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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.AcircularevolutionofthetectonicsintheIzuBorderland 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.
Geology,Geological Survey of Japan。
Environmenta1 Keywor(is:seismotectonics,plate collision,plate bound-
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
1.l Prob亙emset撫gsa翻聖腿r脚sesO量 t恥量sst磁y
The oceanic Philippine Sea plate(PHS)is
underthrusting northwestwar(i beneath the
Eurasian plate(EUR)along Nankai trough
off Southwest Japan.The direction of this
plate boun(lary tums toward north on the
west of Izu Peninsula and extends from Sum・
ga trough through the subaerial region on
the northern bor(1er of Izu Peninsula (Izu
Borderland)to Sagami trough(Sugimura,19721Fig.1).These areas constitute a part
of the major tectonic province calle(i the
South Fossa Magna that comprises thick
accumulation of Neogene sediments and vo1-
canics.These rocks are strongly shortened
and a northward curving fo1(i zone is formed
along the northem margin of Izu Peninsula
(Matsuda,1962).This structure has been
caused by repeated subduction of the buoyant
Izu-Ogasawara arc including Izu Peninsula,
the volcanic arc in the eastem margin of the
PHS,beneath the EUR (Sugimura,19721Matsuda,1978).
On the other hand,many active faults with
high slip-rate occur in the Izu Borderland as
a result of strong shortening (Yamazaki,
1979)。Th“term active fault denotes a fault
having evidences of repeated fault move-
ments in the present tectonic stress臼:field an(1
1iable to be activated in the future.The
author thinks that the active faults constitute
a major basic element of the Quatemary
tectonics in Japan, and configurations of
active faults,their activity an(i history of
faulting represent the actual con(1ition an(i
evolution of tectonic setting in this region.
The intense activity an(i the strikes of these
faults suggest that the fault movements have
close relation to the plate convergence
betweenthe PHS andtheEUR plates(Yama-
zaki,1984)。However,nothing is definite on
the implication and role of each active fault
in the setting of plate tectonics.Therefore,
this paper first describes the history of fault-
ing and of related crustal movement in the
following three areas along the subaerial
Plate convergence boundary:1) the area to
the north of Sumga Bay inclu(1ing the south-
westem foot of Mt.Fuji,2)the southem end
of the Tanzawa Mts.,3)Oiso Hills and
Ashigara Plain area.As Izu block is migrat-
ing to the northwest,each area seems to show
the various tectonic histories to correspond
to the (iirection of convergence boundary.
Then,based on these data,the author dis-
cusses the relationship between each active
faults and plate tectonics through the com-
parison of the local tectonic evolution.
Figure2is the index map of locality name
used in this paper.
1.2 Sig聾量f量cance &聾雌 {e我t甑re o£ 意擬宜s
s重掘y
The stu(1y areas include the inferred
hypocentral region of Sumga Bay for the
future great Tokai earthquake (lshibashi,
1981),and Sagami Bay for the west Kanag-
awa earthquake(lshibashi,1988a,b).Study
on the implication of active fault in the set-
ting of plate tectonics will contribute to the
progress of the effective earthquake predic-
tion that was proposed by Ishibashi(1978).
The investigated area has a unique tectonic
feature:the convergence boun(iary at the
northem margin ofthe PHS crosses an island
arc system continuing from Northeast Japan
arc to Izu-Ogasawara arc,and the intersec一
一604一
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瀞
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欝 φ、
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瓢、1
置UR
Nagoya
1π㌧ “
Suruga /Troug/
8儘 亀…;懸
Northemmarginof
量
騨
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懸麟
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9矯留PAC§O
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o一
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D
0 100kmo
G、
0、、ぴ 亀138。 140。 140
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∂
【
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//
}
〆
M
1\
/、./哲
Σ
雇 ノ Fuji ! 。/ SWfoot
論
酬’
volcano
麗熈麟Fuji1蹟・x茎圭)H。miya!Aミhitaka・
馨聡F、j 熱F % Area to、the north
of Suruga Bay
SurugaBay
穐nz鴨/ Matsuda
1
1い i l N
lzu Pen唾n.
(IZ曲IOCさ)
σ
ノ
Sagami
0
Bay
嘔Okm
、
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
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
on the Quaternary geology and faulting in the
study area, this paper introduces the
following new i(iea. Previous stu(iies on
active faults have used the long-term slip-rate
to evaluate their activity.This rate means
the average rate of the relative slip of two
blocks bounded by :fault and indicates the
1 Fault scarp
Fig.3
2 3
Relationship between the formation of the
fault scarp or tectonic landform and absolute
crustal movement of each block divided by a
fault.Arrows indicate the direction and rate
of tectonic block.Dotted parts represent the
fill sediments.
BLE:Base level of erosion
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
fault.The author,however,argues that the
long-term slip-rate is not always adequate to
interpret the evolution of tectonic landforms.
For instance,as shown in Fig.3,the tectonic
lan(iform caused by dip-slip faulting greatly
varies with the motion of individual block
bounded by fault toward the base level of
erosion。 If a faulting occurs in the subsiding
area,most of the fault.topography wou1(1
soon be buried by basin fill se(1iments.The
absolute rate of uplifting or subsiding on each
faulted block seems to be a more important
factor in the formation of tectonic landforms
than the long-term slip-rate of the fault.
Furthermore,the absolute crustal movement
is fmdamentally significant for the study of
tectonic evolution in the plate convergence
region where the behavior of cmst changes
from subsiding to uplifting in a short period.
Therefore,to recoustruct the fault evolution,
the author examines not only the relative
displacement of fault,but also the absolute
crustal movement(1e(1uced fromthe elevation
of faulted surfaces, the thickness of the
Quatemary sediments,their ages andpaleoenvironment and so on.
2.PREVIOUS WORKS
2.1 St聡岨y o箪act置ve f我眠亙ts亘聾a盟〔丑aro魍nd
IZ聰Pe】臣且恥S聰且a,
In1920’s and1930’s,several investigations
carried out in South Kanto an(l Izu Peninsula
on crustal deformations associated with the
great Kanto earthquake of 1923 and the
Kita-lzu e&rthquake of1930.These surveys
depicted the outline of major active faults in
these regions such astheKozu-Matsuda fault
and the Kita-lzu fault system. Yamasaki
(1926)and Otuka(1929,1930)estimated the
existence of Kozu-Matsuda fault from a
prominent straight scarp between the Oiso
Hills and Ashigara Plain,and(iiscussed the
Quaternary tectonics in these areas.Although existence of the Tanna fault,a part
of northern portion of the Kita-Izu faults
system,had been known by the geologica1
survey for a tunnel construction before1930,
investigations to trace the earth(luake faults
of1930by Ihara&Ishii(1932),Otuka(1933)
and others revealed the cumulative tectonic
features along the fault system.Particularly,
Kuno(1936)pointed out that the Tanna fault
had l km of total left-1αteral displacement
based on the length of valley offset and the
distance of separated same lava flow.
In the late 19307s to early 1940’s, the
tectonic histories of the Iriyama thrust,the
faults in southwestem foot of Mt.Fuji and
the Kannawa fault at so“themmargine ofthe
Tanzawa Mts.were described by Otuka(1984),Tsuya(1940)an(i Tsuya(1942)respec-
tively based on the progress of strati-
graphical and chronological studies on the
Quaternary formation.
