea level the surface of Fujikawa alluvial fan，the｝ate when DRH were deposite（1． Pleistocene to Holocene se（iiments beneath b．Om量ya fa磁（Tsuya，19401Yamazaki， the fan are vertically displaced by faulting。 1979）：Tsuya（1940）inferre（1the existence Comparing the displacement reference hori－ of an arcuate fault along the eastem margin zons（DRH）on both sides of the fault，1ess of Habuna an（1Hoshiyama Hills and named than100m，46mand31．5mofthewest－side it the Omiya fault．Suzuki（1968）estimated upthrown are recognized for SSWl lava of that the Omiya fault was a part of normal 14，000years ago，peaty layers of8，000years ring fault surrounding Mt．Fuji，causedbythe

一617一 B％ll6渉勿z（～プ オh6 （360109づoσl Sz6卿¢y （～プ∫のごz％， Vlol．43，〈b．10

A α Z b → Fuli volcano 100 Iwabuchi terrace m ／RFuji ’o』。・。 SSW1 13，ス60土300Y・B。R ．・i奪・ 』…i蓬粟・ 』・：ll＝』。誇』 』・二。ぷ。』，・’・』6・・： Ofuchi O o ． Fandep． ’0 lava z’噛鞠1，1，， （SSW1）

’1自6istocene 一100 gravelS B勿 〇 1Km 100 α lwabuchi c m 須 terrace Ukishimagahara A R．Fuji O I器躍曼。・・造 ’／／／／／／／ 瀬』論茎 ay

A 30 28．77m Peat 4430：±120Y．B．P． Mean s巨p rate 13・系 7660±230Y．B．P。 B 8090土380Y．B．R 4。054m 5％ 3 ρ・σ 当3

O K9P iΩシ呂 m 、轟 一6m 5m ．o’1・． 臼 ’o臼 Sand＆（i

｝短嘱曹 －1Z8m LAh ’o∋＝’σ． Clay 一30 8180土210 Y．B．P．YBP Vbrtical displacement

Fig．8 Displacement of the Holocene sediment and the Fuji Iava by the Iriyamase fault in the lower part of River Fuji． A：DisplacementofOfuchi lava（SSW1）．Bl Displacement ofthe Holocenetransgression sediment．Section lines correspond to those in Fig．4

Table2 Block movement on the each side of the Iriyamase and Kozu－Matsuda faults（after Yamazaki，1984）．

Faulmame Ref◎rence Age Sea Up㎞wnside Down山mwnside F副t long－telm level displace－ slip－rate Hei菖ht Up1血 Uplift Hei帥t Subsidcnce Subsidence m㎝t rate rate

（Ka） （m） （m） （m） （m／103y） （m） （m） （m！103y） （m） （m！103y）

Aha5h 6．3 2－15『 20．5 18．5 29 1．8～23 3．8～43 0。6～0．7 2玉～23 3．3 KOZU－ Matsuda Mゑsu面ace 60 90 105 1．8 －43．5 30．3 0．5 135 2．3 鉛ult －500 250 曙 一 500 2 OS2 『 2．9 昌13．711．015250 一6 躍 0 一 一 6 2 myamase－ Agoyama Ahash 6．3 2 11．7 1．9 －17．8 19．8 3．2 31．5 5．0 £ault Hol㏄㎝epeat 8．0 －10～一15 21～26 2．6～3．1 －35 20～25 2。5～3．1 46 5．8

SSW－11ava 13．7 －50 65 4．7 －90 40 2．9 105 7．6 OFmudflow 80 －3～一9 260 3．2 －160 150 1．9 410 5．1

一618一 丁召o孟oアz20s ごzJo％g 渉h6％oγ孟hεηz ηz6z堰わz （≧プZ2z6Pθ％勿zs％1ごz6θ． y～zア％ごz26漉の

differential settlement accompanied with the surfaces．From the straight trace of the fault groWth o：f volcanic cone． and northeastward accumulative tilting of On the other hand，Yamazaki（1979） the hills，the Omiya fault seems to be a thought that the northeastem margin of normal fault with steeply－dipping fault plane。 Hoshiyama Hills was a trace of normal fault e。Agoyama£躍旦t（Tsuya，19401Yama－ to comect the Iriyamase and Agoyama zaki，1979）：The Agoyama fault is a reverse reverse faults having a sinistral en echelon fault bounding the eastern margin of Habma configuration．He redefined this normal fault Hills with high vertical slip－rate．Remark－ as the Omiya fault．The height of the fault able features along this fault are the exis－ scarp of the Omiya fault（1ecreases from tenceofsteepfaultscarpof160minheight southeast to northwest．This fault dislocates and the large westwar（1tilting of the hi11s． the Mf一皿surface of20，000yr B．P．and has a The geomorphic surface on the Habuna Hills scarp80mhighinthesouthwestemportion consists mainly of mudflow deposits o1（1er of the fault trace．Whereas in the northwest－ than Mf－I surface of50，000yr B．P．These ern part，the steep scarp turns into gentle surfaces are steeper than those of the alluvial flexurescarp40to50mhigh（Fig．9）．The fan atthe southwestemfoot of Mt．Fuji．The long・term slip－rate along the Omiya fault is （1eformation of Bessho gravels mderlying the estimated to be4to2m／103years（1uring the mudflow deposits is also concordant with the last20，000years． tilting of mudflow surfaces． In the Hoshiyama Hills，the younger mud－ From the accumulative tilting of upthrown flow surfaces dip toward the River Fuji（to side of the fault，the Agoyama fault is esti－ SSW）but onthe contrary，the oldersurfaces mated to be a thrust fault with a relatively dip to the opposite direction （to NNE）． Iow－angle fault plane．Figure10illustrates Iwabuchi andesite and the Bessho gravels the geological cross section（（1－d7）traversing appear on the southwestem margin of the the Habuna Hills and Fujinomiya lowlan（1in hi11s also have a NE－SW strike and NE dip the E－W direction．The Agoyama fault dis－ concor（1ant with the tilting of older mudflow places the base of mudflow deposit more than

SW一［osnIyamahl”s 200m ■ 一 a， a 一＿一＿一一一一一 130m △晒“所』 一戸一一 『1P 5m 9 σ o ＿ノ妄そ．喋奮蕊一 Fan d6P．

，〆’ 幽 6、 100 ．／ OlderFuji／ ’∋・ m 3bム ， ρ ρ o ．！・／ mudfbW 一 。㌔ 5 ρ 5 ▲ 』、 ム 萎OlderFuji ．！ ．！ bウ 轟 6 幽 9 吾f－ mudfbw 会ゐ つ 0 2 ム （ ム

Fuji v。 Mt。Myojo b’ b Bessh・ 惚～ OIder Fuji mudflow ゆ 200m b 遡、 1 一一＿＿＿一一一一一 ’飢 4 100m ｝ ”‘6…ゆ㌔． ト慈 6い 6144・4｝瀞㌔曽 冒一一、 声、， 4 80m ・羅 葭、露厘！箆＝一一’甲甲二3嘉鰯一：勘爺茅倉叩τ・・。．～P曹 o 50酌 r・ρβ 、、 100 ．い ム ”轡欄・二一骸一一一 宕二 4二4じ ＼ R．Urui lwabuchi an eSl e 0

Omiya fault 0 2km

Fig．9 Geologic sections across the Omiya and Agoyama faults（part1）， Section－lines of a－ay and b－b’coπespond to those in Fig．6

一619一

網 βz611（劣∫刀 （～プ 孟hθ （360109づo召l Sz67z7⑳ （ゾノ砿）αアz， Vbl．43，〈石o．10

SW NE C HoshiyamaHiIls C， ヌ 200m ・霧㍗’㌔％ 1暢k4 へあ 4 3 ニレのてコロニ 全、、あ凸，， 940－50m 壁轟陰 100 SWl lava 衰，4 。 4 。 ’，載， ・・2 P～’へ9 H、UrUlR Urui Flluvialdep． R．Fuji 〆l 0 Omiya fauIt Habuna Hi”s 300mEd’ d W ．．費．4．．． OFM ヒ ・靴 R．Urui Mt・Fuli 礁篇蕪 o 200m σ o’ R．Shiba ・1 50m SSW3 ’DriIling kawa ・’蟹呈こ9” 翻 晶棚義へ 贈侵繰灘灘鶴轟霧籍漁一 垂．！。 100m擦i鞭纏 驚瞬 灘繍灘1灘灘馨捌 ．．SW5 蹴論h6錨鱗 ▲ 轟 な 臼 △ 彿 ，： A ． o ▲ Om 410m ， ， ， 「 4 旭Older Fuji mud刊ow I ’ ム 4ム．ゲ （OFM） Agoyama fault 4 ム ， 100m “・ 4 ・ d 4 ． 4

2km ■ △ △

Fig．10 Geologic sections across the Omiya and Agoyama faults（part2）． Section－line of c－c’ and d－d’coπespond to those in Fig．6

410mbetweenthehillsandlowland．Assum． thrust to the north of the river． ing that the base of mudflows was formed a。 亙riyama thr盟st （Konno ＆ Otuka， after the initiation of the activity of older 1933）：Along10km of the Iriyama thrust， Fuji volcano80，000years ago（Machida6渉 Pliocene Fujigawa Group to the west over－ α1．，1975），the minimum mean slip－rate of the thrustsup ontheQuatemarygrouptotheeast． Agoyama fault is estimated to be5．1m／103 Awidefracturezonecanbeobservedalong years．On the other hand，the Agoyama fault the bomdary’between the Hamaishi－dake vertically dislocates the top surface of the Mts．and Kambara Hills． Although the SSW31avaflow（ca．10，000years ago）about Iriyama thrust displaces the Quaternary 50masshowninFig．10。Themeanvertical deposits in the Kambara Hills and marks slip－rate of the SSW3is calculated to be l their westem boun（1ary，no significant fault m／103years．The same slip rates of the two scarp or tectonic lan（1forms are recognized different DRH indicate that the Agoyama along the fault trace．From these character－ fault has constantly repeated the movement istics，Yamazaki（1979）considered that the during the late Quatemary time． Iriyama thrust had ceased its recent activity． （2）hiyama－SMめakawafa眠ltsystem But some evidences of the recent faulting The Iriyama－Shibakawa fault system with were fomd later on several man－made out－ N－S strike extends from the coast of Suruga crops in the Kambara Hills．Sugiyama＆ Bay to the westem foot of Mt．Fuji approxi－ Shimokawa（1982） estimated that the mately25km long．This system is composed Iriyama thrust has continued its movement at of two thrusts dividing the mountain area on a vertical slip－rate of O．25m／103years during the west from the hilly land on the east．The late Pleistocene an（i Holocene． segment to the south of the River Fuji is Although the recent fault slip－rate of the called the Iriyama thmst and the Shibakawa Iriyama trust is less vigorous than that of the

一620一 T66渉o％σ6s σlong 孟h6 ％07オh6ηz盟zごz怨才％ （ゾノ2％P（グz勿zsz61α 侭． Kz〃z〔zzσたの

Iriyamase and Agoyama faults，thewide frac－ which recor（1s the recent activity of the ture zone along the thrust an（11arge（1eforma－ Shibakawa thrust is the older Fuji mudflow tion of the early Quatemary suggest that the of50，000yr B．P．（Mf－1）and the younger Fuji Iriyama thrust had a long history of faulting lava about10，000yr B．P．（SW－l and SW－3）． an（1there was a more active period than the Figure ll illustrates the west－upthrown verti－ present one．Presumably，the Iriyama thmst cal displacement of50m for the Mf－I surface had moved vigorously as a boun（iarybetween andof20mfortheSW－11avasurfaces．Then the uplifting Hamaishi－dake and subsiding mean slip．rate using every DRH is estimated Kambara Hills during the depositional period to be l m／103years．These data indicate that of the early Pleistocene Kambara conglomer－ the Shibakawa thrust has repeated its move－ ate and Iwabuchi andesite． ment more vigorous than the Iriyama thmst b．Sh童蝕誼＆w＆臨臓st（Yamazaki，1979）： during the late Quatemary time． In the The name of the Shibakawa thmst is as－ northemhalf ofthe Shibakawathrust，SW－3 signe（1to the15km long segment of the 1ava serves only a DRH for the recent move－ Iriyama－Shibakawa fault system to the north ment．This is subject to30to40m vertical of River Fuji．The west side of the fault is displacement of W－upthrow．Assuming that upthrown．In the northem extreme of this SW－31avaflowedout10，000yearsago，mean thmst，the strike tums from N－S to NE and slip－rate is estimated to be3to4m／103years． the fault trace seems to plmge into the These facts indicate that the activity of the younger Fuji lava field（Fig．4）．The DRH Shibakawa thrust gradually increases toward

e SW NE e’ 200m 4、’ 50m ♂’40 4 ぐ 一9『ヲρ／．’ ρ7・9ろ ヘロ ペリ R．Shiba 3猫一一一1・・ 陰ξン・ andesite 0 乳。 領。 Fuligawa GrouP BesSり・gravel一。 Shibakawa fault 0 2km f W HabunaHills E f’ 300 300m

〃．急△

R．Shiba 、鴛・ムヘ Mt．Shirao 200 、釜グ凸一ム4 200 △’，ラ SW1 窺㌧

100 100

Fujigawa． Group andesite Bessho Agoyama fauit Shibakawa graveIS fault

Fig．11 Geologic sections across the Shibakawa fault． e－e’section：The50m of fault displacement on MH surface of50，000yr B．P．is observed across the Shibakawa fault． f－f’section：The20m offault displacement on SWI lava flow is observedbetweenthe two sides of Shibakawa valley・． Section－lines of e－e’and f－f’correspon（1to those in Fig．6

一621一 B％ll6！勿z（ゾ 差h6 G60Jo9乞o召l Sz6γzノ（ヨ：y げノ4ゼ）ごz％， ylol。43，ハろo．ヱ0 the north．This characteristics can be inter－ glomerate has accumulated in this new sub－ preted as that the Shibakawa fault gradually sid．ing basin and was soon after covered．by inherits the vigorous activity of the Agoyama the Iwabuchi andesite． During the active fault when they come to each other to the stage of this volcano，the subsidence in the north o：f Fujinomiya city． present Kambara Hills area has gradually （3）0軌erね出s decreased and thesubsi（1ing center hasmoved 我．M＆ts撒o f踊亘t：The NWtrending and northward resulting in the accumulation of SWupthrownMatsunofaultisinferredalong the Bessho gravels in the area of Hlabuna and the River Fuji from the stmctura1（iifference Hoshiyama Hills．This migration is consid－ of Saginota gravels between the Kambara ered to correspond with the development of and Habuna Hills．This fault is interpreted the Iriyamase fault． to be a normal fault which have been forme（1 In the middle Pleistocene，the subsidence in to comect the two en echelon arrange（1 the Habma and Hoshiyama Hills area reverse faults，i．e．the Iriyamase and Shiba－ became less intense，an（i subsequently the kawa faults，during the Middle Pleistocene area has gra（lually change（1into the uplifting time．But this fault has no evidence of recent area．Then，theAgoyama，Omiya andMatsu－ activity． no faults commenced their activity，result－ む．Nosh亘ta 癒臓st ＆翻 亘重s 聾or騒e照 ing in the eastward migration of the subsid－ ext鎌s亘o髄：A conspicuous scarp，whose ing center toward the Fuji－Fujinomiya low－ height excee（1s50m，is recognized at the land．About O．5to O．4Ma，the alluvial fan of eastem margin of the older Fuji mudflow the River Fuji spread over the Kambara and deposit（Mf一皿）distributed alongthe eastem Habuna Hills area to deposit the Saginota flank ofthe Tenshu Mts．Thisscarpseemsto gravels．The s亡bsequent uplifting of Kamba－ exten（1southwestward to the Noshita thrust ra Hills has left the Saginota gravels on the dividing the Miocene Nishi－Yatsushiro hills as terrace deposits． As the Saginota Group and the Pliocene Fujigawa Group in gravels in the Habuna Hills have not under－ the Tenshu Mts．area．As the Noshita thrust gone strong uplift，it seems that the uplift of shows a sinistral en echelon configuration to Habma and Hoshiyama Hills commenced the Shibakawa thrust，the activity of the late，compared to that of Kambara Hills． Shibakawa thmst seems to transfer to the The tectonic inversion in Kambara Hills northern extension of the Noshita thrust． from subsidence to uplift has tumed the char－ acter of Iriyama thrust from a major fault 4．4Geo亘09童ca丑翫量story蹴df蹴亙t腿ove・ between the uplifting and subsi（1ing blocks to me通 a fault representing the relative slip between In Pliocene time，the area of the Hamaishi－ the both uplifting blocks．As a result of this dake and Tenshu Mts．was a part of ancient change，the Iriyama thmst has decreased its subsiding Suruga trough where Fujigawa activity． Group，consisting of thick coarse clastics During the late Middle Pleistocene to the derived from northem mountains，has been early Late Pleistocene，the Iriyamase，Omiya deposited． and Agoyama faults have acted as the bound－ During the early Pleistocene，the west－ ary faults between the upliftinghilly land and upthrown Iriyama and Shibakawa thrusts subsiding lowland．Consequently，the Kam－ have commenced their motion to divide the bara Hills has become a（1issected hilly lan（1 se（1imentary basin of the Fujigawa Group。 due to uplifting and erosion． The west side of the thrust，i．e．the Hamaishi－ On the other hand，the accumulation and dake and Tenshu Mts。，was uplifted gradu－ erosion of the upPer Quaternary have been ally and the axis of subsidence has shifte（i to repeate（1in the Habuna an（1Hoshiyama Hills the east of the thrust．The Kambara con一 where uplifting has been delayed as compar一

