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Hamo Yamazaki＊ Bulletin of the Geological Survey of Japan，vo1．43（10），p．603－657，1992 Teet健ies o£泓：P且鵬e Co且且量si磁我且磯g臨e N鯉翫er恥M蹴g且醜 o餌躍：P輔鵬櫨我，Ce醜ra盈」脚蹴 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． 一603一 B％ll6渉」％ ρプ 云h6 （36010916αl Sz67z76：y （～プノ41）ごz％， Viol．43，ノ〉io．ヱ0 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 一604一 T60渉o多z乞6s 召lo％9 云h6 ％07≠h6刀zηzα怨」％ （ゾノ2％P6％乞％s％乙ごz侃 yとz盟zごz忽zた∂ 【 を 麟マ ⑭な鞭 G魅 89 噛 q 懸 o 380 鏡 懇 ぜ鯵 虜 瀞 購曝 ⑭麟講 調 ⑭ 麟 麟 轄響 ．覧馨 購 灘⑭ 醜 傷 360 φ、 欝 Tbkyo 8儘 Northem 置UR marginof 姻Sa9 亀…；懸 lzuP β £10c％ Nagoya Suruga 9矯 ／Troug 量 ／ 騨 留PAC 瓢、1 1 π㌧ 懸麟 §Ooα “ ’ 00 鱒 OU・お 340 鞭 o＿＿＿一 AgG ℃ O o O 愈o 0① 過 q 0 Oo 気楚 8卜 麟凶圃 4c％ Q Q“ 300 ▽ ・O 一《‘ 32。 Oo 麟 “ P HS 戸 も ！矯 D b Qo一 。 9伊 聴 “ 。 ＆ づ o N 0 100km ’ G、 o 0 、、ぴ 亀 140 138。 140。 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， 一605一 Bz6116オ勿z （～ブ 渉h6 G6010gJ66zl Sz67四6ッ （～プノψごz％， Viol』43フ．〈流o。10 Yκ一／づ多ぢン／／ 穐nz鴨／ ／ ｝ 、．／哲 Matsuda 〆 Σ 論 ノ Fuji 雇 ！ ノ M volcano 。／ SWfoot 麗熈麟Fuji1蹟・x 茎圭）H。miya！Aミhitaka・ 馨聡F、j 熱 Sagami Bay 1＼ Area to、the northF ％ 酬’ of Suruga Bay 1 σ Suruga 1い i l N Bay lzu Pen唾n． 嘔Okm （IZ曲IOCさ） 0 、 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
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