In the late1960’s,the active fault study has
been introduce(1 in the program of earth-
quakepredictionasoneofthenationalpro-jects.By the end of1970’s,most of the major
active faults in Japan have been discovered
through fault survey using aerial photo-
interpretation.In the study areas,the quanti-
tative information on active faults,such as
long-term slip-rate,vertical and latera1(iis-
placements of the fault,was accumulate(1by
means of progress of tephrochronology an(1
radiometric dating method. Machida&Moriyama(1968)reported geomorphic his-
tory around the Kozu-Matsuda fault。Matsu-
shima&Imanaga(1968),Machidaαα1。(1975) and Kano 6♂ α1.(1979) revealed the
fault movement on the Kamawa fault in the
mid(11e to late Quatemary.Yamazaki(1979)
reveaIed the fault behavior an(i the tectonic
significance on the active faults in southwest-
em foot of Mt.Fuji.All these data obtained
by1980were compiled in “Active faults in
Japan”which is an inventory book on active
faults in Japan by Research Group for Active
Fault Japan(R.G.A.F.」.)(1980).
Since then,the active fault stu(1y in Japan
branched off into two fields.One is the study
on the(1etailed history of each fault move-
ment during Holocene time.Many trenches
were excavated across various faults to
一607一
Bz61」6渉勿z(ゾ 渉h6 G60109づoαl Sz67z7¢y (~プノ砿)召銘, 1V≧)1.43,ノ〉io.10
know the age of recent faulting an(i the recur-
rence interval that provide us useful informa-
tion about the long-term earthquake pre(iic・
tion. Archaeological records an(1ancient
manuscripts were also used to specify the age
and locality of faulting that caused historical
earthquakes.Another study is on the clarifi-
cation of the implication o:f each active fault
in the regional tectonics.With increase of
the information on plate collision at Izu
Borderland,Yamazaki(1984)and Itoαα1.(1989)discussed the meaning of active faults
in the framework of plate tectonics.This
paper lies on the field of the latter.
2.2Re且aせio蒐s恥恥めe意wee恥襯童ve蝕磁s
蹴齢late恥o撒ぬry
Sugimura(1972)first delineate(1that the
plate boundary in the South Fossa Magna
region extended from Suruga trough through
the inland area to Sagami trough.He also
thought that the buoyant Izu block on the
PHS collided with the EUR at the northem
margin of Izu Peninsula an(1northwardmigration of Izu block bent the plate bound-
ary in an arc.The:Kozu-Matsuda and Kan-
nawa faults to the north of Izu Peninsulawere
thought to be the plate-boundary faults
where the PHS directly contacted with the
EUR. However,as these faults had extraordinar-
ily smaller slip-rate than the relative plate
motion between the PHS an(1the EUR,it was
unnatural to think that these faults represent
directly the plate boundary.Ishibashi(1976)
thought that the commencement of new sub-
duction zone off the east coast of Izu Penin.
sula diminished the inter-plate convergence
rate in Izu Borderland. According to this
thought,some transform faults across Izu
Peninsula are needed to connect Suruga
trough with a new subduction zone.There-
fore,he argued that the numerous NW-SE
trending faults in the peninsula acted as the
transform fault comecting the both subduc-
tion zones,an(1named the group of these
faults as Izu transform belt.Ishibashi(1977)
further suggested the possibility that the
active fault along the new subduction zone
cause(i perio(iically large earthquakes and
moved coseismically at the event of1923
great Kanto earthquake.
Contrary to this hypothesis, Somerville
(1978) discusse(i that the crustal strain
caused by plate collision was accommo(iated
by the slip of intraplate faulting in Izu block.
He argued that the subduction zone of:f the
east coast of Izu Peninsula was not needed
for the explanation of the decrease of inter-
plate convergence rate at Izu Borderland.
Although the above mentione(1hypothesis
postulated that the active faulting in Izu
Bor(1erland directly represente(i the plate
motion along the convergence bomdary,Nakamura6渉α1.(1984)proposed a new i(1ea
to classify the plate boun(iary into mechani-
cal b6undary zone and material bomdary.
They showed that the inland plate bom(iary
in Izu Borderlan(1,pointed out by Sugimura
(1972),was the material boundary,an(i the
mechanical boundary zone correspon(1e(1to
the active faults zone to the north ofmaterial
boundary.
Yamazaki(1984)investigated the history
of fault movement and its tectonic feature
along the mechanical boundary zone. He
showed that the crustal evolution of this
region was caused by the formation of ac-
cretionary prism associated with the plate
subduction. He also pointed out that the
active faults in this region corresponded to
the imbricated thrusts in the accretionary
prlsm。
Recently,Ishibashi(1988a,b)reconstructe(i
his original hypothesis and showed that the
west Sagami bay fault was not a interplate
fault at the subduction boundary but a kind
of intraplate hinge fault boun(1ing the buoy-
ant inner arc from the subducting outer arc in
the Izu-Ogasawara arc. He renamed the
fracture zone as the west Sagami fracture
zone(WSF).He wamedthe reactivation of
WSFinthenearfuturebasedonthefactthatthe periodical movement of the WSF had
repeatedly damaged Odawara area every70
years through the historical time.
一608一
T60渉oκ露s ごzJo7zg 孟hθ 7zo7孟h67% 窺召碧2% (ゾ Z2z6P6%2nsz61ごz の. 】己αηzα2σ泥∂
2.3 Re互寵童ve mot亙o聾o£騒e PHS toward
t恥e EUR
Many earth scientists in Japan supported
the NW direction for the relative motion of
the PHS toward the EUR along Nankaitrough.However,two estimations:1)N to
NNW direction and2)NW to WNW direc-tion,were proposed for the motion of the
PHIS in Izu Bor(ierlan(i (lshibashi,1984).
The former was derived from the studies on
the geological history and landform evolution
in the South Fossa Magna region(Kaizuka,
19751Matsuda, 1978). The later wasobtained from the plate motion mo(1el on the
basis of the focal mechanism of great earth-
quakes along the plate boun(iary(Seno,19771
Minster&Jordan,1978,1979).To solve this
inconsistency,Nakamura & Shimazaki(1981)conclude(i that the PHS had moved
toward north before l Ma,and then changed
its direction to the northwest. Kaizuka(1984)showed many geological data to sup.
port this idea.
Seno (1985)tried to interpret the above
mentioned inconsistency using the newhypothesis proposed by Nakamura(1983)and
Kobayashi(1983)that northern Honshu is a
portion of the North American plate(:NAM)
and migrating to the west.He considered
that the migrating direction of the PHS
changedfromNNWtoWNWatabout1.5Ma,andthenat O.5Ma theboundarybetweentheEUR and NAM which originally rm through
the central Hokkaido,jumped to the eastem
margin of Japan Sea and the central Japan.
He thought that this jumping of plate boun(1-
ary caused the westward migration of North-
east Japan as a portion of NAM.Onthe other
hand,Ishibashi(1984)who shared the view
point that Northeast Japan was a indePen-
dent microPlate,conclude(1that many phe-
nomena on the tectonic evolution in late
Neogene Japan,such as the different conver-
gence direction between Nankai and Sagami
trough,are interpreted consistently with the
idea that the Southwest Japan commenced its
eastward migration at about l to2Ma.Although this controversy remains inconclu一
sive,recentlyYamazaki(1988)andMaritimeSafety Agency(1988)proposed a(10ubt for
the existence of this new plate boundary
along the eastem margin of Japan sea on
which the both hypothesis are based.
Consequently,the author thinks that the
interpretation of tectonic evolution in the Izu
Borderland is indispensable to solve t五e prob-
1em of different direction of plate conver-
gence between Nankai an(1Sagami troughs.