一622一 T80診oη乞os ごzlo％g 渉h8η07渉h8ηzηzごz堰づ％ げ12z6Pのz2％s％16z但．yヒzηzごz2ご漉の ed with the Kambara Hills．At this time，the 5．ACT亙VE EAULTS亙N THE SOUTH・ Matsuno faulthad alreadyceasedits activity． ERN MARGIN O量THE TANZAWA Although the Southem Shibakawa fault MOUNTA亙NS has decreased its slip rate due to the develoP－ ment of the Agoyama fault，the northem The area of the southem margin of the Shibakawa fault has maintained relatively Tanzawa Mts．is divided into two portions，i． high slip－rate because no active fault has e．，westem part and eastem part，（1ue to the appeared to the east of the Shibakawa fault． difference o：f topographical configuration． The o1（1er Fuji volcano started its activity In the westem part，as shown in Fig．12，the ca．80，000years ago．Since then mudflow steep Tanzawa Mts．ranging in altitude from deposits from this volcano have filled the 1，500mto800mcontact（1irectlywiththe subsi（iing area to the east of the Iriyamase， Ashigara Mts．（less than1，000m high）and Omiya and Agoyama faults，and frequently Hakone volcano． No obvious depression overflowe（i into the Habma an（1Hoshiyama develops between two mountains except the Hills where the strong uplift had already westem extreme of the area where a small started．． dissected hilly land is situated．While，in the About50，000yFears ago，the Habuna Hills eastem part，the Tanzawa Mts．bounds on completely emerged and the broad Mf－I sur－ Ashigara plain with steep slope facing south． face was forme（10n the hills．In and aroun（i This topographical distinction between the the Hoshiyama Hills，the River Fuji intensely west and east part seems to reflect the char－ incised its chamel associated with the uplift－ acteristic of tectonic evolution． ing and northward tilting of the hills．Head－ erosions from the valley of River Fuji devel－ 5．1 Geology a醜dl c「腿s意＆旦move贈e璽意a丑o遡9 oped some tributaries northward in the hills． 儘eweste膿腿rto量蝕esO鵬恥e膿 Before long，these tributaries exten（ie（i to marg五聾oftheT撒z＆waMts． Fujinomiya lowlan（1，traversing the Ho－ （1） 0臆tli聾e of geology an岨to基）ogr瑚）地y shiyama Hills．Consequently the tributaries a．丁躍zawa Mo撒ta亘薦：The Tanzawa captured the upper reaches of the river which Mts．consist mainly of volcanic rocks of the had flown to the southeast in southwestem Miocene Tanzawa Group（Mikami，1958） foot of the older Fuji volcano．Then mud－ which is bounded on the south by the N－E flows and Iavas from the volcano filled the trending Kamawa fault system（Matsushima narrow valleys in the hills again． ＆Imanaga，1968）．Recent studies（Amano， When the hills uplifted by the recurrent 198611shibashi，1986a，etc．）in（iicate that the fault movement，these valley floors remained Tanzawa Group was originated as a mass of as flat surfaces in the hills． Subsequent head volcanics constituting a ridge on Izu－Ogasa－ erosions from the River Fuji extended to wara arc far south，which approached the Fujinomiya and cause（1stream piracy once Japanese Islands following the northward more．As a result of these processes，i．e． migration of the PHS plate and finally collid． uplifting，head erosion，stream capture，valley ed with Honshu at about5to6Ma（early fill an（i uplifting，a flight of terraces （ieve1－ Pliocene）．In late Pliocene，a new subduction oped on the Hoshiyama Hills during late zone occurred along the southern margin o：f Pleistocene time． the Tanzawa area due to the northward shift Thus，geological and geomorphological of Izu block which also belonged to an off－ stu（1ies in the region to the north of Suruga shore volcanic ri（ige on the Izu－Ogasawara Bay reveal that the NF－S trending tectonic arc．Consequently，the uplifting of the Tan－ landform has been formed by the eastwar（i zawa Mts．was initiated and the Ashigara migration of faulting an（1subsiding since the Group accumulated in the subsiding（1epres－ Pliocene． sion（trough）betweenthe Tanzawa Mts．and

一623一 β％lJ（訪勿z（ゾ 孟h（2θ60109づαzl Sz67∂6ツ （ゾノ41）6z％， Vbl．43，．〈石o．ヱ0 》 難…………1………1………1曝欝 7

Gotenba

0 Mt．HAKONE Kozu 遡 ノミげ ぬ ロ 籔 懸 勘難σ Odawara 9 “ 2 0 G SAGAMI 1、屡、箭渇、，髪， B細 ＼、

O 10km Mlshima f，旧，，「” 癒 Atami

Fig．！2 Geological sketch and locality index map of the southem margin of the Tanzawa Mts。 and the Ashigara Plain－Oiso Hills area． The oblique stripes，fine dots and coarse dots indicate the area of the Neogene Tertiary， the Plio－Pleistocene Ashigara group and late Quatemary sedimentsrespectively．Fault二 L Kannawa fault system，2。Kozu－Matsuda fault，3．Shibusawa fault．

Izu block． Kamawa fault system consists of the In the early Pleistocene，the upheaval of Ashigara Mts．，Hakone volcano and the hilly Tanzawa Mts．was accelerated by the colli－ land in the northwestem extreme of the area． sion and subsequent accretion o：f Izu block The Ashigara Mts．are composed ofthe Plio－ toward Honshu． The exposure of the Pleistocene Ashigara Group． The deep Neogene metamorphic rocks in the central gorges an（1steep slopes in the mountains part of the Tanzawa Mts．indicates that the indicate that this area has experience（1 this range has been subjected to intense intense uplift and denudation process uplifting and denuding during Quaternary together with that of the Tanzawa Mts。due （Matsuda，1978）．The uplifting rate is esti－ to continuous upheaval during the middle to mated to be3to4m／103years by Kaizuka late Quatemary． （1987）．Thus，the Tanzawa Mts．has experi－ The Ashigara Group is composed of coarse enced continuous upheava1（iue to the colli－ and thick clastics from the Tanzawa Mts．， sion－accretion tectonics at the northern having accumulated in a paleo－trough locat－ extreme of the PHS plate during the late ed between Honshu and Izu block associate（1 Pliocene to Quaternary． with the plate subduction．The total thick－ b． The As瞳gara M重s．蹴d亘ts醐jace虚 ness amounts to1，000m． ＆rea＝The topography to the south of the Since the1980ヲs，many much attention has

一624一 T60ホo％∫os ごzlo多z9 孟h6 ％07孟h6ηz％zごz篁～勿z（ゾ12z6P6％勿zsz4ごzσ烹．y4ηzごz2ご漉の

been paid to the stratigraphy，structure and gin of the Tanzawa Mts．gradually tumed evolutionary process of the Ashigara Group， into an upheaval area since after the begin－ because this group has recorded tぬe bulk of ning of the Middle Pleistocene（0．7Ma）． evi（ience on the c』011ision o：f Izu block an（i the Consequently，the upheaval of this area has subsequent upheaval of the Tanzawa Mts． promoted extensive denu（iation to form the （Amanoαα1．，19861Huchon＆Kitazato， steep mountains an（1the hilly land． 1984，etc）．Consequently，the Ashigara Group （2）・Activ童亡y o£the K継醜awa f躁亘意system was divided into four formations and their The Kamawa fault system divi（1ing the Sedimentary ageS were eStimated tO range Tanzawa Group and the Ashigara Group runs from the late Pliocene to the early Middle along the southem margin of the Tanzawa Pleistocene based on the yields of Rz名とz－ Mts．Hitherto，this system was thought to be s孟6go40％側zo名α6，assemblage of molluscan a north－dipping thmst （Kannawa fault） shells， foraminiferal biostratigraphy an（i because the northem block is upthrown and paleomagnetic stratigraphy． the trace is winding （Matsushima ＆ The Ashigara Group shows a dome－1ike Imanaga，1968）．But recently，it is presumed structure dippingto NWandNE．Moreover， that the Kamawa fault is a fault system this group has been subjected to large defor－ compounded by E－W trending thrusts and mation by faulting． For instance，the dips NW and NE trending strike－slip faults gra（1ually increase towar（1the northwest by （Kano6渉α1．，1979）．After this，the author the drag deformation associated with the calls the E－Wtrending thrust as the Kan－ thmsting of Kannawa fault system（Amano nawa thmst，and names the composite fault 6砲1．，1986）． system including the thrust and lateral slip The hilly lan（i to the northwest of the faults as the Kamawa fault system．Machida Ashigara Mts．，where the geology is com－ αα1．（1975）recognized that the Kannawa posed of the Suruga gravels in late Pleis－ thrust has continued its thrust movement at a tocene and overlying late Pleistocene tephras， rate of l m／103years since begiming of the is also well dissected．The accumulation（as Late Quaternary base（10n the vertical（1is－ muchas100minthicknessofcobble－rich placement of Suruga gravels as much as100 facies） of the Suruga gravels in（1icate that mbytworeversefaults（KsandKnfaults） this formation was originally fluvial sedi－ branched from the Kamawa thrust． ments derived from the Tanzawa Mts．and On the other han（i， Kano α α1． （1979） filled a relative depression between the Tan－ pointed out that the Ks fault was not a thrust zawa and Ashigara Mts． but a NE trending strike－slip fault offsetting Referring to the origin of the accumulation the Kamawa thrust．Furthermore，they sug－ of the Sumga gravels，Machida6短1．（1975） gested that the E－W trending Kannawa in：ferred that the gravels have been（ieposited thrust of the Kamawa、fault system has com－ in a relative depression formed by the growth pletely ceased its activity in the late Quater－ of volcanic cone of Older Fuji volcano．Kano nary（ltoαα1．，1986）．But the author does α召1．（1984）suggested that some of the lat－ not agree with the above opinions because eral slip faults，obliquelyF cut the Kannawa there are several south facing steep slopes as thrust，have also participate（i in the（ievelop－ much as100m high，which are likelyto be the ment of that depression．The dissected Sum－ fault scarp representing the recent activity ga gravels indicate that the area has under－ along the Kannawa thrust． gone intense uplift during Late Quatemary The above mentioned data derive the fo1－ time． 10wing history on the movement of the Kan－ Thus，the Pliocene to early Pleistocene nawa fault system．During the sedimentation sedimentary basin of the Ashigara Group of the Ashigara Group in Pliocene to early along the westem part of the southem mar一 Pleistocene time，the Kannawa thmst was

一625一 Bz61」6渉♂％ （～プ 孟h6 G60109ぢαzl Sz67〃6＝ソ （ゾノ41）召％， Vbl。43，〈石o．ヱ0 vigorous as a bom（1ary fault between the describe the intense crustal deformation uplifting Tanzawa Mts．and subsiding sedi－ along the active faults during the late Pleis－ mentary basin． But the Kamawa thrust tocene and Holocene． gradually diminished its activity because the （1） Str我tigraphy an｛董geological struet覗re se（1imentary basin of Ashigara Group tumed O墨the footh童llS into an uplifting area（iue to the progress of The geology of the narrow hills on the the Izu collision in the mid（lle Pleistocene． southemfoot ofthe Matsuda Mts．consists of Ito6オσ1．（1986）pointed out that the NE and the middle to upper Pleistocene clastics un－ NW trending strike－slip faults offsetting the conformably overlying the Pliocene Ashigara Kannawa thrust initiated its activity at that Group．The stratigraphy of the foot hills is as age．The reason for the commencement of follows． the strike－slip faulting oblique to the thrust is a．Ash置gara Gro囎：The Pliocene Neishi ascribed to the decrease of fault mobility on Formation of Ashigara Group unconformably the Kannawa thrust，which is cause（1by the underlying the Pleistocene sediments consists intensi：fication of crustal shortening of the mainly of altemating beds of sandstone and Ashigara Group and steepening of the fault siltstone，andesitic lava and volcano－clastics． plane associated with the Izu collision． This formation strikes nearly NW－SE and Ito6オα1．（1987）conclu（led that the activity dips、40 tg 50 northeast・ Consequently，the of the Kamawa fault system has also wea－ younger and gravel rich formations of kened since the early Late Pleistocene based Ashigara Group appear in the Matsuda Mts． on an outcrop data of the NE trending Hira－ to the north．These formations were main yama fault（Amanoαα1．，1984）．However， source of the coarse talus deposits in the hills． the author disagrees with this thought め．M量雌eto肱teP旦eistoce盟ede脚S亘ts： because the faulting （1ata from an outcroP These Pleistocene deposits are composed of camot represent the whole tectonic move． the fluvial sediments of the River Sakawa， ment on the Kamawa fault system． mudflow deposits from Hakone volcano， talus deposits from Matsuda Mts．and some 5。2 Geo且ogy＆恥〔丑cr覗sta丑moveme聾t a亙o血9 pile of tephra layers from Hakone and Fuji 意恥e easter醜 憂亘a，且£ 0£ 重he sO眼t地ern volcano（Fig．13）．Table3summarizes the m麗g童盟of臨eT膿zaw我瞭s． stratigraphy of this area and the facies of Along the southeastem margin of the Tan－ eachlayer． zawa Mts．，there is a strip of the Matsuda The Kananzawa gravels （Yamazaki＆ Mts．which is a fault－bounded and flat－ Machida，1981）consist of pyroclastic flows， topped block：consists of the Ashigara Group air－fall tephras，mudflow layers from Ha－ and Quatemary sediments．The mountains kone volcano an（1the fluvial se（iiments of the are in fault contact with the subsiding River Sakawa in the late Middle Pleistocene． Ashigara Plain on the south and the uplifting As shown in Fig．13，the Pleistocene sedi－ Tanzawa Mts．on the north．As illustrated in ments in the hills has mdergone the strong Fig12，the two faults bounding both rims of shortening to form a foldstructure with NW the Matsuda Mts．correspond to the north－ trending axial plane． westem extensionoftheKozu－Matsuda fault Kananzawa loam（Yamazaki＆Machida， and the eastem part of the Kannawa fault 1981），overlying the Kananzawa gravels，is a system respectively．The southem margin of pile of tephra layers including some pumice the Matsuda Mts．is fringed with an area500 falls from Hakone volcano in the early Late mtolkmwideofdissectedfoothills100to Pleistocene（0．12Ma）．This loam is overlain 300m high．This narrow foothills comprises mconformably by the Matsuda gravels． mainly middle to late Quatemary sediments． The Matsuda gravels（Yamazaki＆Ma－ The geological history of this foothills wi11 chida，1981） are a sequence of coarse talus

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萎 馨 萎

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Fig．13 Detailed geologic map of the hilly area in the southem foot of the Matsuda Mts。 Locality is shown in Fig．12 Legend1：Ashigara group，2：Kananzawa gravels，3：Kananzawa loam，4：Matsuda gravels，5：Pyroclastic flow（1eposit，6：Fuji tephra Iayers 馨 Black arrows show the direction of the North Matsuda fault and the Matsuda－sanroku fault（MSF）．Section lines A－A’to C－C’correspond to those in Fig．14

Table3 Stratigraphy of the hilly area in the southem foot of the Matsuda Mts． （after Yamazaki6オσ1．，1982）．

蒸 Facies Features Age Hol ene F両iTephras Scoria ash fall Stratified魁1 A丘er30，000yrago o罵 UpPerMats臆dagravel Talusdeposits B祀ccia（1erived丘om 30，000一 一 A血gara group 50，000yrB．P． 血the Matsuda Mts． Cmss－beddedAWtype tephras鵬intercalated

Hakoneyounger Black－graycolored TP飢1（1minthic㎞ess） c＆50，000yrB。P． oqの pyroclas丘cflow Pumice flow underlies the flow QO （Tpnow） 一の o お霞宅℃●自 LowerMatsuda Talusdeposits B1㏄cia derived丘om ca．50，000－ gravelS （Blecciaandloam） A曲匝gara gmup 80，000yrB，P． ill theMatsuda Mts． o一d ■ Kananzawaloam A廿剣ltephra Pumice咄s（KIP6－14） ca。120，000yrB。P。 銚一鵠 mainly加mHakoneVolcano Last血ter－ 飢e血tercalated glaci謡peh（x1

の Kananzawagravels Huvial sedtments Boulder－hchgravels ca．130，000一 Mud∬owdeposits oftheSakawaRiver， 200，000yrBP。 Middle Aadesi丘c mud田ow Pleistocene P皿丘cenow，aεhin andPm丘ceflow MiddlePleistocene 丘Dm H欲one Volcano