3.OUTLINE()F G・E()LOGY
3.1Setti醜gi恥蝕e幽tetecto漉s From the viewpoint of island arc system,
Izu Borderland is a intersection between the
East Japan island arc system(Sugimura&
Uyeda,1973)exten(1ing from Kuril Islands
through Northeast Japan to Izu-Ogasawara
islands and the plate convergence zone at the
northern end of the PHS(Kaizuka,1975).Izu・
Ogasawara arc is divided into the non-vol-
canic outer arc and the volcanic.inner arc by
the volcanic front.
In and around Izu Peninsula,the N-S trend-
ing volcanic front runs in Sagami Bay.Izu-
Ogasawara imer arc to the west of the
volcanic front contains Mt.Fuji,Hakone,
Amagi and some other Quatemary volcanoesin Izu Peninsula and Izu Borderland(Fig.1).
Because of this imer arc is made ofrelatively
low density volcanic rocks,the crust is thick-
er than that o:f non-volcanic outer arc.
Conse(1uently,the subduction of the inner arc
beneath the EUR becomes difficult and the
convergence boundary is pushed northward.
Therefore,Izu Bor(1erland is a place subject
to extreme crustal deformation.
3.2 To夢ogr我夢恥亘cal a醜d geo亙09童e我且str聡c-
t櫨eo貴藍eS側騒FossaM&9黙 The Izu Borderland constitutes a part of
the Fossa Magna which is a huge depressed
zone(1ividing the Japanese Islands into South-
west Japan and Northeast Japan.The Fossa
Magna region is estimated to initiate its
deformation in the middle Early Miocene to
early Middle Miocene(Kato,1992). The
一609一
βz61」6あη (ゾ’渉h6 (1}(ヲolo9乞αzl Sz67∂6ッ α〆ノ砿)ごz%, Vbl.43,.八b.10
thick accumulation of Neogene Tertiary and
Quatemary volcanics fill the depression of
the pre-Neogene rocks.The Fossa Magna is
divided into two sedimentary basins,i.e.the
North and South Fossa Magna,by the rise of
the Central Upheaval Zone in the central
part.Izu Peninsula and Izu Borderlan(i con-
stitute some portion of the South Fossa
Magna.In and around Izu Peninsula,the
South Fossa Magna is divided mainly into
two tectonic provinces l1)the stable Izu
Peninsula(lzu block)consisting of the Terti-
ary an(i Quaternary volcanics and2) the
extra peninsula fo1(ied belt(the South Fossa
Magna foldedbelt)consisting ofsedimentary
rocks and volcanics since the Miocene time
(Matsuda,1962).
The Izu block consists mainly of the Yuga-
shima Group(Lowerto Middle Miocene)and
the Shirahama Group(Upper Miocene toLower Pliocene)underlying Quatemary vo1-
canics.Shirahama Gfoup(11to8Ma inage)
yielded the Foraminiferum L吻40の61乞鰯(〈吻hzolの観%)ノのo勉oσof which has not
been fomd in the contemporaneous Tertiary
rocks o:f Japanese islands except for Izu
Peninsula (Saito,1963). 五のりづ40のノolズ按z has
lived in tropical shallow waters indicating
that the Izu block comprise(1a number of
volcanic islands situate(i to the south of
Japanese Islands at the late Miocene(lbara.
ki,1981).The Miocene sediments in this
Peninsula do not show any large scale defor-
mations cause(1by faulting,fo1(iing or uplift-
ing.The topographic relief exceeding1,000m
in Izu Peninsula has made by the growth of
Quaternary volcanoes.
On the other hand,many steep momtains
such as the Misaka,Tenshu and Tanzawa
Mts.1ie on the South Fossa Magna folded
belt in consequence of vigorous uplifting.
This folded zone consists of the Tanzawa,
Koma and Nishi-Yatsushiro Groups of the
lower to middle Miocene and the Fujigawa,
Nishi-Katsura,Aikawa and Ashigara Groups
of the upper Miocene to Pleistocene(Ma-
tsuda,1958).The former is mainly marine
deposits interbe(1ded v守ith volcanic materials
and their thickness exceeds1,000m.The
latter is sedimentary rocks composed of
thick mudstones and conglomerates.Their
sedimentary facies and structure indicate
that some of them were the subsiding trough
fill se(iiments(lto,19851Amano6砲1.,1986).
These rocks experienced the most extreme
shortening of more than20km in Tertiary
formations ofthe JapaneseIslands(Matsuda,
1962,1984). The folded structure extends
around the Izu block with a strong convexity
towards the north.In the folded belt,the
lower to middle Miocene crops out in the
portion of anticlinal axis and the upPer
Miocene to Pliocene fillsthesynclinalportion
(Matsuda,1962).
These geological structure in(iicates that
the South Fossa Magna folded belt is the
Iocus of collision an(1accretion of the buoy-
ant crust,such as Izu block,towards the
EUR。 The formation of sub(1uctive trough
between the PHS and the EUR plates has
been repeate(1through the Neogene perio(i
(Matsuda,19781Amano6齢乙,1986)。Theactive faults in this region extend along the
southem margin of the South Fossa Magna
folded belt to surround the northem margin
of Izu Peninsula.
4.ACTIVE EAULITS AND CRUSTAL DEFORMATIONS IN THE AREA TO THE N(》RTH OF SURUGA BAY
4.1 0utl亙聾e
The geological an(l topographical structure
in the area to the north of Suruga Bay is
characterize(1by a N-S trending zonal struc-
ture as shown in Fig.4.Two N-S trending
fault systems with W-upthrow divide the
region into three blocks. The east fault
system is composed of the Iriyamase,Omiya
and Agoyama faults.They consist the geo-
10gical and topographical boundary between
alluvial lowlan(1s of Fuji an(1Fujinomiya in
the east, an(1 the Kambara, Habuna an(i
Hoshiyama Hills in the west.The west one
consists of the:N-S trending Iriyama and
Shibakawa thmsts and divides the area into
一610一
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、あ
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b
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0 5
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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
Tenshu Mts.to the west.
4.2 Tee亡o醜童e evo亘腿髄o恥 o£ eac恥 £a翻te《丑
腿o磁
(且)H:蹴ais恥亘・ぬ鼠e蹴西丁錨曲髄Mo翻t廊S
The Hamaishi-dake and Tenshu Mts.situ-
ated on the westem margin of study area are
steep and dissected mountains ranging in
altitude from500to1β00meters.These twomomtains are composed of the upper Fuji-
gawa Group of the Pliocene,which consists
mainly of conglomerate and sandstone with a
maximum thickness of more than4,000m.
Fujigawa Group is the thick fi11(1eposits of
the ancient trough located at the Pliocene
Plate boundary to the south of the Kanto and
Misaka Mts. This subsided trough haschanged to uplifting area since the beginning
of the Pleistocene(Matsuda,1984).
(2)臨麟ara,H曲撒a蹉d Hosh量y&m我 Hi且亘s
The Kambara,Habuna and HoshiyamaHi11s,10cated to the east of the mountains,
are bounded on west and east by the Iriyama
-Shibakawa thrust system and the Iriyamase
-Omiya-Agoyama fault system respectively.
a.Kambam H量亘亙s:The Kambara Hills,
10cated in the southwestem side of River
Fuji,is a(1issected hilly area400-500meters
in altitude without flat surfaces on it.The
geology of these hills consists mostly of the
lower Pleistocene Kambara conglomerate,
Iwabuchi andesite an(i mi(idle Pleistocene
Saginota gravels which uncomfortably rest
on the former tWo fQrmations(Table1,Fig.
4).These Quatemary units have been verti-
cally displaced by the reverse faulting at both
margins of the.hills and also have undergone
small scale deformation caused by faulting
and folding in thehills(Sugiyama&Shimo-kawa,1982).Duetotherecentuplift,anarrow
Holocene terrace㌦has been formed on the
eastem margin of the hills.
The Kambara conglomerate (Konno&Otuka,1933),extensively distributed in the
southem Kambara Hills,consists mostly of
600mthickpebblygravelswithsand-en-riched matrix.The relatively monotonousfacies despite the large thickness of this for-
mation indicatesthat the Kambara conglom-
erate is fluvial and shallow water deposits
originally accumulated in the subsi(iing
depressi・n畦suc血astheplace・nthen・rthem
bor(1er of present Suruga trough.
The Iwabuchi.andesite (Otuka,1938),
remains of the early Pleistocene volcano,is
composed mainly of tuff breccia and lavas
and encloses a series of sedimentary beds
correlated with the Bessho gravels in the
Table1 Quaternary stratigraphy of the area to the north of Suruga bay.
GeologicalAge KambaraHills
(Sugiyama&Sh㎞lokawa,1982)Habuna-Hoshiyama H且ls
Hol㏄ene Anuvhm一 一 一 一 一 一 一 一 一 _ _ Iwabuchitenlacedeposit一 一 一 一 一 一 一 _ _ _ _
F司ilava且OWS
一 一 一 一 一 一 噸 一 _ _ _ _
A皿uvium
一 一 一 一 一 一 一 一 一
YoungerF㎎曾i’lavaflows
一 一 一 一 一 一 一 一 一
〇lder珂imudflows一 一 一 一 一 一 一 一 一
Saginotagravels
一 一 一 一 一 一 一 一 一
Iwabuc1丘andesite
Pleistocene
Late
Middle一 一 陶 一 一 一 一 一
肋ly
Saginotagravels
一 一 一 藺 隔 一 一 一 一 _
Iwabuchi andesite
Sed㎞ents Besshogravels
Iwabuchi andesite
Kambaraconglomerate一 一 一 一 一 『 噌 一 _ _ _ _
珂ikawaGroup
隔 一 一
一 一 一 一 } 一 嘲 一 _
F雨ikawaGroupP互iocene
一Con蚕01血ty一一一一Unconfbmlity
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. Thisformation overlies the Kambara conglomer-
ate conformably and mderlies the middle
Pleistocene Saginota gravels unconformably.
The age of this volcano is estimate(i to be the
Matuyama reversed magnetic epoch in theearly Pleistocene from the reversed magneti-
zation of lava魅blocks obtained from the
upPer part of the formation.
The Saginota gravels.(Konno&Otuka,
1933)arefluvialfan(1epositsof200minthickness distribute(i in the westem half of
Kambara Hills an(i in the southem Habuna
Hills.The age is estimated to be O.4to O.5
Ma on the basis of yielding of S惚040η
0吻磁擁s and the occurrence of widespread
vitric ash which is potentially correlated to
Ks-10ash in the mi(1dle Pleistocene Kazusa
Group,Boso Peninsula(Machida6孟α1.,1980).
Thelwabuchiterraceof30to40minaltitude,on the eastem margin of Kambara
Hills,is an uplifted Holocene terrace com-
posed of coarse fluvial and fine brackish
water sediments.Akahoya ash(Ah)empte(1
in Southem Kyushu at6,300years ago was
found in the brackish water sediment at13.7
m in height(Yamazakiαα1.,1981).These
facts indicate that1)Iwabuchi terrace is a
filltop terrace created by the postglacial
transgression,and2)assuming the paleo-sea
level of6,000years ago was about2m above
the present one,the eastem margin of Kam-
baraHillswasuplifted11,7matarateof1.9m/103years since after the fall of Ah ash.
The following geological evolution of the
Kambara Hills is obtained from the above
mentioned stratigraphical data.
In the early Pleistocene,the Kambara con-
glomerate accumulated thickly in the subsid-
ed depression similar to present bottom of
Suruga trough. The activity of Iwabuchi
volcano has also occurred in the(iepression
following the deposition of Kambaraconglomerate and a thick series of andesitic
lavas and agglomerate has consequently
accumulated. In the early Middle Pleis-
tocene,the Saginota gravels overspread the
present Kambara Hills an(i its adjacent area
as fluvial deposits.Then,subsiding of these
areas gradually changed into uplifting.As a
result o:f the change,the Saginota gravels
emerged and the erosional process began in
the Kambara Hills area.
め. H&恥聡髄a-Hos血量yam我 H亘亙亙s:The
Habuna and Hoshiyama Hills ranging in
altitude from100to300m,in the northeast
bank of River Fuji,are bounded by the
Iriyamase,Omiya,Agoyama and Shibakawafaults.The gentle inclination of depositionaI
surfaces preserved well on both hills in(1icate
that the Habma an(1Hoshiyama Hills weretilted to the west and to the north,respective-
1y. The geology of these hills consists of
Iwabuchi andesite intercalating with the
thick Bessho gravels,th6 1ate Pleistocene
Older Fuji mudflow units and several sheets
of Younger Fuji Iavas in the late Pleistocene
to Holocene.
Hitherto,the Bessho gravels extensively
distributed in both hills were assume(1to be
equivalent to the Sagi耳ota gravels in the
Kambara Hills because the Bessho gravels
seemed to overlie the agglomerate of Iwabu-
chi andesite mconformably at several out-
crops along the River Fuji(Tsuya,19401
0tuka,1938).But this study has confirmed
that the Bessho gravels can be classified into
two formations l upper formation which is
relative to the Saginota gravels in the Kam-
bara Hills an(110wer one enclosed in Iwabu-
chi an(iesite.
The former,gravels of200m in thickness,
occupies the southem Habuna Hills with N-S
or NE-SW strike and E dip.The mud layer
bearing abun(iant plant fossils at the base of
upper formation intercalates with a thin
glassy marker tephra identical with the ash
layer in Saginota gravels. The Saginota
gravels in the Habuna Hills have undergone
more intense deformation than those in the
:Kambar皐 Hills. This fact implies theexistence of a fault(the Matsuno fault)mn-
ning through between two hills along the
River Fuji.
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
Hoshiyama H皿s is composed of totally more
than1000m thick and well stratified cobble-
pebble-sized layers intercalated with thin
sand beds.The extremely large thickness
and monotonous facies of this formation
suggest that the Bessho gravels are resulted
by the accumulation of coarse sediments in a
contimous depression.They were transpoted
from Misaka Mts.by the Paleo-River Fuji.
The geological structure of the Bessho
gravels is different largely between that of
the Habuna Hills with NW-SE strike,SW
(1ip of40to60degrees and of the Hoshiyama
Hills with:NW-SEstrike,NE dip of30to50
degrees. This fact also indicates that the
Agoyama fault has commenced its activity
after the accumulation of Bessho gravels
traversing its sedimentary basin,and conse-
quently the blocks on either si(ie of the fault
have(1eveloped(1ifferent tilting from each
other.As there has not been observed any
erosional surface on the contact between the
Iwabuchi andesite and the un(ierlying mud
layer top of Kambara conglomerate(Sugiyama&Shimokawa,1982),the Bessho
gravels and the Kambara conglomerate are
assumed to have accumulated in a series of
subsiding basin. The thickness of Bessho
gravels is increasing toward the north in-
dicating that the center of subsi(ience has
migrated to the north during earlyPleistocene.
The Older Fuji mudflows in the late Pleis-
tocene (Tsuya,19681Agglomeratic mud-flows of Tsuya,1940)are composed mainly
of original mudflow(1eposits derived from
Older Fuji volcano,their secon(iary(1eposits
and alluvial fan gravels at volcanic foot.As
the depositional surfaces Qf these mudflows
are well preserved on both hills,they can
subdivide into several units through the
Om
5m
Key亡ephra
OS_伽KgP
MS一噸
AT一{
scoriaゆ
Pum臨nOW
Agex104y.B-」》.