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deposits derived from the conglomerate of The other is the end of the accumulation of Ashigara Group in the Matsuda Mts．The the Matsuda gravels．Since then，the south－ gravels contain several intercalated key te－ em flank of the Matsuda Mts．has been ter－ phras such as Pm－I pumice of O．08Ma from raced to form the foothills．This also sug－ central Japan and the Younger Pyroclastic gests a weakening of the activity of the Flow of Hakone volcano（TP flow）of O．05 Matsuda－sanroku fault and the onset of the Ma，and underlie the scoria ash from Fuji new faulting（North Matsuda fault：Yama－ volcano．The age of the gravels is estimated zaki＆Machida，1981）along the present to be middle to late Late Pleistocene time． margin of the foothills． Accor（iingly，the （2）Fa磁movementa亘o聾gthesouthem fault scarp rapidly developed during the late foot of Ma重s題〔丑a、Mts． Late Pleistocene to Holocene at the southem The sedimentary sequence of the southem margin of the foothills．The eastem exten－ foothills of Matsuda Mts．shows two major sion of this fault scarp seems to continue to turning Points on the tectonics in this area． thenorthemtrace oftheKozu－Matsudafault One is the begiming of the accumulation of mentioned later． Matsuda gravels in the early Late Figures13and14illustrate that the foot． Pleistocene．This rapid increase of the sedi－ hills area has undergone a short wave－1ength mentary supply implies the accelerated up－ folding with NW axis associated with the heaval of the Matsuda Mts．，associated with activity of the North Matsuda fault．This the activation of the fault（Matsuda－sanroku folding accompanies several reverse faults fault l Yamazaki＆Machida，1981）alongthe with same strike to the fold axis． The southem margin of the Matsuda Mts． arrangement of these structures suggeststhat

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B’ 暴撃・＼Ma，Sud、 麟鷲誤欝 ．認編 200m 一25・ノ 、graveIS＼ 典 Kananzawa Ashigara ［oam＼ ・三・、㍗・マ1：1’1マ・9・ギ弼灘 150 Group， 稀 B 簑無，熱戯≧講隅 川1川川IIl Tc Ioam ρ く〉 むゆゆ ク ワ ロロ つ ま ッ。 懸唱 ．鑓 t 200 Nor†h 。・。23 DOP ρρ’σ一G 100 リ ロ くン ひ ゅ ロゆ Matsuda ・D。 Kananzawa fauI grave！S ，診．9・’！．9≦←’～』o，’r 150 c 無 TP flow 郷色ノ～ソ≧砲マぐζ一蔑考！ 100 ノ’）狸’多 O 300m 、秩肺《 l／ヤ

Fig．14 N－S geologic cross sections of the hilly area in the southem foot of the Matsuda Mts． Sections A－A’to C－C’correspond to those in Fig。13．

一628一 慧

T60渉o耽s召lo％9孟h碑o席h翻耀顧銘げ伽P召痂s吻侭，y魏σ9剃 the North Matsuda faulthas a leftlateralslip of faulting in this area is the migration of component as well as N upthrown．From this fault movement toward the south with time sense of the fault movement and E－W strike and the associated uplift in the backward of the fault，the stress field in this area is area of the active fault．These characteris－ estimated to be NE compression during Late tics are very similar to the ones in the south－ Pleistocene to Holocene time． Northeast westem foot of Mt．Fuji described in chapter 馨 compression is also pointed out from the 4，though the time scale and magnitude of the micro stmctures developed in the Ashigara movement in this area are a much lesser Group．That is，Huchon＆Kitazato（1983） （1egree than in Fuji area． inferred that the stress field in the Matsuda The spatia1（east－west）difference of the Mts．area shiftedfrom a NWcompressionto tectonic features along the southern margin a NNE at about O．3Ma，based on the minor of Tanzawa Mts．might be ascribed to the faults analysis of the Ashigara Group． difference of relative distances from the However，the orientation of stress field northem tip of Izu block where the PHS derived from the geological evidence（1iscords co皿des with the EUR．That is，the westem withthe expected NWcompressionfrom the part of the Tanzawa margin，10cated on the plate motion of the PHS．This fact suggests north to northwest of Izu Peninsula，is that the complicated local cmstal movement thought to be the plate collision area．The including the rotation of large block has extensive upheaval of the Tanzawa and the occurre（1along the plate convergence bound－ Ashigara Mts．might be caused by the inten－ ary during the late Quatemary． sive crustal shortening and uplift，1ike a pres－ sure ridge，due to the collision of Izu block 5．3 Character亘s髄cs o蛋tぬe crust＆l move－ which camot subduct under Honshu． menta互0醜gt恥esO鵬he騰marg量聾0£ In the eastem part，there are some tectonic 蝕e T癩zawa Mts． features such as the salient topographical Along the southem margin of the boundary between the Tanzawa Mts．and persistently uplifting Tanzawa Mts．，the Ashigara Plain，and the migration of fault style of cmstal movement shows a spatiaI movement． The existence of a subsiding （1ifference between the eastem and westem depression，the Ashigara Plain，perhaps indi－ parts．In the westem part，the se（1imentary cates that the crustal shortening of the east－ basin of the Ashigara Group has been exten－ ern part is less a（ivanced than that of the sively uplifted since the Middle Pleistocene． westem part due to the endurance of plate No subsi（ience has occurred except on the subduction despite the apProach of Izu block． westem extreme where the fluvial deposits Although the migration of fault movement accumulated in the small basin formed by shows a similar pattem to that of the south－ local tectonics or environmental change． westem foot area of Mt．Fuji，there are con－ During the Middle Pleistocene，the activity spicuous differences between two areas for of the E－W trending Kamawa thrust dimini－ the occurrence interval of migration events． shed，andNEandNWtrendingfaultsconcur－ As previously mentioned，the Tanzawa and rently initiated the strike－slip movement． Ashigara Mts．areas are thought to be the The indistinctness of the topographical plate collision zone during the Quaternary boundary between the Tanzawa and Ashiga－ period． raMts．is ascribedtothisdiminishingvertical Therefore，the shorter interval for the fault movement since the Middle Pleistocene． migration in the eastern Tanzawa area as On the other hand，the E－W trending fault compared to that of Mt．Fuji area suggests has created a salient topographical boundary that the interval relates to the rate of crustal between the Tanzawa Mts．and the Ashigara shortening and the convergent style between Plain in the eastem part．The characteristics twO Plates．

一629一

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6．ACT亘VE FAULTS AND TECTONICS time． IN T翻E ASH亙GARA P：LAIN－OISO a． Pre－Quatern鑓y rocks：The pre H亘LLS AREAS －Quatemary rocks whose outcrops are boun（ie（1by faults are distribute（i in several 6．1 0盟t亘量聡e parts，especially in the northwestem part of In the northem extension of Sagami the hills（Fig．15）． These rocks consist of trough，which is generally believed to be the altemating Miocene volcanics，mudstone and plate convergence boundary between the Pliocene conglomerate（Otuka，192910taα PHS and EUR，there are several topographi－ α1．，198211to，1985）． cal provinces such as the Oiso Hills，the 蝕．Q舩te膿麗y sed匡me艶ts：The Quatemary Ashigara Plain，Hakone volcano and the sediments in the Oiso Hills consist mainly Tanzawa Mts．in northem extreme（Figs．2 of the middle Pleistocene Ninomiya Forma－ an（i12）． The Oiso Hills，an uplifte（1block tion，middle Pleistocene to Holocene marine surroun（ied by several active faults，is located and fluvial sediments and an overlying thick in the northwestem extension of Okinoyama tephra sequence from several volcanoes to banks，which extended aIong the northeast－ the west．With reference to these tephras， ern side of Sagami trough，and is thought to Machida α α1．（1974）established the be a part of a tectonic zone cause（i by the chronology of tephra，marine an（1fluvial vigorous crustal movement associated with sediments since the Middle Pleistocene the plate convergence（Kaneko，1971）． At （Table4）． the northwestem extension of trough axis， The underlying Ninomiya Formation there is the subsiding Ashigara Plain which outcrops in the southem part of the （Ashigara depression）on the west of the hills consists mainly of sand an（i mud layers， Oiso Hills，which is thought to have been the total thickness of which amounts to200m． extension of Sagami trough filled with the The age of this group is estimated to be the thick deposits（1erived from the Tanzawa early Middle Pleistocene because the Mts．The NW tending Kozu－Matsuda fault Matuyama－Brunhes magnetic boundary（0．7 rms between the Oiso Hills and the Ashigara Ma）is fomd at the lowest part of the Plain forming the distinct fault scarp of300 Ninomiya Formation（Koyamaαgl．，19861 m high． Honnla （3渉ごzl．，1985）． On the basis of ben－ thonic foraminifera，Yano（1986）estimated a 6．2 Geology and cr腿sta旦 moveme盟t 畳n shallow sea（less than a few hundred meters O量so Hills蹴αAs聴g躍a Pl＆i聖 in depth）was the sedimentary environment （1）Oiso照1且S of the Ninomiya Formation．The relatively The geology of the Oiso Hills consists of small thickness of the Ninomiya Group indi－ marine and fluvial sediments and air fall cates that this group is a non－basin fill sedi－ tephras，which were （1eposited since the ment accumulated in the exteriors of the beginning of Middle Pleistocene underlain by subsi（iing area． the Neogene－Tertiary．The geological struc－ The Quatemary sediments inthe Oiso Hills ture of the hills is characterized by a basin contain8units of marine and fluvial terrace stmcture with an anticlinal axis along the sequence，namely the T－f，T－e，T－d，T－c， westem margin of the hills and a depression T－b，T－a，K and H formation in order of at the central part（Uesugi，1986）．With decreasing age（Table4），and basin fill sedi－ reference to the tectonic evolution of the ments of the Sogayama formation in the hills，the Association of Kanto Quatemary Middle Pleistocene．The age of each terrace Research（A．K．Q．R．）（1980）pointe（i out that deposit is estimatedtobe O．5Ma，0．4Ma，0．35 the tectonic movement change（i from subsi－ Ma，0．28Ma，0．23Ma，0．18Ma，0．13Ma and dence to upheaval during Middle Pleistocene O．06Marespectivelybasedonthestratigraphic

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下a Ashigara 7 1霞K。 Sagami Bay Plain K 8 日ノ9が 0 m

Fig．15 Map showing the distribution of pre－Quatemary rocks，faults and marine terrace strandlines in and around the Oiso Hills（after A．K．Q．R．，1987）． Legend：1．Miocene Tanzawa Group，2．Plio－PleistoceneAshigara Group，3－9。Strandlines of the T－e to H marine terraces． Fault number l LKozu－Matsuda fault，2．Shibusawa fault，3．North Matsuda fault，4． Kannawa fault system，5。Kawaotagawa fault，6．Hadano fault，7．lsehara fault，8．Komu－ kai fault，9．Ikusawa tectonic line． relation with the dated tephras（Machida6渉 1ain by the middle Pleistocene：Ninomiya and α1．，1974，Machida，1975，A．K．Q．R．，1987）． Sogayama Formations and terrace deposits The Oiso Hills can be（1ivided into the younger than T－c formation．The Sogayama following four areas，due to the inequality of Formation which constitutes the Sogayama crustal movement in the Oiso Hills． upheaval block，occurs along the westem and In the northern area of the Oiso H：ills，the southwestem margin of the Oiso Hills．This T－f to K fQrmations of terrace（1eposits， formation consists of mud layers inthe lower－ composed of sand beds and gravels less than most part and pebble to cobble size uncon－ a few tens of meters in thickness，show accu－ solidatedgravelsabout500minthickness． mulative tilting toward the east． Hitherto，the correlation of the Sogayama Accordingly，the older formations are we11 formation in the stratigraphic sequence were exposed in thewest part ofthe area．Most・of not confirmed because of various opinions on the formations are intercalated with the air its age（Kojima，19541Kikuchi＆A．K．Q．R．， fall tephras towar（i the east．The altemated 19771Yano，1986）． sedimentary an（i eolian（1eposits in this area Recently，a glassy ash layer which can indicate that the eastwar（1tilting occurred presumably be correlated to the Handa tuff since after the accumulation of T－D tephra in the upper most Osaka Group，in south west formation（0．4Ma）． Japan was（1iscovered at the lower part of the The westem area of the Oiso Hi11s is under一 Sogayama formation（Mizuno＆Kikkawa，

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Table4 Chrono－stratigraphy of tephras in the Oiso Hills and its relationship to the geomor・ phological surfaces and the terrace deposits．After Machida6砲1．（1974）and A．K．Q・ R．（1987）．

transgr regr Age 羅辞 Surface 「k）rrace depo． Mqrine sed。←一炉 ×104y 軒の9 一坐Ω Tc 「：r皿 r I H M2 一Φ一〇」 一 10 S Ksurface u 盈 「臨tion K

IT－Am 下aI 幽 聴uchiya Φ⊆Φoo彰の■ 隔Φ一ユΦ一でP：Σ 20 Φ一〇陶ΦT－B 下b π Akisawa u T・CU T－C l 下cl 一 一 下du 「颪「一 30『 u T－D 一 l 下dI Sogayamq

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50

1991）．As the Handa tuff has already been The Quatemary geology of the southem recognized in the middle part of the T－D area contains the Kuzukawa（Hamkawa6！ tephra formation of the Oiso Hills（Uesugi，α α1．，1977），Akisawa，Tsuchiya（Kikuchi＆A． α1．，1985），the age of Sogayama Formation is K．Q．R．，1977），K and H formations overlying estimated to be the age of T－E to T－D tephra the Ninomiya Formation．The first three formation． formations correspond to the Sogayama，T－c The similarity of lithofacies and thickness to T－b，and T－a formations respectively． of the Sogayama Formation to that of the Each of these formations consists of marine Bessho gravels in the southwestem foot of sand and mu（11ayers intercalating with thin Mt．Fuji indicates that this formation was a gravel strata． The thickness of each subma血e basin fill sediment accumulated in formation is less than100m．The K（Kis－ the subsided trough close to the coast．The sawa）formation is the transgressive sedi－ deposition of Sogayama Formation implies ments of the last inter－glacia1（0．13Ma）． the occurrence of the NW trending subsided The H formation is the marine terrace sedi－ trough in the westem part of present Oiso ments formed by the post glacial transgres－ Hills during middle Middle Pleistocene time． sion．The depositional surface of this forma－ As the deformed Sogayama Formation is tion is called the Nakamurahara surface unconformably covered by the less（1eformed （Yonekura6齢1．，1968），emerged6，300years T－c terrace deposits，the subsiding movement ago．The top of marine sediments is at an in this area is estimated to have tumed into altitudeofabout20malongthesouthem upheaval before the accumulation of T－c coast of the Oiso Hills．The mean uplift rate formation（about O．3Ma）． in the southern area of the Oiso Hi11s is

一632一 購

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calculated to be about3m／103years． after the Last Interglacial Stage（about O．13 Therefore the gentle subsidence continued Ma）． through the Middle Pleistocene age and this （2） A曲量gara P互a量聡 area was frequently inundated with sea water The Ashigara Plain，10cated in the north－ 魏 at high sea－1evel stages．The change from westem extension of Sagami trough，is subsidence to upheaval might have occurre（i composed of extensive alluvial fan develo－ since after the last interglacial period ih the ping in the lower reach of the River Sakawa southem area．Machida＆Moriyama（1968） and the Holocene terrace with small relative 舞 pointed out that the upheaval of the Oiso height．This plain，bounded on the eastem i慧 Hills became active in the．recent geological and northem margin by the active faults，is a

time，based on the elevation ratio of K to H kind of fault angle depression developed in 雛 terraces．The recent increase of uplifting is the eastem foot of Hakone volcano． As in goo（1agreement with the above estimation． shown in Fig．16，the geomorphic features of In the eastern area，the last interglacial K the Ashigara Plain are characterized by formation composed of sand and pebble extensive development of the HoIocene layers extensively distribute（i to fringe the Kamonomiya terrace and a lack of the last i差 protruding Pre－Quatemary rocks．As most interglacial terrace （K terrace）． The of Quaternary se（iiments un（ierlie the K for－ Kamonomiya terrace is the depositional 嚢 mation（Oka6渉α1．，1979），it seems that the surface of the Gotemba mudflow（GMF： 蓬 subsiding in this area had exceed before the Machida，1964）from Fuji volcano at2β00 accumulation of the K formation．This fact years agO。 垂 also suggests that the tectonic inversion oc－ The gravity map of South Kanto and 群 curred from subsidence to upheaval in the Tokai district（Komazawa，1982）shows the き 萎 beginning of Late Pleistocene． continuous low anomaly zone which stretches き The following tectonic evolution of the from the Ashigara Plainto theAshigara Mts． Oiso Hills is deduced from the geological This anomaly pattem suggests that the thick 嚢 data mentioned above． accumulation of the Plio－Pleistocene き The Oiso Hills was subjecte（1to slight Ashigara Group fills the（iepression basin subsidence during the early Middle beneath the Ashigara Plain．After this the Pleistocene（0．7to O．5Ma）．About O。5Ma， author calls this subsiding depression the the NE tren（iing（iepression occurred in the “Ashigara depressionヲ’． northem and westem areas of the hills to According to the drilling data（Yamazaki deposit the thick Sogayama Formation．But 6砂1．，1982，Fig．17），the Holocene sediments the stable tectonic condition was still continu－ in this plain are composed of the alluvial fan ing in the southem and eastem areas．The gravels distribute（i in the northern part and Sogayama Formation accumulate（1mainly in along the river course，and marine fine sedi－ the westem area of the hills． The crustal ments of the Kamonomiya terrace． The movement of the northem area，where subsi－ Kamonomiya terrace deposit is intercalated dence was originally not intense，gradually with several key be（is such as peat layers， changed into upheaval an（i cause（i the east－ volcanic ash，and GMF etc．，which are pos－ ward tilting． sible to yield the age of the layers。Figure18 About O．3Ma，the crustal movement in the shows the relationship between the ages of westem area converted from the intense sub－ these layers an（1their（iepth． sidence to upheaval and consequently the The younger Hakone pyroclastic flow Oiso Hills tilted towar（i east．Although the （1eposits（TP flow：50，000years o1（1）an（i the slight subsidence in the southern and eastern M2terrace deposits（60，000years old）under－ areas still contimed after O．3Ma，the whole 1ie the alluvium ofthe Ashigara Plain（Yama－ Oiso Hills changed into active uplifting area zakiαα1．，1982）．These deposits are under一