0.3
Mudflowsurfaces
Fuji Iavas
羅拗o o o o o o
▲ ▲▲ ▲ A ▲
%
SSW4,SW6
___Mf-WSW1,2,5,SSW1,2
一Mf一皿2.15
4.9
1一…
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-
chi(1a(1977)divides the mudflows into older
an(i younger units on the basis of thickness of
the covering tephra.Yamazaki(1979)andYamazakiαごzl.,(1981)also studie(i the strati-
graphy of tephra and distinguished five sur一
faces on the mudflow deposits(Figs.5,6).
These surfaces contain severa1(1epositional
surfaces of original mudflows from the older
Fuji volcano and fluvial surfaces of tributary
flow of the River Fuji.
These numerous surfaces have been form一
獺i難 □ 望彰 錫 Alluvial fan
欝撚 翻 啄 Sbpes げロおえ
監.纐‘孟
雛’、ノー甲
#堀
縮
4
躁’彰き一,.響.
d’
Mudflow I’
》
er lavas Mudfbw IV,1輌導
甥ゑ纏?
。慨’三羅一 ▲A▲▲▲
}
//
Mudflow U Mudflow III
曙▲ A
≡三 ※/
ow I’
Mudflow l婁騒き 》4》4
Xl i
號現
夢%
霧欝
f
d〆
黛 e灘 露
3,羅集.
・難μ、糀
騒灘 ・一.
羅瀦
i難
鮎
錫
Younger Iavas
\
瑛:i
Fujinomiya l~ 2c
諺
0.3
0S
MS
ATn2」
5.O
TPflo
x104yr.
i難
Om
1
2
3
4
5
6
旨
.霧
..:::欝
・鵜::
……u藤:灘
護
澱
,難
鰍う1
e唖’z摺
1.lriyamase fault
2.Omiyafault3.Agoyama fault
4.Shibakawafault
“
賦River Fuji
ざ
a
ba
灘
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,
cause(1by faulting along the eastern margin
of the hills system. But the (1istribution
pattem of mu(iflow surfaces shows the
regional characteristics,that is,the surfaces
older than50,000years ago(Mf-I and I’)
(1evelop well on the Habuna Hills,while
younger surfaces,especially the surface of
20,000years ago(Mf-III)extend over the
Hoshiyama Hills.This contrast is due to a
relatively higher uplifting rate and earlier
emergence of the Habma Hills than those of
the Hoshiyama Hills.Each of surfaces onthe
Hoshiyama Hills extends from north to south
in direction with narrow width.The younger
Fuji lavas(SSW2,SW5)eruptedabout10,000
years ago flew over the hi11s an(1reached
River Fuji.Although Tsuya(1940)assumed
that these lavas have not been(1isplace(1by
Omiya fault at the eastern margin of the hills,
the large gap of lava height,exceeding50m,
between Fujinomiya Basin and HoshiyamaHills indicates that the Omiya fault has verti-
cally displace(i these lavas.
Above data lead to the following conclu-
sion on geological evolution of the Habuna
and Hoshiyama Hills. The Habuna an(1
Hoshiyama Hills were a place intensely sub-
si(1ing where the thick Bessho gravels had
accumulated during early Pleistocene time.
In the Middle Pleistocene,this area was still
subsi(1ing contemporaneously with the accu-
mulation of Saginota gravels.But around
80,000years ago when older Fuji volcano
commenced its activity,the crustalmovement in this area had gradually changed
from subsidence into uplift.Since50,000yr
B.P.,though both h皿s have undergone the
same significant uplifting,the difference of
distribution pattem of mudflow surfaces
became prominent between two hills(1ue to
the difference of tilting direction.
(3)Fuj旦一Fujinomiyalowla織 Fuji-Fujinomiya lowland,10cate(i in the
northern extension of axis o:f Suruga trough
and extending over the east si(1e of Habuna
an(i Hoshiyama Hills,is composed of the
Fujigawa alluvial fan, Ukishimagahara
swamp onthe southemflank of Mt.Ashitaka
and Fujinomiya basin。
&. E臆」亘g我w&&亘旦朋v量a旦ね醜:Thick fluvial
gravels transported from the Misaka and
Akaishi Mts.by the River Fuji are the con-
stituent material of this fan.These gravels
intercalate the SSWl lava of Fuji volcano
with the radiocarbon age of14,000yr B.P.
(Yamazaki,αα1.,1981). The maximum
depthofthislavareachesupto-140matthemouth of River Fuji(Murashita,1977).
b. Uki曲imaga恥ara:Ukishimagahara isa lagoon formed by the continual sub(1uction
and the formation of barrier alon墓the south-
em flank of Mt.Ashitaka. The markertephra of KgP(2,900yr B.P.)and K-Ah ash
(6,300yr B.P.)were found in this lagoon
sedimentsatdepthof-6mand-17.8mrespectively(Yamazakiαα1.,1981).The
depth of these tephras approximately indi-
cates the subsidence of Ukishimagaharasince after the deposition of these tephras
because the sea level has been relatively
stable during last6,000years. The mean
subsiding rate is estimated to be2m/103
years since the KgP age and to be3m/103
years since the Ah ash age.Figure7shows
the relationship between the altitude of sam-
pled peats in the lagoon sediments and their
radiocarbon age.
c. F細亘聾om量ya ba,s沁:This basin is a
fault angle depression formed on t五e south-
westemfootofFujivolcano.A300mdri1-1ing revealed that this basin was composed of
thick Holocene Fuji lavas an(101der Fuji
mudflows younger than80,000years ago.The bottom of drilled core did not reach the
Bessho gravels uhderlying Older Fuji mu(1-
flows(Yamazaki6渉σ1.,1981). This fact
indicates that Fujinomiya basin has under-
gone intense subsiding at a rate of2to3m/
103years since the late Pleistocene.
4.3 Moveme恥t of aetive f&賜且ts
(1) hiyamase-Om量ya-Agoy&ma鉛腿亘t sys・
tem
This fault system divides the region into
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
日) J
I I 『 コ
… , ’
I I I l
??幹
’!ノ ,
ノ
46
ノ
磯 //i/
嬢
!!,轡
ノ
31m
♂ノ
iノ
毒’’ Ah’’ i
O I陥BuCHl
欝UKISHIMA GAHARA□(Hatan。αai.,
1979)
10RADlOCARBON AGE 5 30
ω > ζ 一〇
rO mm>
5 ε
需
一30
一50
Fig.7 Comparison of the sample heights and their
ages between two drillings on the both sides
of the Iriyamase fault.
hilly land to the west.This system exten(is
from the mouth of River Fuji to the north and
disappears into the younger lava field。The
Iriyamase and Agoyama faults inthis system
are reverse faults showing a sinistral en eche-
10n configuration. The Omiya fault is a
normal fault comecting the Iriyamase an(1
Agoyama faults(Fig.4).
a. 敢亘yam&se飴翻t(Tsuya,1940):This
fault,bounding the eastem margin of Kam-
bara and Hoshiyama Hills,is a thmst having
theWpartupthrown,andextendsfromthemouth of Riyer Fuji to the north。Southern
extension of the fault is also assume(i to
continue to the submarine active fault along
the base of westem slope of Suruga trough.