一633一

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國V・ICaniCS 囲硲rtiary 0 2km 懸 Dril畦ng site 饗／ ダBeachridge4Bars

Fig。16 Geomorphological map of the Ashigara plain，southwest Kanto．

1ain by the thick gravels inclu（iing cobbles （Machida，1975）．Accordingly，mean subsi－ derived from the lava of Old Somma stage2 dence rate of the M2terrace deposit is esti－ （OS2） （Kuno，1950，1951）of Hakone volcano mated to be O．65m／103years．Further，the Ah up to500m in depth。 ash，deposited in the brackish water environ－ As the beginning of the OS2stage proned ment near the coast，apPears at－1．8to－2．3m to be about O．3Ma by tephrochronology in depth（Matsushima，1982）．This depth （Machidaαα1，1974），the age of the sedi－ indicates4m of subsidence an（i O．6m／103 ments filling the（1epression is assumed to be years of mean subsidence rate during last yomger than O．3Ma．Therefor，the calcu－ 6，300years ifthe origin＆1sea－level is assumed 1ated long－term subsidence rate of the tobe2mabovepresent． Ashigara depression is more than1．7m／103 The difference of the mean subsidence rate years，While，the mean subsidence rate ofthe between the older and younger markers indi－ younger sediments has a smaller value than cates that the subsidence movement in the that of the older ones．For instance，the M2 Ashigara depression gradually became weak terrace deposit，which appears at－45．3m in through the Late Pleistocene．The depth of depth beneath the eastern Ashigara Plain， Ah ash below the present sea－level means shows39mofsubsidenceatmostifthe that the subsidence of the Ashigara depres－ altitude of the original sea－1evel of 60，000 sion has still continue（i in the Hlolocene， yearsagoisassumedtobelowerthan－6m though the rate has decreased． On the other

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Kαmonomiyα500m boring RADIOCARBON AGE 103y．B．P 10．493m 12 10 10 8 6 4 2 yB。R Om 30 m 臼還88’ζ鑛 G《へF ＋130 f 4930土130 O f i 、：一㎜ 8 … Ah －2．3m ？ 8170±210 ’。㌔09● … ．o 21 fineSGnd si睡 醗95ぞ£臼 ＞ m ’． OO α1奮， ρ’oゐ cobble q si睡 o ’o・6 ．壷 嚇 一10 ・』．P， 罰 o・ρ《》’ grqveI q’■ ’ （＝ ．0 ’ρ ’O 100 マ’9・；o・ 一 一 Om O ρ． o’Or宅 輪／ i 雛、1 ・ρ ■』ρ． m 0，000 ／／ ／ ’ρ，o o 臼 O 輔 0’o，o．臼 「 甲0，0．0 一20 0甲 9730ま200 o臼σ・臼 f．s， 一〇’．’o．o ◎ i → O臼 OP O 工 ・i鷲無罰．一。 ㈱一／ si． ・9◎o・ ／ 9る98』 m f．s． ’o 撤蛭． ・ρ’q O ’0 0． 刀 r鞭蹴自． o・4擾 Ω 一30 sil督y正S． ，ρ 0． 0．甲0・0 A ＞ 蝋麟蚕 ．σ ひ’ …… ・oρ←0 ＞

0’ cby 09’ρ，ク 300 ’o 7 ’0■0’ ／ o・ρ ・ OOiso 婁 o．o’o 一40 ．9．ρ ・ ／ （MArSUSHIMA， ・o■o ●o o． pebble l6℃b＝◎； 10460土170 σ㌦θ 懸Odaw鼎） 円 ’δ121固o 9・ αぞlo green奮u秤g・， 欝 ，ρ．◎1 田 ・？o：沸4 ondesi看e g， ク9・σ ／ ◎（㎜SuSHIMへ マ’00 1982） 一50 0 φ 50 泰癬鰻一 σ・ρ ρ 300 0 0 ， 11060土190 〇一’？ 幽 ．oレ 一10000±200 019■o’ ’ A A A ．づ・o・ sc◎riα q臼α’q 一60 馨膿 o’g． o o O 0，ρぴOl ・。o∫’ O g oρ 』ρρ1

o ρ σ o o ’ Fig．18 o：マ’ク0 00 Comparison of the sample heights and their

・杢・幽 ages of the H610cene sediments between the dqrkbrown あ9窪9 coqrse s． both sides of the Kozu－Matsuda fault．The ρ蓼6イ pebble dated samples of Odawara were obtained 二》画い from the Kamonomiya500m drilling．Ah ．・ρ鱒『 shows the Akahoya ash layer． 二夕ρ．？

（1uction along the Sagami trough．After this Fig．17 Geologicallogsofthe500mdrillingcarried out by GSJ at Kamonomiya in the Ashigara author calls this type of earthquake the plain． “Kanto－type earthquake”，The relationship GMF：Gotemba mudflow deposit of2，300yr between the long term accumulative subsi－ B．P．from Mt．Fuli．Ah：Akahoya glassy ash dence of the Ashigara depression and the layer of6，300yr B．P．from south Kyushu． recent uplift associated with the Kanto－type earthquake will be discussed in Chapter8． hand，the Ashigara Plain，together with the OisoHills，wereuplifted1．8to1．2mbytheco－ 6。3 Koz腿一Ma蕊s腿dla量a朋亘t seismic crustal movement associated with In the westem Oiso Hills area where the the1923Great Kanto Earthquake（Omura， Sogayama Formation thickly accumulated 1925）． during T－D tephra stage，the subsidence Furthermore，the Kamonomiya terrace weakene（l and partly changed into a slight which is a part of the uplifted flood plain of upheaval in the T－C tephra stage（ca．0，3to 2，300yearsagohas3to4mrelativeheight O．25Ma）．Then，the Sogayama block has from the present flood plain．These facts emerged to form the hilly Iand．This tectonic suggest that the formation of the change was presumably caused by the activa－ Kamonomiya terrace was closely related tion ofthe vertical slip onthe：Kozu－Matsuda with the co－seismic cmstal movement of the fault with east side upthrow．Consequently， great earthquakes cause（i by the plate sub一 the origination of the Kozu－Matsuda fault is

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inferred to be at the T－D tephra stage（0。3to tion of fault movement for a few hmdred O．4Ma）． thousand years．This delay suggests that the Inthe laterhalf ofMiddle Pleistocene（025 mean－slip rate which represents the relative to O．13Ma），the T－a marine terrace of O．15 movement of the two blocks divided by the Ma locally extend over the southem part of fault is not always adequate to reconstruct the Kozu－Matsuda fault．This means that the fault evolution and related geological despite the vigorous activity of the Kozu－ history in detai1．The author thinks the most Matsuda，・the fault scarp did not have such a important factor to indicate the degree of large relative height at that time compared fault evolution and to affect the growth of the with present．Accordingly，the scarp hもight fault scarp is not the mean slip－rate of the of the present Kozu－Matsuda fault is thought fault but the absolute movement of each to have mostly grown（1uring the late Pleis－ faulted block。 tocene to Holocene． Table2shows the absolute displacement Withreferencetotheverticaldisplacement an（1（iisplacement－rate of each faulte（1block of the Kozu－Matsuda fault，no precise esti． based on the present altitude and original mation can be derived from the DRH formed height of the M2terrace and Holocene sedi－ before60，000years ago because of the lack of ments．The data on the table reveal that the detaile（i stratigraphical information in the displacement of the Kozu－Matsuda fault dur－ Ashigara depression．But if the author broa（i－ ing mid（ile Pleistocene time was mostly 1y estimates the Iong term slip rate of the compensated by the subsi（1ence of the Kozu－Matsuda fault，it can be said to have a Ashigara depression an（1the upheaval of the value of more than2m／103years since O．3 Oiso Hills was relatively smal1．This means Ma． That is，the sediment beneath the that the faulting of the Kozu－Matsuda fault Ashigara Plain at500m in depth is possibly hardly contributed to the growth of the fault correlated with the T－c to T－a（0．3to O．15 scarpinthemiddlePleistocene．Whileduring Ma）terrace deposits which lie in the Oiso late Pleistocene to Holocene，although the Hi11sabove100minaltitude．Accordingly， faulting on the Kozu－Matsuda fault the displacement of the T－c terrace deposit continued at a constant relative slip rate，the bythe：Koz比Matsuda fault is estimatedtobe absolute movement of each block varied more than600m and the mean slip－rate is widely．That is，the subsidence rate of the calculated to be more than2m／103years． Ashigara depression（1ecreasedwithtime an（i， As mentione（1in the previous section，the on the contrary，the uplift rate of the Oiso M2marine terrace deposits in the Ashigara Hills increased to form the conspicuous fault Plain and Oiso Hills apPears－45．3m and十90 scarp・ m in altitude respectively．Meanslip－rate for In spite of the recent activation of the the Iast60，000years is calculated to be2．3m／ movement that formed the fault scarp，no 103years．With reference to the Holocene fault scarplet displaying the recent faulting is sediment，Matsushima（1982）showed22m of found along the major scarp．The lack of a vertical fault offset during last 6，300 years fault scarplet can presumably be interpreted and calculated at3．5m／103years of slip－rate． as a result of flexure appearing as the surface Thus the mean slip・rate on the Kozu－ expression of the reverse faulting l this may Matsuda fault，which is obtained from the be （iue to the （iistribution of thick uncon－ various fault DRH，indicates that this fault solidated deposits along the Kozu－Matsuda has recurred to the movement at the slip－rate fault． of2to3．5m／103years since the middle Along the northem trace of the Kozu－ Middle Pleistocene． But，as previously Matsuda fault，the obvious fault scarp with mentioned，the appearance of fault scarp on 50mofrelativeheightappearsatthewestem the Kozu－Matsuda fault fellbehindthe initia一 margin of the fluvial M3terrace of50，000

一636一

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T60ホo沈sσJo％9云h卿oπh醐脚顧％げ伽P6痂s吻硯．y4耀9剃 years ago．This M3terrace is also bounded to siding（1epression forme（10n the westem Oiso the east by a graben younger than the M3 Hills area about O．5Ma where the thick terrace．These facts indicate that the west－ Sogayama formation accumulate（1．One plau－ ward migration of faulting has occurred in sible reason for this tectonic change is the the northem part of the Kozu－Matsuda fault transition of the convergent style of plate during the later half of Late Pleistocene to boundary from transform faulting to sub（iuc－ Holoce纂e。This feature（therecent migration tion along the Sagami trough． of the fault movement）is well similar to that Circa O．4to O．3Ma，in the middle Middle of the North Matsuda fault in the southem Pleistocene，the Kozu－Matsuda fault com－ margin of the Matsuda Mts．，10cated to the menced its activity in this subsiding northwest of the Kozu－Matsuda fault．This depression or westem margin of it． The tectonic similarity between two faults indi－ westem side of this fault subsided at a rate of cates that the northern part of the Kozu－ more than2m／103years to form the Ashiga－ Matsuda fault and the North Matsuda fault ra depression．In spite of the vigorous activity have behaved as a single fault in the recent of the Kozu－Matsuda fault，the fault scarp geological time． dividing the Oiso Hills an（1the Ashigara Nakamura6オ召1．（1984）supposed that the Plain was not overt because of the upheaval WNW moving PHS caused the eduction of the hills was still small at that time． along the westem part of Sagami trough and In the late Middle Pleistocene（0．2to O．15 consequently the Kozu－Matsuda fault was Ma），the upheaval rate of the Oiso Hills forme（1as a normal one．But，the develop－ gradually increased and the Sogayama block ment of numerous thrusts with NW－SE composed of the Sogayama Formation in the strike and the anticlinal structure with N－S westem Oiso Hills area emerged． On the axis in the westem Oiso Hills area do not other hand，the subsi（1ence of the Ashigara supPort the above hypothesis and， to the （iepression slowly weakened．Since after the contrary，it indicates that the Kozu－Matsuda last interglacial period（0．12Ma），the uplift fault is a thrust fault with some left lateral of the Oiso Hills accelerated according as slip component． subsi（1ence in the Ashigara Plain has de－ creased．As a result of this tectonic change， 6．4 Tec窟o聾亘c evo且盟t量on the scarp of the Kozu－Matsuda fault has The crustal movement took place towar（i rapi（ily developed，though the relative slip the middle Middle Pleistocene in this area． rate of the fault remained unchanged． The lithofacies and sedimentary structure of the Ninomiya formation which is the lowest 7．SUMM：ARY OF THE QUATERNARY Middle Pleistocene underlying the terrace SYSTEM ANI）THE ACT亙VE STRUC． deposits of the Oiso Hills（io not contain any TURES AL（》NG THE亙ZU BORDER－ evidence of the vigorous tectonic movement LANゆ during the accumulation of this formation． As the plate boundary connecting southem 7．l Q聡舗e職蹴ysyst艦s Tanzawa region and the Japan Trench ought The Quatemary chrono－stratigraphy and to have existed in the early stage of the the evolution of fault systems shown in Fig． middle Pleistocene，the calm crustal move－ 19reveal that the Quatemary system of the ment at that age indicates that the style of Izu borderland is classified into two cate－ the tectonic movement along the plate bound－ gories：the basin－fill type and the non－basin－ ary was possibly not plate subduction but fill type．The former was deposited in sub－ transform faulting or eduction without verti－ siding tectonic basins and accumulated to cal displacement． more than several hundred meters in thick－ Although the reason is uncertain，the sub一 ness．The latter was deposited outside the

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Fig．19 Quatemary stratigraphy of the plate convergence region in the northem margin of Izu Peninsula．Bold line under fault names indicates the duration of the fault movements． Screened and hatched parts indicate the marine and Iacustrine deposits respectively。 subsiding areas．The （1istribution of each for volcanic rocks，are mostly composed of deposit is shown in Fig．20 monotonous conglomerate intercalating with The regional evolution of crustal move－ thin sand or mud beds． The sedimentary ment brought about two groups of sedimen－ environment of such sediments is considered tary basins． One is un（ler continuous subsi－ to be a rapidly subsiding area around amouth dence up to present inclu（iing the fill deposits of river which has transported a large in Fuji－Fujinomiya lowlands and Ashigara amount of coarse materials． plain．Thus the deposits in the basins of this On the other hand，the non－basin－fill type group，namely l a type，are foun（i along the deposits are subdivided into two groups continuation of trough axes on land． The according to their thickness an（i（iuration of crustal movement in the other group（1b deposition．The thicker deposits of the2a type basin）was converted from subsidence type，200to300minthickness，havebeen to uplift，and consequently the se（iiments in accumulated continuously for a long time at these basins are now expose（1and eroded． a low sedimentary rate，without affecting the The Kambara conglomerate，．Iwabuchi an－ intense subsidence．The thinner deposits（2b desite and Bessho gravels in the］Kambara， type），1essthan100mthickareterrace Habuna，and Hoshiyama Hills，the Ashigara deposits，that covered the l b type deposits Group in the southem margin of Tanzawa unconformably during the uplifting period． Mts．and the Sogayama Formation in the The Ninomiya Formation，the Akisawa－Tsu－ Oiso Hills are deposits of the l b type basins． chiya Formations in the Oiso Hills belong to The deposits of the l a and l b type，except the2a type，an（i the Saginota gravels in the

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ノ Present subsiding 圏 depression （1a）

Depression converted ㌧． ／ 猛 to the upheavaI（1b） 〆 Non basin fi［I type ＼1） 騒 deposits（2a，2b） ’》 Quatemary い＼ volcanic rocks フ）驕．“・㌃』聾r1欝e 團 い Active fauIt l！l ノ ＞5m／103y． 〉1m／103y， ノ＼1 2 1汰＼＼ ／ 1 戒聾， ノ く1m／103y 511ノ ．甲1蹴既．甲 ノ・／ non aCtive fault ．細二．。1隔．ρ二・合

0 10km 漫』＼＼i・

、 ’

Fig．20 Distribution of the Quatemary sediments and of active faults in the plate convergence region of the northem margin of Izu Peninsula． Fault name：1。Iriyama－Shibakawa thrust，2．Iriyamase－Omiya－Agoyama fault（sys－ tem），3，Kannawa fault system，4．Kozu－Matsuda fault，5．Kita－lzu fault system， 6．Hirayama fault，7．Shibusawa fault．