Although no tectonic landform is found on
the surface of Fujikawa alluvial fan,the}ate
Pleistocene to Holocene se(iiments beneath
the fan are vertically displaced by faulting。
Comparing the displacement reference hori-
zons(DRH)on both sides of the fault,1ess
than100m,46mand31.5mofthewest-sideupthrown are recognized for SSWl lava of
14,000years ago,peaty layers of8,000years
ago and Ah ash fall respectively(Fig.8)。The
long-term slip-rate of the fault(ierived from
these data is estimated to be constant with a
value of7to5m/103years during Holocenetime.This is the largest value of vertical slip
rate of the active faults in the lan(i of Japan.
At the event of the Ansei-Tokaiearthquake that occurred in Suruga trough in
l854,a small mound called“Jishinyama”(earthquake mou項(1) apPeared on the floo(玉
plain near the mouth of River Fuji(Omori,
1919).Judging from the fact that a vast area
of cultivated land was(1eveloped to the west
of the mound after the earthquake,it is con-
clu(1ed that the formation of“Jishinyama”is
not a small local event but a tectonic land-
form accompanied with extensive uplifting of
the westem side of the Iriyamase fault.This
means that the southem segment ofIriyamase fault has moved at the Ansei-
Tokai earthquake of1854.
SSWl lava of14,000yr B.P.outcrops to
the west of the Iriyamase fault.This lava
seems to have(1eposited on the river floor
because it rests directly on the flood loam.
Assuming that the sea level of14,000yr B。P。
was50to60mlowerthanpresentlevelandthe height of the outcrop above sea level at
that time was the same as the present value
(十15m),the original altitude of this lava
might be-35to-45m。Consequently,thewestem block of the Iriyamase fault has been
uplifted50to60matarateof3.5to4.4m/103years since the deposition of SSWl lava,
and the eastern block has subsi(1ed55to45m
with the rate of3.3to3.9m/103years.
Table.2shows the relative slip rate of the
faults and the absolute rates of block move-
ment which were obtained from the heights
of several DRH and the heights of sea level
when DRH were deposite(1.
b.Om量ya fa磁(Tsuya,19401Yamazaki,
1979):Tsuya(1940)inferre(1the existence
of an arcuate fault along the eastem margin
of Habuna an(1Hoshiyama Hills and named
it the Omiya fault.Suzuki(1968)estimated
that the Omiya fault was a part of normal
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
100m
O
一100
100m
α
Z
Iwabuchi terrace
/RFuji’o』。・。 SSW1 13,ス60土300Y・B。R
.・i奪・
→Fuli volcano
b
O
』…i蓬粟・
z’噛鞠1,1,,
B勿 α lwabuchi
須 terrace A
I器躍曼。・・造
』・:ll=』。誇』
Fandep.
』・二。ぷ。』,・’・』6・・:
o .
’0
’1自6istocene
gravelS
Fig.8
R.Fuji
瀬』論茎
4430:±120Y.B.P.
Ofuchi lava(SSW1)
〇 1Km c
Ukishimagahara
’///////
30
Om
一30
A 28.77m
Peat
7660±230Y.B.P。
Table2
Mean s巨p rate
B
5% 4。054m 3
ay
13・系
ρ・σ
8090土380Y.B.R
Y.B.P.
Vbrtical displacement
当3K9P iΩシ呂
、轟 一6m5m
LAh.o’1・.
臼 ’o臼
}短嘱曹
Sand&(i
-1Z8m
8180土210
’o∋=’σ.
Clay
YBP
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
Block movement on the each side of the Iriyamase and Kozu-Matsuda faults(after
Yamazaki,1984).
Faulmame Ref◎rence Age
(Ka)
Sealevel
(m)
Hei菖ht
(m)
Up㎞wnside
Up1血
(m)
Uplift
rate
(m/103y)
Hei帥t
(m)
Down山mwnside
Subsidcnce Subsidence rate
(m) (m!103y)
F副t long-telmdisplace- slip-rate
m㎝t
(m) (m!103y)
KOZU-Matsuda 鉛ult
Aha5h
Mゑsu面ace
OS2
6.3
60
250
2-15『
20.5
90
『
18.5
105
曙
29
1.8
一
1.8~23
-43.5
-500
3.8~43 0。6~0.7
30.3 0.5
500 2
2玉~23 3.3
135 2.3
myamase-Agoyama £ault
躍
Ahash
Hol㏄㎝epeat
SSW-11ava
OFmudflow
2.9
6.3
8.0
13.7
80
0
2
-10~一15
-50
-3~一9
昌13.711.015250
一
11.7
21~26
65
260
一
1.9
2.6~3.1
4.7
3.2
一6
-17.8
-35
-90
-160
6 2
19.8 3.2
20~25 2。5~3.1
40 2.9
150 1.9
31.5 5.0
46 5.8
105 7.6
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
groWth o:f volcanic cone.
On the other hand,Yamazaki(1979)thought that the northeastem margin of
Hoshiyama Hills was a trace of normal fault
to comect the Iriyamase and Agoyamareverse faults having a sinistral en echelon
configuration.He redefined this normal fault
as the Omiya fault.The height of the fault
scarp of the Omiya fault(1ecreases from
southeast to northwest.This fault dislocates
the Mf一皿surface of20,000yr B.P.and has a
scarp80mhighinthesouthwestemportionof the fault trace.Whereas in the northwest-
ern part,the steep scarp turns into gentle
flexurescarp40to50mhigh(Fig.9).Thelong・term slip-rate along the Omiya fault is
estimated to be4to2m/103years(1uring the
last20,000years.
In the Hoshiyama Hills,the younger mud-
flow surfaces dip toward the River Fuji(to
SSW)but onthe contrary,the oldersurfaces
dip to the opposite direction (to NNE).
Iwabuchi andesite and the Bessho gravels
appear on the southwestem margin of the
hi11s also have a NE-SW strike and NE dip
concor(1ant with the tilting of older mudflow
surfaces.From the straight trace of the fault
and northeastward accumulative tilting of
the hills,the Omiya fault seems to be a
normal fault with steeply-dipping fault plane。
e。Agoyama£躍旦t(Tsuya,19401Yama-zaki,1979):The Agoyama fault is a reverse
fault bounding the eastern margin of Habma
Hills with high vertical slip-rate.Remark-
able features along this fault are the exis-
tenceofsteepfaultscarpof160minheightand the large westwar(1tilting of the hi11s.
The geomorphic surface on the Habuna Hills
consists mainly of mudflow deposits o1(1er
than Mf-I surface of50,000yr B.P.These
surfaces are steeper than those of the alluvial
fan atthe southwestemfoot of Mt.Fuji.The
(1eformation of Bessho gravels mderlying the
mudflow deposits is also concordant with the
tilting of mudflow surfaces.
From the accumulative tilting of upthrown
side of the fault,the Agoyama fault is esti-
mated to be a thrust fault with a relatively
Iow-angle fault plane.Figure10illustratesthe geological cross section((1-d7)traversing
the Habuna Hills and Fujinomiya lowlan(1in
the E-W direction.The Agoyama fault dis-
places the base of mudflow deposit more than
SW一[osnIyamahl”sa 一_一_一一一一一
■ 一
130m200m
a,
△晒“所』 一戸一一
_ノ妄そ.喋奮蕊一『1P 5m
Fan d6P.
9 σ o
,〆’ 幽 m6、 100
./ OlderFuji/ ’∋・ .!・/ mudfbW 一 .!.!
ム5 ▲ 』、
bウ 轟
。㌔, ρ ρ o
萎OlderFuji
3bム
5 ρ
6 幽 9 吾f- mudfbwつ 0
2
ム (
会ゐ
ム
Mt。MyojoFuji v。
ゆb Bessh・ 惚~ OIder Fuji mudflow bト慈遡
、
1 一一___一一一一一 } ”‘6
…ゆ㌔.
6い
’飢 4
6144・4}瀞㌔曽 冒一一、 o
100m
80mr・ρβ
200m
・羅 、、
声、, 4
葭、露厘!箆=一一’甲甲二3嘉鰯一:勘爺茅倉叩τ・・。.~P曹50酌
.い 骸一一一”轡欄・二一
\ lwabuchi
宕二 4二4じ R.Urui
ム100
an eSl e 00
b’
Omiya fault
0 2km
Fig.9 Geologic sections across the Omiya and Agoyama faults(part1),
and b-b’coπespond to those in Fig.6
Section-lines of a-ay
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
CSW HoshiyamaHiIls
200m
NE C,
ヌ
壁轟陰SWl lava 衰,4 。 4 。
’,載, ・・2 P~’へ9
R.Fuji
・霧㍗’㌔%
1暢k4 4 3あ へ ニレのてコロニ 全、、あ凸,, 940-50m
〆l Flluvialdep.
H、UrUl
Habuna Hi”s
R Urui
Omiya fauIt
100
d
0
W ..費.4... ・靴
OFM ヒ
礁篇蕪 o
R.Urui Mt・Fuli σ o’
R.Shiba kawa 翻
200m・1 50m SSW3 ’DriIling ・’蟹呈こ9”
100m
晶棚義へ
擦i鞭纏
贈侵繰灘灘鶴轟霧籍漁一灘繍灘1灘灘馨捌垂.!。
驚瞬
蹴論h6錨鱗 △な 臼
A .
..SW5▲ 轟
o ▲
410m I
Agoyama fault
彿 ,:
, , 4 「 , ’ ム
4
Om
旭Older Fuji mud刊ow
4ム.ゲ (OFM)
ム , 100m
2km
“・ 4 ・ d
△ ■
4 . 4
△
300mEd’
Fig.10 Geologic sections across the Omiya and Agoyama faults(part2).
and d-d’coπespond to those in Fig.6
Section-line of c-c’
410mbetweenthehillsandlowland.Assum.ing that the base of mudflows was formed
after the initiation of the activity of older
Fuji volcano80,000years ago(Machida6渉α1.,1975),the minimum mean slip-rate of the
Agoyama fault is estimated to be5.1m/103
years.On the other hand,the Agoyama fault
vertically dislocates the top surface of the
SSW31avaflow(ca.10,000years ago)about
50masshowninFig.10。Themeanverticalslip-rate of the SSW3is calculated to be l
m/103years.The same slip rates of the two
different DRH indicate that the Agoyama
fault has constantly repeated the movement
during the late Quatemary time.
(2)hiyama-SMめakawafa眠ltsystem The Iriyama-Shibakawa fault system with
N-S strike extends from the coast of Suruga
Bay to the westem foot of Mt.Fuji approxi-
mately25km long.This system is composed
of two thrusts dividing the mountain area on
the west from the hilly land on the east.The
segment to the south of the River Fuji is
called the Iriyama thmst and the Shibakawa
thrust to the north of the river.
a。 亙riyama thr盟st (Konno & Otuka,
1933):Along10km of the Iriyama thrust,
Pliocene Fujigawa Group to the west over-
thrustsup ontheQuatemarygrouptotheeast.Awidefracturezonecanbeobservedalongthe bomdary’between the Hamaishi-dake
Mts.and Kambara Hills. Although theIriyama thrust displaces the Quaternary
deposits in the Kambara Hills and marks
their westem boun(1ary,no significant fault
scarp or tectonic lan(1forms are recognized
along the fault trace.From these character-
istics,Yamazaki(1979)considered that theIriyama thrust had ceased its recent activity.
But some evidences of the recent faulting
were fomd later on several man-made out-
crops in the Kambara Hills.Sugiyama&
Shimokawa(1982) estimated that theIriyama thrust has continued its movement at
a vertical slip-rate of O.25m/103years during
late Pleistocene an(i Holocene.
Although the recent fault slip-rate of the
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-
ture zone along the thrust an(11arge(1eforma-
tion of the early Quatemary suggest that the
Iriyama thrust had a long history of faulting
an(1there was a more active period than the
present one.Presumably,the Iriyama thmst
had moved vigorously as a boun(iarybetween
the uplifting Hamaishi-dake and subsiding
Kambara Hills during the depositional period
of the early Pleistocene Kambara conglomer-
ate and Iwabuchi andesite.
b.Sh童蝕誼&w&臨臓st(Yamazaki,1979):
The name of the Shibakawa thmst is as-
signe(1to the15km long segment of the
Iriyama-Shibakawa fault system to the north
of River Fuji.The west side of the fault is
upthrown.In the northem extreme of this
thmst,the strike tums from N-S to NE and
the fault trace seems to plmge into the
younger Fuji lava field(Fig.4).The DRH
which recor(1s the recent activity of the
Shibakawa thrust is the older Fuji mudflow
of50,000yr B.P.(Mf-1)and the younger Fuji
lava about10,000yr B.P.(SW-l and SW-3).
Figure ll illustrates the west-upthrown verti-
cal displacement of50m for the Mf-I surface
andof20mfortheSW-11avasurfaces.Thenmean slip.rate using every DRH is estimated
to be l m/103years.These data indicate that
the Shibakawa thrust has repeated its move-
ment more vigorous than the Iriyama thmst
during the late Quatemary time. In the
northemhalf ofthe Shibakawathrust,SW-31ava serves only a DRH for the recent move-
ment.This is subject to30to40m vertical
displacement of W-upthrow.Assuming that
SW-31avaflowedout10,000yearsago,meanslip-rate is estimated to be3to4m/103years.
These facts indicate that the activity of the
Shibakawa thrust gradually increases toward
e SW NE e’
0Fuligawa GrouP
200m
50m
ヘロ ペリ
陰ξン・乳。 領。
f W300
Shibakawa fault
ぐ 一9『ヲρ/.’
ρ7・9ろR.Shiba
4、’
3猫一一一1・・ andesite
♂’40 4
BesSり・gravel一。
0HabunaHills
200
100
R.Shiba
〃.急△
、鴛・ムヘ
、釜グ凸一ム4
Mt.Shirao
2km
E f’ 300m
Fujigawa.
Group
SW1 窺㌧△’,ラ
200
Fig.11
Bessho graveISShibakawa fault
andesite Agoyama fauit
100
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-
preted as that the Shibakawa fault gradually
inherits the vigorous activity of the Agoyama
fault when they come to each other to the
north o:f Fujinomiya city.
(3)0軌erね出s 我.M&ts撒o f踊亘t:The NWtrending and
SWupthrownMatsunofaultisinferredalongthe River Fuji from the stmctura1(iifference
of Saginota gravels between the Kambara
and Habuna Hills.This fault is interpreted
to be a normal fault which have been forme(1
to comect the two en echelon arrange(1reverse faults,i.e.the Iriyamase and Shiba-
kawa faults,during the Middle Pleistocene
time.But this fault has no evidence of recent
activity.
む.Nosh亘ta 癒臓st &翻 亘重s 聾or騒e照
ext鎌s亘o髄:A conspicuous scarp,whoseheight excee(1s50m,is recognized at the
eastem margin of the older Fuji mudflowdeposit(Mf一皿)distributed alongthe eastem
flank ofthe Tenshu Mts.Thisscarpseemsto
exten(1southwestward to the Noshita thrust
dividing the Miocene Nishi-YatsushiroGroup and the Pliocene Fujigawa Group in
the Tenshu Mts.area.As the Noshita thrust
shows a sinistral en echelon configuration to
the Shibakawa thrust,the activity of the
Shibakawa thmst seems to transfer to the
northern extension of the Noshita thrust.