Kambara Hills，01der Fuji mudflow deposits other major active faults in Japan，such as in the Hoshiyama and Habma Hills，Suruga the Atera Fault（70km）and the Median gravels in the southem margin of Tanzawa Tectonic Line（300km）． Mts．and from the T－e to H formations in Active faults except for the fault system in the Oiso Hills belong to the2b type． Izu Peninsula are intensely developed along the northern border of Izu block（Figs．2and 7．2 Ch鍵acter亘st童cs o£Aetive Fa腿且ts 23）． They comect Suruga and Sagami The main active faults in the studied area troughs surrounding Izu Block from the are compiled in Table5．Figure20shows the north．This fault zone consists of many short distribution of major active faults in the faults．The geometric and kinematic pattem South Fossa Magna around Izu Peninsula． of these faults，such as the strike of the faults The thickness of each fault line in the figure an（i the sense of fault movement，differs from indicates the long term slip－rate of the fault． place to place systematically． The vertical components of slip rate of the In the western part，to the north of Suruga faults in Table5are so large as to be classi－ Bay，a number of N－S trending faults have fied as the most active faults in Japan．Their （1eveloped inthe Neogene（1eposits since early net slip will be Iarger if strike－slip compo－ Pliocene．Among these faults，the active ones nent is taken into account．Notwithstan（iing are found in the eastem side． This active their high activity，the length of each fault is fault system is expresse（i as zigzag lines extraordinarily short in comparison with running from south to north，where N－S

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Table5 Data list of the active faults in the Izu Borderland．

Nameof血e Faulmame F＆ultlength Faultre蚕erence Displacement Age Sliprate Paper £aultsystem Imdstdke （m） （Ka） （m！103y） Feat田es

（km）

1。猛yama－Shibaka 30＋ N－S wa五aultsystem 1，1㎞yama㎞st 12＋ N－S Riverteπacedeposit 5 W 20 025 Sug1yama＆Shimokawa （1982） 1。2Shibakawath皿st 18→・ N－S F両ilavaSW－1 20 W 10 2 Yamazaki（1979） F吋ilavaSW－3 3（ト40 W 10 3－4 Nonhempa直 OlderF吋iMudnow－1 50 W 50 1 nearShibakawa （Mf。1）

1．3No血tathmst 6＋ N－S Mf一皿 50＋ W 2．5＋ 2。myamase－Agoya 23＋ N－S mafaultsys些e皿 2．1歴yamasefault 10＋ NNE F両ilavaSSW－1 100 W 14 7．1 Yamazaki8∫4此（1981） A皿uvium 46 W 8 5．7 Ahash 31．5 W 6．3 5 22Qmiyafauh 7 NW Mf－m 80 SW 20 4 Noma1五ault F可il＆vaSW－5 80？ SW 10 8？ 2。3Agoyama£ault 14＋ N。S OMFbase 410＋ W 80 5＋ F切ilavaSSW－3 50 W 10 5 2．4Matsuno£ault 6 NW SぱuctuleofSaginotagr． Nomalfault？ 3．Kannawa£ault 25 E－W system 3．1Kalmawa血ust 一一 E占W TanzawaGIAshigaraG N boundaly

3．2Shiozawafault 5＋ NE Sumgagravols 100（V）NW 80 1．2 MachidaεごαZ．（1975） 33Naka嘘ugawaf 5＋ NW TanzawaG／A血garaG 34MatsudaSanK）㎞ 5－ N－S A血garaGIMatsuda N Yamazaki＆Machida 蜘皿t gravels （1981）

3．5Nor血Matsuda 5－ N－S Matsudagravels ？ N Yamazaki＆Machida fault （1981）

3．6Hirayamafault 10＋ NE HakoneOS く500 NW 300 1．7 Itoε鳳（1989） 7 NW 21．5 0．3 4．Kozu－Matsuda 11÷ NW T－c釣皿ation 600＋ NE 300 2 蝕』1t M2deposit 135 NE 60 2．3 Ahash 22 NE 6．3 3．5 M3terrace 50＋ NE 45 1．1＋ tren（1ing reverse faults arrange（1in sinistral The Kozu－Matsuda fault has approximate－ en echelon configuration are connected by 1y straight NW－SE trending fault trace with NW－SEtrendingnormalfaults．The activity a slight undulation．Reversal dip－slip disloca－ of individual faulting increases from west to tion seems to be predominant on this fault． east． Many minor faults rm in the Oiso Hills In the central part，to the south of Tan－ parallel or diagonally to the Kozu－Matsuda zawa Mts。，a major reverse fault trending E－ fault，but most of them are not evident in the W（：Kamawathmst）anditsconjugate fault topography． Therefore，these minor strike－slip faults trending NE－SW and NW－ faults can be regar（ie（i as insignificant SE compose an active fault system（Kan－ subsidiary faults，such as branching or super－ nawa fault system）．The strike－slip faults ficial ones，induced by the movements of the stretch into the Tanzawa and Ashigara Mts． Kozu－Matsuda fault． across the Kannawa thrust．Several active In all these three regions，the active fault faults run parallel to the Kannawa thmst in systems are located exactly on the bomdary its southern si（ie along the border zone between subsi（1ing areas on Izu side and between Ashigara Plain．The southem most uplifting areas fringed with older basin－fill one，extending to theKozu－Matsuda fault is type（1eposits． the most active among them．

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7．3 丁蝕e evo互眠tio醜o｛cr腿st＆且moveme喪ts the basin，or the area of deposition．b）Deve1－ estimated£rom t塵e c血＆醜ges o£fa躍lt opment of terrace（1eposits（type2b）or

’ove ＆ctiv童ty rall erosion evi（1ences the predominancy The change of fault activity in time was of uplifting in the area．c）Accumulation of reconstructe（1 0n the basis of the relative non・basin－fill type deposits （type 2a） indi－ dislocation of two blocks bounded by each cates the transitional period between subsi－ fault．Figure21shows the track of vertical dence and uplifting． In all three cases above， movement rate in each tectonic block．The the duration of each crustal movement is Quatemary se（iiments can evidence the crus－ indicated by the（1epositional age of the sedi－ tal movement，subsidipg，stable，or uplifting， ments．The affects of the eustatic sea level 0f the covered areas（1uring their depositional changes are not taken into account・ ages．The tectonic evolution ofboth sedimen－ The screened spaces between two lines in tary basins and fault systems in time an（1 Fig．21indicate the rate of relative displace－ space was revealed as in Fig．21． ment，of adjoining blocks．For instance，the The relation between sedimentation and change of the space between the Kambara crustal movement is summarized as follows・ Hills and the mountainous area to the west フ a）Basin－fill sediments（type la and lb） represents the decline of the activity of the in（iicate the predo卑inancy of subsi（1ence over Iriyama－Shibakawa faults l the westem 0 50 70 x104yBP

1． 1Hamaishidake ，1～2πゾ103チ TbnshuMts， 1 lRl㌔AMA，SHIBAKA陥 十 弘U口S 一 2 2KanbaraHiUs Iwabuchi肱Kanbaragr 皿SaginotagraveI ＿甲』一一pp IP彫謙一燃』｛『『 皿 1． 3 HabunaHoshiyama 【ニコOldFujimudflow ＿爾塵－．一蹴灘燃一羅聯 Hi11s Saginotagr． Besshogr lwabuchi鴨lca． 3 lRl漁MASE，AG（フYAMA 十 i Kanbaragr 一・i瀧 酬U口S 灘§ll∫ 4Fuli Fulinomiya OldFujimudrbw

2． 5 5拍nzawa M姶． KANN触A FAU口國 十 6 一 6AshigaraMts AshigaraGoup ⊂ゴSurugagravei

9寧．9あ十 皿囲礪罵撒 3． 7Mt、Matsuda Ashigara Gゆup area 7 SANROKU訊UlT 一十liMatsudagr跳8 8MatsudaHi”s 灘 8 黙㌔Ashigaraplainfillsediments Sediment MA「SUDAKl「際 囮羅〔¶ransiti㎝period 9『 plainfi”sediments 弘ULr 9AshigaraPlain一 、煮弊 Subsidenceperiod Others 10 KOZU MA『SUDA 一 9 弘UL『 十 100isoHiIls 1 135m－03y驚rraces Sogayamag暁 Ninomiya Group 一 【＝＝＝二＝二＝＝＝二＝二二＝コ

Fig．21 Diagram showing the regional variation of tectonic evolution along the Izu Borderland． Bold－solid line represents the trace of the absolute crustal movement on each tectonic block． Screened part show the transition of relative slip rate of the active fault bounding two blocks．

一641一 Bz61」4ぎ％ （ゾ 煮h6 （360109乞αzl S％7〃6：y （Zプノ4ゼ）ごz％， Vbム 43，〈ηo。ヱ0 mountains have been continuously uplifted in the uplift of the Oiso Hills．This change can the Quatemary．So the track remains always explain the discrepancy between the constant the plus side．On the other hand，the track of activity of the fault and the topographic the Kambara Hills stays on the negative side evi（1ence． in the early Pleistocene ju（1ging from the The above－mentioned examples show the deposition of basin・fill type Kambara migration of the front of active faulting conglomerate，which evidences subsidence of which is commonly observed in Izu Border－ the region．Then the movement of the Kam－ 1and．Ayoungerfaultdevelopsalongthelzu bara Hills was converted to uplift in the side of an older fault which becomes inactive middle Middle Pleistocene．The alluvial fan or less active after the emergence of younger deposits of the Saginota gravels in the Kam－ one．Such migration of active faults induces bara H皿s marks the transitional epoch．The the shift of tectonic basins，as the active Saginotagravelsisasthickas200mto faults are usually located between the uplift－ suggest gentle subsiding during the de－ ed basin－fill deposits on the Honshu si（1e and positional age．However，it was emerged just the active subsiding basin on the Izu si（ie． after the deposition and have never been covered with yomger se（iiments mtil pres－ 8．：DISCUSS亙ON ent．The Holocene marine terraces in the hills also indicate that active uplifting has 8．1 P丑就e tecto脈ic豊mBl亘catio簸o£a，ct量ve been continuing from the mi（1dle Pleistocene． 歪a，魍亘ts量n the IZUl Bord！er且a，鷺dヒ The track of the：Kambara Hills，therefore， （1）A model of tectonic evolution shifts from minus to plus in the middle Pleis－ Case stu（iies of active faults and crustal tocene as shown in Fig．211t is thus concluded movements in the Izu border land led to a that the activity of the Iriyama－Shibakawa common feature of tectonic evolution in this fault had culminated in the Early Pleistocene region．It is the Izu－oriented migration of and have declined since the Middle Pleis－ active faults and sedimentary basins． The tocene．The Kamawa thrust and the Matsu－ typical field of this migration is the area to da－sanroku fault have also declined in the the north of Suruga Bay． In this area，the Pleistocene．On the contrary，the Iriyamase－ following processes of tectonic evolution Agoyama faults and North Matsuda fault were recognize（1．1）Thick sediments of the have become more active．Though any dras－ Kambara－Bessho gravels were deposited in tic change has not occurre（i to the relative basin developed on the（iownthrown Izu si（ie movement alongthe Kozu－Matsuda fault，the ofafaultsystem．2）Anewfaultsystem absolute movements of the Ashigara Plain （lriyamase fault）was forme（i inside or along and the Oiso Hills have changed in the late the Izu．ward margin of the basin．3）A new Pleistocene． In the middle Pleistocene，the subsiding area（Fuji lowland）has developed Ashigara Plain ha（1subsided rapidly beside in front of the Iriyamase fault replacing the the Oiso district which had been stable rela－ older basin of the Kambara Hills，where tive to sea leve1，but in the late Pleistocene， fluvial Saginota gravels were deposited un－ the Oiso Hills starte（1to uplift accor（iing as conformably to the former basin・fill sedi・ the decline of subsidence in the Ashigara ments．Yamazaki（1984）proposed a mo（1el Plain． of the tectonic evolution in Izu Bor（ier lan（i The growth of Kozu－Matsuda fault scarp accor（1ing to the three processes above． also in（1icates this change．The fault scarp Here，the author proposes a new model con－ ha（i not appeared in the mi（idle Pleistocene sisting of four stages in a cycle and two due to the one－sided subsidence of the substages，improving the previous model in Ashigara Plain，as in画e case shown in Fig．3． order to make it applicable to the entire Izu Then it emerge（1in the late Pleistocene with Bor（1erlan（i（Fig．22）．

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1 2 3 4

欝一 薪，

奮 形「『ノ↓ づ夕 一獅且

／／ A A B A B 嘗． A B

欝 ◎ ◎ い s、． 幽 藷 sぬ且搾騨s蝋奮

嚢 A A

Fig，22 The cyclic development of the geological structure associated with the Izu－ward migration of faulting and tectonic depression．Arrows represent the direction and rate of the absolute crustal movement．Fine dotted part show the same formation．

Those stages and substages are as follows： fault A is divided into two blocks by a new Sねge1：The upthrown side of a thrust fault occurred in the basin．The upthrown faultAinFig．22upheavesconstantlyandthe sideofthefaultAcontinuestorise．Both downthrown side subsides，where the terms blocksboun（1edbythefaultBcontinueto upthrow and downthrow denote relative subside． The subsiding rate of the strip motions along the faults and‘‘upheava1”and between faults A and B decreases，while the “subsidence”are measured relatively to the dov財nthrown side of the fault B subsides very sea－1eve1． The upheaval and subsidence rapidly．As to the faults scarps，this differen－ share the net displacement of the fault A． tial movements of three blocks bring about a The active faults systems from Suruga sharp contrast between fault A and B．The trough to the southwestem foot of Mt．Fuji scarpofthefaultAcontinuestogrowasa are now on this stage l l the Iriyamase fault boundary between the uplifting and the sub－ an（1Agoyama fault correspond to the fault A si（1ing area． On the other hand，no scarp in Fig．22．The subsiding portion of the axis emerges along the fault B，as both sides are of Suruga trough，Ukishimagahara，and Fuji subsi（1ing to bury the fault topography． Fujinomiya lowlands are the tectonic depres－ The present Kozu－Matsuda fault sions of the downthrown side of the active correspondstothefaultAinthestage2， faults．The thick basin－fill sediments such as judging from the rapid growth of the fault the Kambara conglomerate，Bessho gravels， scarp in the late Pleistocene and from the Ashigara Groups，Sogayama gravels，and so decreasing subsiding rate in the Ashigara on，which now compose hills or mountains， Plain．A concealed fault，namely B，is expect－ were accumulated in the previously subsiding ed in the Ashigara Plain or on the eastem areas at stage1． foot of Hakone volcano to the west of the Sねge2：The subsi（1ing area in front of the ：Kozu－Matsuda fault．This fault，which has

一643一

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not yet been recognized，grows possibly far－ the Iriyamase－Agoyama fault system in the ther west of the River Sakawa．Ota6渉召1． eastfallunderthefaultAandthefaultBin （1982） supPosed an active fault along the the model respectively．While the three topo－ westem flank of Chiyo Upland （Fig．16）， graphic units bounded by these two fault which is composed of pyroclastic flow systems correspond to thethree zones divided deposits，in the eastem part of Ashigara by faults A and B。、They are the Hamaishida－ Plain．This fault might be regarded as fault ke Mts．，the Kambara－Hoshiyama－Habma B．However，the altitude of K－Ah ash layer Hills，and the lowlands of Ukishimagahara laid down in blackish water（10es not change and of Fuji－Fulinomiya． across the cliff assumed to be a fault scarp． The fault B in the stage4is the equivalent So it seems that this linear feature is not a of fault A in the stage1，with the same fault scarp but the erosional topography of a crustal movement onboth sidesl so，a cycle of fluvial terrace cliff，and no concealed fault tectonic、evolution is complete（i in these four can be expected between the River Sakawa stages． The next cycle is marked by the and the Kozu－Matsuda fault． occurrence of new fault A derived from the Stage3：The block between faults A and fault B in the previous cycle．That is to say， Bisbeinguplifted（inanabsolutesense）by the most active fault zone migrates from the activity of fault B．This makes the fault Honshu side to Izu side accompanying a Aafaultbetweentwoupliftingblockandits sedimentary basin in downthrown side and a slip rate decreases． The uplifting block uplifted area in upthrown side． between faults A and B is covered Some cases of deviation from this cycle led mconformably with thin deposits from the to the information of the following sub－ upthrown side of the fault A，due to the slow stages：substage l follows the stage l with－ rate of rising． out the formation of a fault B or a new The Iriyamase fault near the mouth of subsiding area in the（10wnthrown side of the River Fuji is regarded as the fault B in the faultAlitshangingwallcontinuestouplift． stage3．Alluvial fan type deposits，such as The recent cmstal movement alongthe：Kozu－ the Saginota gravels in the：Kambara Hills， Matsuda fault may correspond to this sub－ the Older Fuji Mudflow deposits in the Ho－ stage，supPosing the concealed faults to the shiyama and Habuna Hills，．fluvial terrace west of Sakawa River do not exist．In this deposits of T－e to T－a Stages in the Oiso region，the fault scarp of the Kozu－Matsuda Hills，were deposite（1in the gently subsiding fault grows continuously without any older or uplifting areas between faults A and B in fault A behind it．Meanwhile，the rate of this stage3． subsidence in the Ashigara Plain decreases． Stage4：The uplifting rate of the block Such evidence may be interpreted as devia－ between the faults A an（1B is accelerate（1and tion from stage l to substage1． consequently the sedimentary surfaces on Substage2is characterize（1by the extinc－ this block・are emerged to be dissected．The tionofthefaultAwithoutmigration．As activity of fault B becomes more intense with compressive stress increases，the down－ the growth of a fault scarp，（iifferentiating thrown side of the fault A（thrust fault）is the former subsi（ling area intQ terraces an（1 defomedseverely，andfinallyrisesfromthe hills in upthrown side．Meanwhile，fault A subsi（1ing basin．This deformation results in becomes less active，．as the （iifference of weakening the activity of the fault A．This is uplifting rate decreases between the a consequence of the augmentation in the downthrown and the upthrown side． The rising of the downthrownblock，which makes present cmstal movement on the westem the rate of displacement along the fault sma1－ foot of Mt，Fuji exactly corresponds to this 1er and the dip of the fault plane steeper． stage．The Iriyama－Shibakawa faults and Excess shortening of a region induced by

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very large horizontal compression explains ascribed to the form of plate convergence in the phenomena described above． Recent this area．Plate dynamics will be discussed in deformation in the Tanzawa Mts．and the the section8．2．（2）． Ashigara Mts．fall under this substage2， The trough fill se（iiments on the northem where the Kannawa thrust corresponds to the extension of Sumga and Sagami troughs， fault A．At last the fault A becomes extinct therefore，are considered to have been accret－ and both adjoining blocks uplift miformly． ed to Honshu and（1eformed along with the （2）P亘説etecto髄量c童卸亘i蝋量o聾of我ct亘ve subduction of the PHS during at Ieast these 亙蹴且ts hundreds of thousands years．And the defor－ In the Izu Borderland，fault systems and mation of the ac6retionary prism along Nan－ tectonic basin have migrated continually kai trough continues through Suruga trough 鑛 towar（11zu Peninsular during mid（ile to late to the Izu Bor（1erlan（i on land．Consequently， Quatemary．This migration resembles to the the crustal deformation in this region can not evolution observed at thrust fronts in mar－ be regarded as interplate deformation，that is gins of inlan（1 basins （lkeda， 1983，0ta ＆ to say，none of the active faults is a superfi－ cial expression of a plate boun（iary，but it is 選 Sangawa，1984）．However，the migration in 羅 the Izu Borderland is quite different from （1eformation of accretiorary prism，where that in inland basins because of the lack of imbricated thrusts system develop in the 慧 the MBF（main bomdary fault），which at hanging wall over the subducting plate depth is singularly expressed and its low－ （Kagamiαα1．，1983）。 angled branch appears as a migrated frontal Seely6砲1．（1974）propose（10n the mecha－ thrust in inlan（1basins．Inthe Izu Borderland， nism of migration that the underthrusting of き the direction of the migration always ori－ wedge－shaped slices composed of younger ente（1to the Izu Peninsula．Rapidly subsiding trench fill sediment ma（1e the dip ofthe thrust zones which extend to the axes of the troughs steeper finally to immobilize it，and then new develop in front of the most active faults． thrust develope（i in front of the older ones． The basin fill sediments in these zones will be Besi（ies this mechanism，the author supposes differentiate（1by new faults，certain parts of that in case of buoyant subduction，strong which will emerge as upthrown blocks on the compressional stress in accretionary prism Honshu side being folded or tilted toward tends to immobilize older thrusts．The stress Honshu。 can only be released by generating new The Izu－ward migration of tectonic depres－ thrusts in front． Tsuchi（1984）concluded sions an（1the（ieformation of uplifte（101（1er that Neogene series had successively accret－ basin fill sediments are the characteristics of e（1to the west at the Suruga trough in the the tectonic evolution in the Izu Borderlan（1． South Fossa Magna region west of Suruga This evolution is the same as the growth of Bay，because the tectonic units divided by N－ accretionary prism which takes place along a S trendihg faults got younger from west to trench axis at the foot of a continental slope， east．Accordingly，the author thought that where trench fill sediments are continuously the formation of accretionary prisms has accrete（10n continental slope and deformed continued since the Miocene until present by imbricated thrusts。The thrust develops repeating the migration and regeneration of successively into yomger accreted mass．In fault systems and sedimentary basins． the Izu Bor（ierland，this accretion process occur not in deep sea bottom but in shallow 8．2Reg童o賊a亙c血＆餓cterist置cso釜c臓sta丑 sea or on land，while the process of sedimen－ moveme虚s躍繊臨e量ror旦9量醜 tation and tectonics are quite similar to nor－ （1）Reg置0総亘置蝕e配e o聲臨e e職sta亘 mal accretion．This exceptional occurrence move贈e盟t of accretion on land or shallow sea is possibly The active tectonics in the Izu Borderlan（1

一645一

鱒賠欄理 β％lJ6云勿z （ゾ’孟h6 G6010gJαzl Sz∫7z杉ッ （ゾノ41》ごz7z， V「ol．43，〈ηo。10 can be regarded as（ieformation of the sedi－ migration in this region are the smallest in mentary prism in an accretion process，as it the studied area． was conclu（ied in the previous section． The tectonic evolution inthe Oiso Hills an（l Nevertheless，the crustal movements here are the Ashigara Plain is characterized by the not uniform in time and space fortheir extent lack of an o1（1er cycle of accretionary prism． or duration and they sometime deviate from No imbricated structure is found behin（i the the mode1．As to the extent and duration of present active zone。 The author considers accretion process，the one on land to the that an accretion process begun in the middle north of Sumga Bay has more closely－spaced Quatemary in this region an（i a（1ifferent imbricated thrusts（as much as a few kilo－ tectonic regime had been predominant before meters）and much shorter time perio（10f the initiation of the accretion． reactivation（about several hundred thousan（i （2）黙eorig量聡o£臨ereg童o聾a亙d榔ere配e years）than that of Nankai trough and south－ The regional characteristics of the crustal em Suruga trough，where it is5to10km movements in the Izu Border land can be （Kato（3渉α1．，1983）an（i several million years， ascribed to the difference in the form of plate respectively（Shimamura，1986）． convergence． In this sense，the Izu Bor（1er－ Typical deviation from the circular mode1 1and is no longer an uniform collision zone as （Fig．22）was observed in the area of the has been supposed by Sugimura（1972）or southem margin of Tanzawa Mts．，where the Matsuda（1978）and others． This widely tectonic evolution has developed from the accepted collision has never been studie（i in stage l to the substages l and2．Notwith－ detailto substantiate．Forexample，Matsuda standing the conversion of the cmstal move－ （1984）define（i the beginning of collision mere－ ment from the subsidence to upheaval in the 1y by the （1isapPearance of the open sea sedimentary depression of Ashigara Group， between Izu an（1Honshu due to the north－ which had occurred during middle Quatemar－ westward shift of Izu Block．Only Ishibashi y time，neither new fault system nor basin （1896a）discussed the evolution of the form of have developed in front of the depression． convergence in time． He conclu（1e（i that C6nsequently，the uplift of the Ashigara subduction，buoyant subduction and collision Group has continue（1to convert it to a dissect－ have been repeate（1in this order since the ed mountainous area （A§higara Mts。）。 Middle Miocene in the South Fossa Magna． Accordingly，the fault between the two uplift－ Now，the author proposes to apply Ishiba－ ing blocks，here the Tanzawa Mts．and shi’s three variation of the convergence form Ashigara Mts．，became less active andfinally not only to the historic development but also ceased to move．This transition of the fault to the regional difference of crustal move－ movement presumably was induced by the ments．On the solid basis of active tectonics， decrease of relative slip （iue to the rapi（i the so－called collision zone of the Izu Border－ uplift of the Ashigara Mts．The dip of the 1and can be sub（iivide（i into collision zone reverse faults has also become steeper sensu stricto，sub（iuction zone an（1buoyanガ because of the shortening of the block with sub（iuction zone． increasing compressive stress． In the area to the north of Suruga Bay，the The crustal movement between Yamakita crustal movement has been subjected to the and Matsuda along the southem foot of the sub（1uction or buoyant subduction of the Matsuda Mts．agreeswiththecircularmode1． PHS．The migration of fault system and the But the width of each block between two formation of sedimentary basin are common faults is very short，up to l km and the tectonic results of both subduction styles． （1uration of one cycle is also very short up to With reference to the these two subduction one hundred thousan（1years．So，the horizon－ style，Ishibashi（1986a）took the cumulative tal spacing and the time interval for the fault uplift of hinterland as evi（ience of the buoy一

一646一 7セo渉o多z彦os ごzloη9 渉h6 アzoπh6刀z 〃zごz響力z（ゾ1ゑz6P6アz勿zsz46z〔E．｝そz％zごz2ご漉の

ant subduction．The sub（1uction style in the Matsuda Mts．，the Oiso Hills and the Ashiga－ area to the north of Suruga Bay is supposed ra Plain indicate the underthrusting of the to be buoyant because the distinctive and PHS beneath these areas．The rapid uplift of extensive uplift（4mm／yr）is recognized in the Tanzawa Mts．area behind the Matsuda the Akaishi Mts．，the hinterland of this area Mts．canbe ascribedto buoyant subductionin （Danbara，1971）． This feature is distin－ this part．However，the spatial intervals of guished from normal subduction such as active faults here are much shorter than along Nankai trough，where coseismic uplift those to the north of Suruga Bay an（i also the has been repeated without conspicuous cumu－ migration of active fault front has occurre（i 1ative upheaval of the accretionary prism an（i more frequently．Therefore，the transition frontal arc．Along the southem margin of from buoyant sub（1uction to collision may be Tanzawa Mts．，both sides of the Kamawa under way in the Matsuda Mts．area．These fault system have been uplifted without regions lack older accreted block in Honshu generating younger basin，and the fault activ－ side behind recent active zone，and now we ity itse1：f is not so high． Intense horizontal can observe the first an（1the latest cycle of compressive stress raises all this area，and accretionary evolution． This cycle com－ the former main fault is no longer a boundary menced in the middle Pleistocenebecauseofa fault between blocks，being truncated by a new plate convergence system that had oc－ number of strike－slip faults．This may indi－ curred at that time．But this convergence as cate that brittle disruption of the blocks is well as the possible transition inthe Matsuda occurring，or about to occur in this region。 Mts．areahasnotbeenevidencedgeologically This style of cmsta1（1eformation exactly to discuss its cause．These regional charac－ correspon（i to the collision s6nsz6 s渉万o孟o teristics above are summarized in Table6． （1efined by Ishibashi（1986a）．Therefore，Izu The arrangement of plate convergence Block colli（1es against Honshu only along the forms seems to be systematic in the Izu southem margin of the Tanzawa Mts．in the Borderland． That is，the collision s6％s％ Izu Bor（ierland． s翻o孟o occurs between the Tanzawa Mts．and The active accretion process around the the northem most part of Izu Block．The

Table6 Regional tectonic variation along the plate convergence zone．

SubducdonZone BuoyantSubduc匝onZbne Transitional Zone Collision Zone Rigion Nankai－Southem Nor血emSumga Southem fbot of No質hem H欲one Smgatmu喜h tmuεhtoSW五〇〇t theMatsudaMts． andS，marginof ofF吋iVolcano W．Tanzawa Mts．

Dip ofsub一 ducdngslab 13－141） More than2301）2） ？ ？ （degree）

Distance of i血bhcated 30－10㎞3） less than10㎞ 1㎞＋ 0 thmsts

Age ofthe Mi㏄ene to Lower to Middle MiddletoUpPer

youngest LowerPli㏄ene4） PlOistocene pleistocene 一 a㏄re豆onaw prism

T㎞e illterval ofmゆrfault afewMa less than l Ma less than O．1Ma 0 mig【ation

0血er fbatures Upheavalofplate boundalylegion

Reference cited 1）Katoε∫α’．（1983），2）Ishida（1986），3）R。GA．F．」．（1980），4）Sh血amura（1986）

一647一 Bz6116ガ％ （ゾ’孟h6 G60Jo91αzl Sz67∂6ツ （ゾノ4プ）ごz7z， Vbl．43，No．10 buoyant subduction occurs both in the west north of central Suruga trough is because the and in the east of the collision zone，north of dip is too steep to recognize the slab surface Sumga Bay and the Matsuda Mts．area by seismic reflection profiles．The relation－ respectively，an（i then normal subduction ship between the forms of convergence and occurs in both extensions of the Izu Bor（1er－ the dip of slab is supposed as follows：as the 1an（1．This symmetrical arrangement of three （iip becomes steeper，the form varies from convergence forms must be originated from normal subduction to buoyant subduction， the steepening of the subducting slab．Figure an（i finally to collision an（i in the meantime 23is an isobath map of the upper surface of the intervals of imbricated thrusts get nar－ the subducting slab of PHS，with submarine rower．The steepening of the slab dip toward active faults（R．G．A．F．J．，1980）overlaid to the collision bomdary should mainly be show the intervals of imbricated thrusts．The originated from the buoyancy of Izu Block． （iip of the subducting slab becomes steeper Besi（1e，the steeper subduction results in tight－ from Nankai trough to Suruga trough（Kato er coupling of the blocks on both sides，where 6渉α1．，1983）．The absence of isobath further the tectonic stress in（1uce（1 by the plate

30 20 ゆ ＿ぢD ρ 勿 一錠09 ε ．祠《亀、 いM、 輩 』」 ．齢 ｛

趨 2 名レ乏 4勇

50km ぢ多、健▽ ！一／一／一 し6・ 8ノ ／ － ／ ／ ！、 ／ ／ （ 、5 7 ／ ご＿ノ ／ 6／

Fig．23 Relationship between regional variation of the subduction angle of the PHS plate and tectonic deformations including the active fault movements． Bold lines：Active faults along the plate convergence zone（after R．G．A．F，」．，1980）．Thin－ solid lines along the Nankai trough l Contours of depth to the basement of Izu Spur． Numeral is in kilometers below sea leve1（after Kato6！α1．，1983）．Thin－broken lines along the Nankai trough：Contours of depth to oceanic basement（after Kato6渉α1．， 1983）．Thin－solid line in land：Contours of the upper surface of the subducting Philippine sea plate （after Ishida，1986）。

一648一 T6。オ。廊σ1・％9渉h碑・πh6甥脚麟げ伽P6痂s吻偉・｝伽伽切

motion is easily transmitted to Honshu to model of the cmstal movement in Fig．24 bring about intense crustal movement in the explains well this overlapping an（1the three hinterlands． pieces of evidence above．The step－like lines in Fig．24show the vertical movements of the 8．3 Seismotecto恥ics o歪t翫e Iz盟Bo「〔翌e「且a醜dl Oiso Hills（upper，uplifting）and the Ashigara Large vertical slip rate disproportionate to Plain（10wer，subsiding）．This demonstrates the length distinguishes the active faults in that the latest Oiso－type earthquake is dated the Izu Borderland from the other major ca．2，300yr B．P．and there after this region inlan（i active faults in Japan．Frequent occur－ has been uplifted only by Kanto－type earth－ rence of large earthquakes is usually－expect－ quakes． ed from faults in order to maintain this high The other active faults seems to have simi－ activity． Nevertheless，no earthquake has 1ar characteristics of seismicity to the Kozu－ been recor（1e（1in this region except for the Matsuda fault ludging from their large slip Ansei－Tokai earthquake of1854，when the rate an（i absence o：f earthquake record in the Iriyamase fault moved around the mouth of last1，000years． These active faults are the River Fuji． imbricatedthrusts in accretionaryprisms and The mode of seismic activity，i．e．recur－ can be regar（ied as subsidiary faults branch－ rence interval of earthquakes from a fault ing from the main fault．The main fault itself and their magnitude or amount of slip，has is the plate boundary between the PHS and been estimated only for the：Kozu－Matsuda EUR and only this fault acts as earthquake fault in the Izu Borderland．Matsudaα召1． source．Therefore，the recurrence intervals （1978）identified the：Kozu－Matsuda fault to of earthquakes cannot be proportional to the the source fault of the Oiso－type earthquake． activity and the length of each surface active Furthermore Matsuda（1985）estimated its faults．The seismicity along the plate bomd－ recurrence interval at l70years and the verti－ ary in t五e Izu Bor（ierland is not so active as cal slip per event at O．5to1．Om，based on the 39 assumption that the magnitude is M＝7con－ ／① si（iering that the fault length（10to20km）is 糀、二擁／／ not large． 恥 On the other hand，Yamazaki（1985） 申 島 ／ ＝》 ／ proposed a quite different estimation that the ／ ／ interval is as long as 2，000 years and the ／ ！ 回α6錦3y vertical slip more than7m．These values ／ 4 ／ were drawn on the basis of geological an（1 0 『『T 6Ky geomorphological evidences．They are：1） 8 議 A△A蝿Ev．。農② The Ashigara Plain has been subsiding con－ 器 stantly at a・rate of O．65m／103years since 罹 AS糊GARA PLAlN 蓋 middle Late Pleistocene．2）Both the Oiso 3 EARTHQUAKE Hills and the Ashigara Pain were uplifted by △OISOfypeぬKANTO骨ype the great Kanto earthquake of1923．3）Three levels of the Holocene marine terrace sur－ Fig．24 Diagram showing the relationship between faces are recognized along the Oiso Hflls． the cmstal movements on the Oiso Hills and Accordingly，two modes of coseismic uplift， Ashigara plain and the occurrence of the Oiso－type earthquakes． the first being the Oiso－type earthquake with L Uplift rate of the Oiso Hills． 2： long recurrence interval and the secon（i the Subsidence rate of the Ashigara Plain．Filled Kanto－type earthquake with shorter interval triangles： Occurrence of the Kanto・type （about800years），were considered to overlap eαrthquakes． Open triangles：Occurrence of in the Ashigara Plain and the Oiso Hills．The the Oiso・type earthquakes．

一649一 β％Jl（3ガ銘 （ゾ 孟h6 0601091αzl Sz67∂6ッ （～プノ吻）ごz％， Vbl．43，〈石o．ヱ0 along the Nankai trough or the southern Izu Bor（ierland an（1 interprete（1 that the Suruga trough．The difference of the form of active faults in the region are characteristi－ plate convergence again well explains the cally the imbricated thrusts with short longer recurrence interval in the Izu region． length and high slip－rafe occurred in the That is to say，the buoyant sub（1uction and accretionary prism．This study also revealed the collision in the Izu Borderland consume that the Izu Bor（1erland，previously believed much less energy in faulting along the plate to be a uniform collision zone between the bomdary than a normal subduction．So，slow PHS and EUR，can be（1ivided into two zones， rates o：f stress accumulation along the plate the zone of collision s6％sz6 sかづ6オo an（i the boundary faults causes the recurrence inter－ buoyant subduction zones，on the basis of the val to be longer than that of the boundary regional features of Quaternary tectonics． along Nankai trough． Besi（1es，the author discussed．the style and Ishibashi（1988a，b）proposed the N－S tren－ recent activity of an active fault in this area ding fracture zone，West Sagami Bay frac－ with respect to the coseismic cmstal move－ ture zone，splitting the PHS onthe east of Izu ment．Consequently，this stud．y may be useful PeninsulaforthesourceofM＝7sizeearth－ in the advancement of the knowledge on the quakes which have caused serious damage on seismotectonics in the Izu Borderland which Odawara city with about73years interval is in（1ispensable for the earthquake pre（iiction during historical times． This proposal is inJapan． mainly base（i on the analysis o：f the crustal The main results are as follows． movement at great Kanto earthquake of 1） The study area to the north of Izu 1923，0f tsmami sources and of damage dis． Block is divi（ie（l into three areas accor（iing to tribution of several historical earthquakes． their tectonic characteristics． In each area， In spite of the obvious structure o：f the the Quatemary chrono－stratigraphy， geo－ proposed model of the fracture zone，no evi－ 10gic structures and active faults，especially dence supPorting the proposal is found except with respect to their history of activity and the uplift of the Hatsushima Island off Atami the crustal movement in both sides of the coast and submarine scarps in Westem fault，were described． Sagami Bay．Although the fracture zone is 2）Distinctive active fault systems have inferred beneath the westem margin of the developed in the Izu Borderland surrounding Ashigara Plain，any supporting evidence can the northem margin of Izu Peninsula．Thrust not recognized in this area at present time． type dip slip faults are predominant in this Accordingly，it is difficult to comment on this region with the highest class slip－rate in fracture zone from the viewpoint of the Japan． In the area to the north of Suruga Quatemary tectonibs on land．． Bay，there are two N－S trending fault sys－ tems which consist of several short faults 9．CONCLUSIONS arranged en echelon．The eastem，Izu side system is younger and more active than the The Izu Bor（1erland is a convergent plate westem system in the Quatemary．This indi－ boundary on land that combines Suruga cates that the place o：f intense fault activity trough with Sagami trough． The tectonic migrates Izu・ward from west to east．The evolution and recent cmstal movement along same phenomena wasrecognize（1inthe south－ this boundary are discusse（i on the basis of em foot of Tanzawa Mts．The length ofeach geological and geomorphological research fault，1ess than20km，is very short dispropor－ from the viewpoint of plate tectonics． tionately to its high activity（slip－rate more Through this research，the author revealed than l m／103years）as compared with the the significance of the regional Quatemary same type of active faults in other regions of faulting in the setting of plate tectonics in the Japan．

一650一 T60渉o％乞os ごzlo／z9 孟h6 7zoπh6ηzηzごz讐歪％ （ゾ1乏z6Pε％1箆sz諾召 偉．y4ηzαzごzた∂

3）The Quatemary system in the study have a recurrence interval of several thou． area is classifie（1into basin fill type and non sand years and the displacement by an event basin fi11－type deposit on the basis of their should be more than several meters．They lithofacies，thicknesses and tectonic features． are essentially imbricated thrusts in the ac－ The active faults develop on the boundary cretionary prisms：therefore，it is the between older uplifted deposits in the Honshu megathrust along the upper surface of the side and present sedi血entary basin in the Izu subducting slab which is the controlling force side．Subsiding areas have migrated towards of the activity of the above mentioned fahlts． Izu Peninsula along with the migration of The seismic activity represente（1by the fault systems． active fault movement along the northem tip 4）The Quatemary tectonics inmost Pa「ts of the PHS（iecreases from the normal sub－ of the Izu Bor（ierland are controlled by buoy－ duction zone of Suruga and Sagami troughs ant subduction of the Philippine Sea Plate to the collision area ofthe southemmargin of except for a small portion where exact colli－ Tanzawa Mts．due to the steepening and the sion occurs．That is，the Izu Borderland is buoyancy of the subducting slab． This not a uni：form collision zone as has been decrease accords with the low frequency of supposedl widely．The buoyant subduction is earthquakes in the Izu Borderland． evi（ienced by the formation of accretionary prism on lan（1in this region．Accordingly，the Ack盟ow亙e〔墨geme麺ts active faults in the Izu Bor（1erland are im－ bricated thrust faults caused in accretion The author expresses his gratitude to Mr． process，as branche（i fault from the concealed Yoshihiro Kinugasa，the chief of the Seis－ boundary fault of plates．This is one of the motectonic Research Section，Geological reasons why the active faults in the Izu Survey of Japan，and Professor emeritus Borderland have higher slip－rate than those Sohei Kaizuka and Professor Hiroshi Ma－ in other region of Japan independent of their chida of Tokyo Metropolitan University，for fault length． their kin（1 supPorts and guidances of this 5）It is impossible to interpret all of the work．An（1he is also in（1ebted to Professor active faults and crustal movement from the Tokihiko Matsuda of the Earthquake viewpoint of the accretion process．The three Research Institute，University of Tokyo and regions divided in the conclusion1）show Dr．Katsuhiko Ishibashi，Building Research distinct regional tectonic difference． This Institute，Ministry of Construction，and co1－ difference is ascribed to the variety of the 1eagues of Seismotectonics Research Section， form of plate convergence． The form Geological Survey of Japan for their discus－ changes from collision s6nsz6 s渉万660 in the sion and suggestion on the Quaternary southem margin of Tanzawa Mts．，to buoy－ tectonics in the Japanese Islands． ant subduction in both eastem and westem flanks of the Borderland，and finally normal Refere亙亙ces subduction along the Suruga and Sagami troughs．Besi（ie this difference in space，the Amano，K．（1986）South Fossa Magna as multi transition of convergence form were reco9－ －collision zone． E6zπhノ臨o％云h砂，vol．8，P． nized：in the southem margin of Tanzawa 581－585．＊ Mts．，buoyant subduction was converted to ，Yokoyama，K．and Tachikawa，T．（1984） Hirayama fault which cuts an older collision at the en（l of Early Pleistocene．In somma of Hakone Volcano．10蹴G601． the Oiso Hills and the Ashigara Plain，buoy－ Soo．ノ41）召％，vol．90，P．849－852．＊ ant subduction commenced in early Middle 一一一r Takahashi， H。， Tachikawa， T．， Pleistocene． Yokoyama，K．，Yokota，C．and Kikuchi，J． 6） The active faults in the Izu Borderland

一651一 B％lJ6あ％ ρプ 孟h6 060109乞041Sz67z杉ッ （～ブノ41）ごz冗， Vηol。43，〈石o。ヱ0

（1986）Geology of Ashigara group－Collisi－ “Jishin－yochi no Houhou（Method of the on tectonics of Izu micro－continent with earthqua．ke prediction）”，U％初．oグ To初o Eurasian plate．Kitamura Commem。E3s砂s P名6ss，P．193－209．＊ G601．，p．7－29．＊＊ （1981） Specification of a soon－to－occur Association for Kanto Quatemary Research（1987） seismic faulting in the Tokai district，cen－ Stratigraphy an（i geologic structure of tral Japan，based upon seismotectonics． Oiso hills．Bz61」且ssoo．Kごz％孟o Qz6ごzオ．ノ～6s．， Earthquake prediction－An introductional vol．13，p．3－46．＊ review，〃碗n’06 E”初g S6吻s 4，P． Danbara，T．（1971）Synthetic vertical movements in 297－332． Japan during the recent 70 years．ノ；oz6名 （1984）Plate motions in south Fossa 0604．Soo．ノ4）召％，vol．17，P．100－108．＊＊ Magna－an application of the Japan Sea Ikeda，Y．（1983）Thrust－front migration and its subduction hypothesis． E召7オh ノ以o多z孟h砂， mechanism －Evolution of intraplate vo1．6，P．61－67．＊ thrust fault systemr Bz611．五）ゆ渉．Gθ097． （1986a）History of the plate motions in U痂∂．To勧o，vol．15，p．125－159． the South Fossa Magna：an essay．E副h Harukawa，M．，Iso，N．，Uesugi，Y．，Mori，S．and κo％渉h砂，vol．8，P．591－597．＊ Nagasaki，T．（1977）On the stratigraphic （1986b）Northeast Japan－North Amer－ classification and correlation of so－calle（1 ican Plate theory and theory of eastward Ninomiya formation （2nd rep．）． β％ll． migration of Southwest Japan， 動πh ／15sooκα盟孟o Qz6ごzオ．1～6s．，vo1．4，p．18－32．＊ 巫o％オh砂，vol．8，p．762－767．＊ Hatano，S．，Tsuzawa，M．andMatsushima，Y．（1979） （1988a）‘West Kanagawa Earthquakeン Vertical movement in Holocene age and and earthquake pre（1iction I．Kごzgαたz6，vol． geodetic time along the northem coast of 58，p．537－547．＊ Suruga Bay．R勿， Coo名ゴ劾σガng Coηz吻． （1988b）‘West Kanagawa Earthquake’ Eごz7オh（7．P名召4．，vol．21，P．101－106．＊ an（l earthquake prediction II．Kごzg召h鋸，voL Honma，M．，Uesugi，Y．，Nakai，H．and Someno，M． 58，p．771－780，＊ （1985）Paleomagnetic study of the Myoken Ishi（ia， M．（1986）The configuration of the formation（Ninomiya Group）in Ninomiya Philippine Sea and the Pacific plates＆s －cho，Kanagawa Prefecture，Japan．B％ll． estimated from high－resolution micro ／1sso6κごz7z云o Qz6ごzオ．み～6s．，vo1．11，P．17－22．＊ －earthquake hypocenter． 1～のり． 地ゑ 1～6s， Huchon，P。and Kitazato，H．（1984）Collision of the 06η彪7Z万sσs．P名6zノ．，no．36，P，1－19．＊ Izu block with central Japan during the Ito，M． （1985）The Ashigara group：A regressive QuaternaryF and geological evolution of the submarine fan delta sequence in a Quater－ Ashigara area．乃6歩o％oゆ勿sJos，voL110，p． nary collision boundary，north of Izu Pen－ 201－210． insula，central HQnshu，Japan。S6（！麹6％如η Ibaraki，M．（1981）Geologic ages of the Neogene 06010gy，vol．45，p．261－292． complex in the South Fossa Magna based Ito，T．， Kano，K．，Uesugi，Y．，Kosaka，K．and Chiba， upon planktonic foraminifera，10蹴G601． T．（1989）Tectonic evolution along the Soo．ノ4卿z％，vo1．87，p．47－49．＊＊ northemmost border of the Philippine Sea Ihara，K．and Ishii，K．（1932〉The earthquake of plate since about l Ma．Tεo競oρh鋤os，voL northern Izu． ∫盟z1）6短ごzl σ60ゐ Sz67zノ．ノ砿）ごzn， 160，p．305－326． 名4）07あno．112，111P．＊＊ ，Uesugi，Y．，Yonezawa，H。，Kano，K，， Ishibashi，K．（1976）East－off Izu tectonic line an（i Someno，M．and Chiba，T．（1987）Analyti・ west Sagami Bay fault as an origin of the cal method for evaluating superficial fault Izu Peninsula uplift．ノ16s渉飢 S6乞sηzol．Soo。 displacements in volcanic air fall deposits： ノ4ψ6zn，no．2，vo1。29。＊ Case of the Hirayama fault south of （1977）Reanalysis of the1923great Kanto Tanzawa mountains，central Japan，since earthquake（2）一tectonic implication of 21，500years B．P。10％プ．G60ρh夕s。ノ～6s．，voL the Nishi－Sagami Bay fault．∠46s厩 92，p．10683－10695， S6丞ηzol．Soo．ノ4ψα％，no．1，129。＊ ， ，Kano，K．，Chiba，T．，Yonezawa， （1978） Practical strategy for the H．，Someno，M．and Honma，M．（1986） earthquake prediction．In Asada，T．（ed．） Paleogeographical and tectonic develoP一

一652一 丁昭0競Sα伽9飾碗07診死翻耀㎎魏0ゾ伽Pε痂S吻偉，y襯磁劔

ment in Ashigara and Oiso area since l part of the Oiso Hills．一as the Result of Ma．E観h〃10窺h砂，vo1．8，p．630－636．＊ the1976，investigated by Research team of Kagami，H．，Shiono，S．and Taira，T．（1983）The westem Oiso Hills，一 B％ll．z4sso6．κ伽渉o． plate subduction and formation of accre’ Qz鶴孟．去～6s．，vol。4，p．2－17．＊ tion prisms along Nankai Trough．κα即肋， Kobayashi，Y．（1983）On the initiation of subduc－ vol．53，P．429－438．＊ tion of plates．E召7オhノ砿o％云h砂，vo1．5，P．510 Kaizuka，S．（1975）A tectonic model for the mor－ 一514．＊ phology of arc trench systems，especially Kolima， N．（1954）Geological study of the Oiso for the echelon ridges and mid－arc fault． district，Kanagawa Prefecture．10蹴G601． 侮）のz6s6∫oz6名G60乙G6097．，vo1．45，P．9－28． So6．ノ碕うα％，vo1．60，p．445－454．＊＊ （1984） Lan（iforms in and aroun（1the Komazawa，M （1982）Gravity map of Shizuoka， South Fossa Magna and their tectonic Omaezaki and Yokosuka（Bougue anom－ process of growth．（2％砿1～6s．如醐，vol． alies map），1：200，000。G6010g磁l S％7昭yげ 23，p．55－70．＊＊ 血勿％，耀s661伽60％s銘砂sS副6s，22． （1987）Quatemary crustal movements in Konno，E。andOtuka，Y （1933）Geology of the area Kanto，Japan．∫oz6飢 0609先 Tohッo，vol．96， between Yui river and Fuji river in Shizuo－

p．223－240．＊＊ ka Prefecture．∫o％飢 （｝601．Soo．ノ砂α銘，vo1． Kaneko，S．（1971）Neotectonics of Oiso Hills and 40，p．408－411．＊ contiguous（iistricts in south Kanto，Japan． Koyama， M．，Kitazato，H：．and Yano，S．（1986） ノio％先 （3セol．So6．ノ砂ごzn，vo1．77，P．345－358．． Tectonic movement of the Oiso Hills Kano，K．， Ito， T． an（1 Uesugi， Y． （1979） deduced from the paleomagnetic survey． Characteristics and （iisplacement of the Eα7オhノ匠o％云h砂，vol．8，P．620－625．＊ Shiozawa fault system which cuts the Kuno，H （1936）On the displacement of the Tanna Kannawa thrust．16オh助彫φ．o％P名ω6彫づo％ fault since the Pleistocene．B％ll。Eごzπhg．

ρプハんz渉％名とzl五）ズsごzsオ6鴬．，p．315－318。＊ 五～6s．1ns渉．，vol・14，p．621－631． ，Someno，M．，Uesugi，Y．and Ito，T．（1988） （1950）Geology of Hakone volcano and Fault movement in the northwestem a（1j acent areas， Part I． 10z6フ・』 Eα6． S（ゴ．フ Ashigara area，central Japan，since the U初∂．To勿o，Sect．II，voL7，p．257－279． Middle Pleistocene．G60s6」．ノ吻．Sh伽o肋 （1951）Geology of Hakone volcano and U勉∂．，no．14，p．57－83．＊＊ adj acent areas， Part II．∫oz6飢 Eごzo． S61．， ，Uesugi，K。，Ito，T．，Chiba，T．，Yonezawa， U勉∂．To勿o，Sect．II，vol．7，p．351－402． H。and Someno，M，（1984）Active faulting Machida，H （1964）Tephrochronological study of in and around the southem Tanzawa Volcano Fuji and a（ijacent areas．∫o雛． mountains and the Oiso Hills，South Fossa G609名To為ツo，vo1．73，P．293－308，337－350．＊＊ Magna region，Japan．⑭砿1～6s．如伽， （1975） Pleistocene sea Ievel of South vol．23，p．137－143．＊＊ Kanto，」＆pan，analyzed by tephrochrono－ Kanto Quaternary Research Group（1980）Stratigra－ logy．Q鰯オ67nごzηSオz64勿sフTh6五～oツαl Soo勿砂 phy and some geologic characteristics of qプ〈セzo Z（gごz乙ごz％4，p．215－222． the middle Pleistocene in the South Kanto， （1977）Kazanbai wa Kataru．Aoki shobou， Japan． Qz6ごz差 ノ～6s． ノ4ゆごzn， vo1．19， P．203 Tokyo，342p．＊ －216．＊＊ and Moriyama，A．（1968）The develop－ Kato，H．（1992）FOSSA MAGNA－A masked ment of Mt．Fuji and Mt．Hakone vol－ border region separating southwest and canoes analyzed from the tephro－ northeast Japan．B％ll．G601．Sz67z7．ノψα7z， chronological study in the Ooiso Hills。 vo1．43，p．1－30． G609名ノ～θ∂．ノ4ウどz％，vol．41，P．241－257．＊＊ Kato，S．，Sato，N．and Sakurai，M．（1983）Multi ，Arai，F．and Sugihara，S．（1980）Tephro－ －chamel seismic reflection survey in the chronological study on the middle Pleisto－ Nankai，Suruga and Sagami troughs，1吻． cene deposits in the Kanto and Kinki 功4zo9名勿hづ01～6s．，vol．18，P．1－23．＊＊ districts，Japan．Qz6ごz差ノ～6s．ノ41）ごzn，vo1。19，P． Kikuchi，T．and Association for Kanto Quatemary 233－261．＊＊ Research（1977）Stratigraphy oftheMiddle ，Matsushima，Y．and Imanaga，1．（1975） Pleistocene series in the eastem Sogayama Tephrochronological study on eastem foot

一653一 βz6116！∫盟 （ゾ 孟h6 060Jo9ぢαzJ Sz67∂¢y （～ブノ吻う召％， 1／『ol．43，〈b。10

ofMt．Fujivolcano－withspecial reference of Professor，H．Fujimoto Sixtieth Birth－ to geomorphological development．Q襯云． day，p．232－244．＊ 五～6s．ノ碗）ごz％，vo1．14，P．77－89．＊＊ Minster，」．B．and Jordan，Tl H．（1978）Present－day ，Arai，F．，Murata，A．and Hakamata，K． plate motion．10獄G60φh夕s．R6s．，voL83，p． （1974） Correlaion and chronology of the 5331－5354． middle Pleistocene tephra layers in south and （1979）Rotation vectors for Kanto．ノ∂z6名 G609名 Toたッo，vol．83，P．302 the Philippine and Rivera plates．E．0。S。， －338．＊＊ vol．60，P。958． Maritime Safety Agency（1988）Seismic reflection Mizuno，K．and Kikkawa，K．（1991）Consideration and refraction survey around the Sa（10 of the extinct of a middle Pleistocene te－ 1sland．ム～（ψ．l Cooz4勿zαだ％g Coηzηz．E召7孟hσ． phra，Ng－！ash in central Japan．Qz薦差R6s． P名64．，vo1．40，69－71．＊ ノ4ゆσ％，vol．30，P．435－438．＊＊ Matsuda，T．（1958）Late Tertiarystratigraphy and Murashita，T，（1977）Salt－water encroachment in development of folding in the upper Fuji Shizuoka Prefecture．1％4z6sか毎1乞仇z彪ろNo． River Valley，Yamanashi Prefecture，cen－ 225，p．30－42．＊ tral Japan．ノioz6冗 G601．Soo．ノφごz錫，vo1．64， Nakamura，K．・（1983）Possible nascent trench along p．325－343．＊＊ the eastern Japan Sea as the convergent （1962）Crustal deformation and igneous boundary between Eurasian and North activity in the South Fossa Magna，Japan． American plates．B％ll．E副勿．ノ～6s．1郷差， G60ρ勿S。〃伽09先 ∠4窺．G60φ勿S．U吻on， vo1．58，p．711－722．＊＊ vo1．6，P。140－150． and Shimazaki，K．（1981）Sagami and （1978）Collision of the Izu－Bonin arc with Suruga troughs an（i subduction of the central Honshu：Cenozoic tectonics of the Philippine Sea plate．Kαgヒzたz6，voL51，p． Fossa Magna，Japan．10礁P勿s．E観h，vol． 490－497．＊ 26，Supp1．， S409－S421． ， and Yonekura，N．（1984）Sub－ （1984）Northward－convex structure in duction，bending and eduction．Present and South Fossa Magna and collision of Izu Quaternary tectonics of the northern block。 Qz6α直 1～6s． ノ切うごz7zク vol．23， P．151 border of the PhilipPine Sea plate．β％IJ． －154．＊＊ So6，060よF名冴％06，vo玉．26，P．221－243． （1985）On the Oiso type earthquake．Eごzπh Oka，S。，Shimazu，M。，Unozawa，A．，Katsurajima， κo％オh砂，vol．7，P．472－477．＊ S．and Kakimi，T。（1979）Geology of the ，Ota，T。，Ando，M．and Yonekura，N． Fujisawa district。Q襯4名朋gl6S67．S6α16L （1978）Fault mechanism and recurrence 50，000．G60」．Sz67z7．（～ズノψα7z．111p．＊＊ time of major earthquakes in southern Omori，F．（1919）A catalogue of the destructive Kanto district Japan，as deduced from earthquakes in Japan， （416－1918）． み～勾り．

coastal terrace data． （360」．Soo．且ηz．βz611．， ∫吻6先Eごzπh（7．1％z7．Coηz簿z．勿zノ碑）α％，no．88，

vol．89，P．1610－1618． p．1－61．＊ Matsushima，Y。（1980）The post－glaciaUohmon Omura，H，，（1925） Change of land－level ac－ tranSgreSSiOn and CrUStal mOVement companied by the Great Kwanto Earth－ deduced from molluscan assemblages in quake．R勿．加勿名E観勿。加∂．Co郷窺．吻 the South Kanto，Japan．Eごzπh ノ砿o％孟h4ソ， ノ4ゆζz7z，no．100B，p．55－59．＊ vo1．2，P．52－65．＊＊ Ota，Y．and Sangawa，A．（1984）Active faults in the （1982） Radiocarbon ages of the alluvial eastern foot area of the Suzuka range， deposits in the Ashigara Plain，northem central Japan．（｝609名 ノ～6∂．ノψα銘，vol．57 coast of the Sagami Bay，central Japan． （Ser．A），p．237－262．＊＊ Q彼z渉．1～6s．ノψごz％，vo1．20，p．319－323．＊＊ ，Matsuda，T．，Koike，T．，Ikeda，Y．， and Imanaga，1。（1968）Note on the Kann－ Imaizumi，T．and Okumura，K，（1982） aWareVerSefaUlt．β％ll．臨彫9側αP縁 Geological report on the Kozu－Matsuda 〃z6s．磁渉．S61．，vo1．1，p．65－73．＊＊ fault．乃zz7召sあ8iαあ07zム～6望）．o多z孟h6α6ガ∂8五zz4孟s Mikami，K．（1958）Igneous activity and tectonic 初κ加αP6％．伽4κo脇一伽醜磁吻％1！ history of the Tanzawa Mountainland． 名6gぎo％s，Office of Kanagawa Pref．，p．82 Jubilee Publication in the Commemoration －194．＊

一654一 7セoホo銘露s ごzJo多zg 云h6 πoπh6仰z％zσ碧づ％ （ゾノ2z6Pのz勿zsz払α 6厚． y4〃zごz2召た∂

Otuka，Y．（1929）Astratigraphicalstudy・oftheOiso sevier，Amsterdam，247p。 block and its．a（1j acent area （part1，part Sugiyama，Y．and Shimokawa，K，（1982）The geo－ 2）．10z侃 G60Z．Soo．ノ1砂ごz％，vol．36，p．435 10gic structure of the Ihar奪 （玉istrict and

－456，479－497よ＊ Iriyama faults system in the South Fossa （1930）Arecentgeologichistoryofthe Magna region，central Japan．B％ll．G601． Oiso block an（i its adjacent area （1，2）． S鋸7”．ノψごz％，vol．33，P．293－320．＊＊ G609飢1～6∂乞6卿げノ¢クごz箆，vol．6，P．1－20，113 Suzuki，T．（1968）Settlement of volcanic cones． －143．＊ Bz611．『Vlol6．Soo．ノ¢飢z％，Ser．2，vol．13，P．95

（1933）The geomorphology and geology －108．＊＊ of northern Idu Peninsula，the earthquake Tsuchi，R．（1984）Neogene and Quatemary struc－ fissures of Nov．26，．1930，an（i the pre－and tures of the area aroun（i Suruga trough post－seismic crust deformations．B％ll． and their neotecto．nics． Qlz6ご訪。ノ～6s．ノ4ψ召銘， Eごz7孟hσ．1～6s， 乃zs渉．，vol。11，p．530－574． vo1．23，P．155－164．＊＊ （1938）Geologic structure of the eastern Tsuya，H．（1940）Geology and petrological studies Ihara district，Sizuoka Prefecture，Japan． of volcano Huzi（Fuji），III．βz611．Eαπhg． Bz611． Eヒzπh（7． 石～6s． 乃zs渉．， vo1．21， P．394 ノ～6s．1盟s渉．，vo1．18，P．419－445．＊＊

－413．＊＊ （1942）Note on the west wing of the Ozawa，K．and Ooki，Y．（1972）Geology and Kannawa thrust－fault．β％1乙Eαπhg．1～6s． groundwater of the Nakai district，the 乃zs抗，vol．20，P．322－336，＊＊ westem part of Oiso Hills．B％ll．∬oオ （1968）1：50，000Geologic map of Mt．Fuji． 5φ7z’％91～6s．乃zsよ K砂z㎎αzoαP泥，vol．3，P． GeoL Surv． ofJapan．＊＊ 73－82．＊＊ Uesugi， Y．（1986） Cover rocks of Oiso Hills Research Group for Active Faults of Japan（1980） －Problems to be solve（i一．伽ll．∠4ssoo． Active faults in Japan：sheet maps and κα％オo Q％σオ．1～6s．，vo1．12，p．11－20．＊ inventories，U勉∂．Toた妙P名6ss，363p． and YonezawaシH。（1987）Active stages of Saito，T．（！963）Miocene planktonic foraminifera the Hirayama fault at the northem fringe from Honshu Japan．710hoh％Un初．S6づ． of Izu Peninsula．∫oz6飢S（第s．Soo．ノiのうα銘，vol。 1～砂．2nd ser。（geo1．），vo1．35（2），P．123－209． 40，p．122－124．＊ Seely，D，R。，Vai1，P；R．and Walton，G．G。（1974） ，Ito，T．，Utada，M．，Someno，M．and Trench slope modeL G6010劇げoo％勧6％一 Sawada，T． （1985） Thrusts outcrops 舌α1耀㎎初s，P．249－260． between Zoshiki and Konuta，westem part Seno，T．（1977）The instantaneous rotation vector ofOisoHills，Bπll．z4ssoo．勧物Q襯オ． of the PhilipPine Sea plate relative to the R6s．，no．11，P．3－15．＊ Eurasian plate．T60孟o％oψhlys乞6sンvo1．42，P． Yamasaki，N （1926）Physiographical studies of the 209－226． great earthquake of the Kwanto district， （1985） ：Northem Honshu Microplate 1923．10蹴 Eαo．S62．1“ψ．Un初．To勿o， hypothesis an（i tectonics in the surroun（i－ Sect．II，vol。1，P．77－119． ing regionl when did the plate jump from Yamazaki，H，（1979）Active faults along the inland centralHokkaido．10％7．G804．Soo．ノψ伽， plate boundary， north of Suruga bay， vo1．31，P．106－123． Japan．Eσπhノ砿o刀オh砂，vol．1，p，570－576．＊ Shimamura，K．（1986）Topography and geological （1984）Neotectonics of the South Fossa structure in the bottom of the Suruga Magna in Central Japan deduced from trough：a geological consideration of the active faults．Qz6α渉．ノ～6s．ノψζz％，vo1．23，P。 sub（luction zone．∫o麗飢 （3609冗 Toたyo，vol． 129－136。＊＊

95，p．317－338．＊＊ （1985）Geology and tectonic movement in Somerville，P．（1978）The accommodation of plate and around the Ashigara plai血．Eαπh collision by （ieformation in the Izu block， 皿o％云hむ，vol．7，P．466－472．＊ Japan．β％ll．Eαπh（1．R6s．1盟s渉．，vo1．53，P． （1988）Geology of the Toyama trough off

629－648． the Noto Peninsula，central Japan as ob－ Sugimura，A．（1972〉Plate bomdaries in and served from DSRV “SHINKAI’2000”． around Japan．K㎎とzh％，vol．42，P．192－202．．＊ ノ且ノ協S7ECT1～ 1）6砂s＆z 」1～6s6召70h ヱ988 and Uyeda，、S。（1973）Island Arcs．EI一 （Technical reports of the4th symposium

一655一 Bz61」4カz（～プ オh6 G60Jo9ズ08J Sz67∂召ッ （～プノ41）σ％， yio乙 431．〈b．10

on deep－sea research using the submersi－ Suruga Bay．Co吻名6h6多zs吻6ノ～砂．o％ オh6 ble“SHINKAI2000”system），p．9－24．＊＊ 6召πhg．餌2漉痂o％，Agency Sci．Techno．，p。 and Machida，H．（1981）Active faults and 177－207．＊ tectonic lan（iforms along the northern bor・ Yano，S．（1986）Stratigraphy，depositional environ・ der of Ashigara plain．∠4ぬ血釦刀。4ssoα ments and geologic ages of the southem Qz薦彪ηzごzη1～θs．，no．11，p．96－97．＊ part of the Oiso Hills，Kanagawa Prefec－ ，Kakimi，T．，Kato，K．，Ikeda，K。，Takaha－ ture．G60sα1～砂．Sh乞2％o肋U窺∂．，voL12，p． shi，M。，Nagata，S．andIto，Y．（1982）Study 191－208．＊＊ on the crustal structure in the plate bound－ Yonekura，N．，Suzuki，1．，Hasegawa，T．，Uesugi，Y．， ary region．乃z彪万盟z 名4）oフ・オ ρプ 渉h6 00％勿名参 Endo，K．，Okada，A．，Kawana，T．，Ishik－ h6？zs吻6s渉z64ソoπ孟h6s6ゑs％zo彪o云onσos（ゾオh6 awa，K。and Fukuda，M．（1968）Radiocar・ 7zo7孟h67％ オ4） ごzzo6z ｛ゾ’Phズ1ゆブ）」％6 S6α カ」ごz渉a bon dates and Holocene marine terraces Agency Sci．Techno．，p．368－393．＊ along the coast of the Bay of Sagami， ，Sakamoto，T．，Hata，M．，Kurasawa，H．， central Japan．（2z66z差 R6s，ノ¢クσ銘，vo1．7，P． Kakimi，T．，Hatano，S．，Machida，H．and 49－55．＊＊ Oyagi，N．（1981）Geological study of the （＊in JaPanese，＊＊in Japanese with English active fault in the region to the north of abstract）

伊豆半島北縁プレート境界域の地殻変動

山 崎 晴 雄

要 旨

フィリピン海プレートとユーラシアプレートの衝突境界と考えられる伊豆半島の北縁部地域は，衝突運 動を反映した第四紀の活発な地殻運動の存在が知られている。しかし，従来の研究では極めて広域的なプ レート運動と，実際に野外で観察される活断層の運動やそれに関連する地殻変動との具体的な関係の解明 は不十分であった。このため，地震予知上重要な地震テクトニクスの研究もあまり進捗していなかった。 広域テクトニクス上での活断層の運動の意味を明らかにするため，本論では伊豆半島北縁部を1．駿河 湾北岸内陸域，2．丹沢山地南縁地域，3．大磯丘陵・足柄平野地域に分け，各地域の第四系の層序，層 相，及び断層運動の調査等を通じて第四紀中・後期の地殻変動史を復元した。この過程では，断層活動の 考察において，従来からの相対的な断層変位速度だけでなく，断層に境される各地塊の隆起・沈降などの 絶対運動にも着目して調査を進めた。これから第四系を堆積時及びその後の地殻変動の特徴によって大き く4つに区分した。 地殻変動史の調査から，各地域に共通するいくつかの広域的な地殻変動の特徴と，他地域とは一致しな い独自の特徴が認められた。広域的な特徴は激しい沈降域が徐々に隆起域に変化することと，活断層や沈 降域などが規則的，周期的に伊豆側に移動することである。この特徴に基づいて地質構造の発達モデルが 構築された。 このモデルの成因の考察，並びにモデルに当てはまらない諸特徴の原因の考察から，伊豆北縁部での地 殻変動は，基本的には陸的な性格をもつ軽いスラブの沈み込み（浮揚性沈み込み）が従来衝突域と考えら れていた内陸地域にも起きているため，そこに付加体が形成されること，そして，更にその運動の進行の 結果沈み込み帯が衝突域に変化することで説明できる。すなわち，現在の駿河，相模両トラフの陸側延長 域では浮揚性の沈み込み運動が進行し，古い沈降域の隆起と新しい沈降域及び断層運動の伊豆側への移動 が起きていると考えられる。これに対し，丹沢山地南縁は伊豆地塊の沈み込みが困難となったために衝突 境界に変り，上記の沈降域の形成などの沈み込み帯の特徴が全く認められなくなっている。

一656一 嚢

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これから，この地域に存在する活断層の多くは付加体中に発達する覆瓦スラストと考えられ，これを支 持するデータやその特徴を提示した。更に，この考えに立って国府津・松田断層の活動周期，変動様式を 考察し，従来の内陸活断層とは断層長と変位量の関係などを同列に論じられないことを示した。

（受付：1992年4月16日。受理：1992年7月13日）

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