4.4Geo亘09童ca丑翫量story蹴df蹴亙t腿ove・ me通
In Pliocene time,the area of the Hamaishi-
dake and Tenshu Mts.was a part of ancient
subsiding Suruga trough where FujigawaGroup,consisting of thick coarse clastics
derived from northem mountains,has beendeposited.
During the early Pleistocene,the west-
upthrown Iriyama and Shibakawa thrusts
have commenced their motion to divide the
se(1imentary basin of the Fujigawa Group。
The west side of the thrust,i.e.the Hamaishi-
dake and Tenshu Mts。,was uplifted gradu-
ally and the axis of subsidence has shifte(i to
the east of the thrust.The Kambara con一
glomerate has accumulated in this new sub-
sid.ing basin and was soon after covered.by
the Iwabuchi andesite. During the active
stage of this volcano,the subsidence in the
present Kambara Hills area has gradually
decreased and thesubsi(1ing center hasmoved
northward resulting in the accumulation of
the Bessho gravels in the area of Hlabuna and
Hoshiyama Hills.This migration is consid-
ered to correspond with the development of
the Iriyamase fault.
In the middle Pleistocene,the subsidence in
the Habma and Hoshiyama Hills areabecame less intense,an(i subsequently the
area has gra(lually change(1into the uplifting
area.Then,theAgoyama,Omiya andMatsu-no faults commenced their activity,result-
ing in the eastward migration of the subsid-
ing center toward the Fuji-Fujinomiya low-
land.About O.5to O.4Ma,the alluvial fan of
the River Fuji spread over the Kambara and
Habuna Hills area to deposit the Saginota
gravels.The s亡bsequent uplifting of Kamba-
ra Hills has left the Saginota gravels on the
hills as terrace deposits. As the Saginota
gravels in the Habuna Hills have not under-
gone strong uplift,it seems that the uplift of
Habma and Hoshiyama Hills commencedlate,compared to that of Kambara Hills.
The tectonic inversion in Kambara Hills
from subsidence to uplift has tumed the char-
acter of Iriyama thrust from a major fault
between the uplifting and subsi(1ing blocks to
a fault representing the relative slip between
the both uplifting blocks.As a result of this
change,the Iriyama thmst has decreased its
activity.
During the late Middle Pleistocene to the
early Late Pleistocene,the Iriyamase,Omiya
and Agoyama faults have acted as the bound-
ary faults between the upliftinghilly land and
subsiding lowland.Consequently,the Kam-
bara Hills has become a(1issected hilly lan(1
due to uplifting and erosion.
On the other hand,the accumulation and
erosion of the upPer Quaternary have been
repeate(1in the Habuna an(1Hoshiyama Hills
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
Matsuno faulthad alreadyceasedits activity.
Although the Southem Shibakawa faulthas decreased its slip rate due to the develoP-
ment of the Agoyama fault,the northem
Shibakawa fault has maintained relativelyhigh slip-rate because no active fault has
appeared to the east of the Shibakawa fault.
The o1(1er Fuji volcano started its activity
ca.80,000years ago.Since then mudflow
deposits from this volcano have filled the
subsi(iing area to the east of the Iriyamase,
Omiya and Agoyama faults,and frequently
overflowe(i into the Habma an(1Hoshiyama
Hills where the strong uplift had alreadystarted..
About50,000yFears ago,the Habuna Hills
completely emerged and the broad Mf-I sur-
face was forme(10n the hills.In and aroun(i
the Hoshiyama Hills,the River Fuji intensely
incised its chamel associated with the uplift-
ing and northward tilting of the hills.Head-
erosions from the valley of River Fuji devel-
oped some tributaries northward in the hills.
Before long,these tributaries exten(ie(i to
Fujinomiya lowlan(1,traversing the Ho-
shiyama Hills.Consequently the tributaries
captured the upper reaches of the river which
had flown to the southeast in southwestem
foot of the older Fuji volcano.Then mud-
flows and Iavas from the volcano filled the
narrow valleys in the hills again.
When the hills uplifted by the recurrent
fault movement,these valley floors remained
as flat surfaces in the hills. Subsequent head
erosions from the River Fuji extended to
Fujinomiya and cause(1stream piracy once
more.As a result of these processes,i.e.
uplifting,head erosion,stream capture,valley
fill an(i uplifting,a flight of terraces (ieve1-
oped on the Hoshiyama Hills during latePleistocene time.
Thus,geological and geomorphologicalstu(1ies in the region to the north of Suruga
Bay reveal that the NF-S trending tectonic
landform has been formed by the eastwar(i
migration of faulting an(1subsiding since the
Pliocene.
5.ACT亙VE EAULTS亙N THE SOUTH・ ERN MARGIN O量THE TANZAWA MOUNTA亙NS
The area of the southem margin of the
Tanzawa Mts.is divided into two portions,i.
e.,westem part and eastem part,(1ue to the
difference o:f topographical configuration.
In the westem part,as shown in Fig.12,the
steep Tanzawa Mts.ranging in altitude from
1,500mto800mcontact(1irectlywiththeAshigara Mts.(less than1,000m high)and
Hakone volcano. No obvious depressiondevelops between two mountains except the
westem extreme of the area where a small
dissected hilly land is situated.While,in the
eastem part,the Tanzawa Mts.bounds onAshigara plain with steep slope facing south.
This topographical distinction between the
west and east part seems to reflect the char-
acteristic of tectonic evolution.
5.1 Geology a醜dl c「腿s意&旦move贈e璽意a丑o遡9
儘eweste膿腿rto量蝕esO鵬恥e膿 marg五聾oftheT撒z&waMts.(1) 0臆tli聾e of geology an岨to基)ogr瑚)地y
a.丁躍zawa Mo撒ta亘薦:The TanzawaMts.consist mainly of volcanic rocks of the
Miocene Tanzawa Group(Mikami,1958)which is bounded on the south by the N-E
trending Kamawa fault system(Matsushima
&Imanaga,1968).Recent studies(Amano,
198611shibashi,1986a,etc.)in(iicate that the
Tanzawa Group was originated as a mass of
volcanics constituting a ridge on Izu-Ogasa-
wara arc far south,which approached the
Japanese Islands following the northwardmigration of the PHS plate and finally collid.
ed with Honshu at about5to6Ma(earlyPliocene).In late Pliocene,a new subduction
zone occurred along the southern margin o:f
the Tanzawa area due to the northward shift
of Izu block which also belonged to an off-
shore volcanic ri(ige on the Izu-Ogasawara
arc.Consequently,the uplifting of the Tan-
zawa Mts.was initiated and the Ashigara
Group accumulated in the subsiding(1epres-
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
2
籔9
G
》 7
Gotenba
Fig.!2
難…………1………1………1曝欝
0
懸
1、屡、箭渇、,髪,
Mlshima f,旧,,「”
“
0
Mt.HAKONE 遡
勘難σ
\、
Kozu
ノミげ ぬ ロ
Odawara
SAGAMI B細
癒 O
Atami
10km
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.
In the early Pleistocene,the upheaval of
Tanzawa Mts.was accelerated by the colli-
sion and subsequent accretion o:f Izu block
toward Honshu. The exposure of theNeogene metamorphic rocks in the central
part of the Tanzawa Mts.indicates that the
this range has been subjected to intense
uplifting and denuding during Quaternary(Matsuda,1978).The uplifting rate is esti-
mated to be3to4m/103years by Kaizuka(1987).Thus,the Tanzawa Mts.has experi-
enced continuous upheava1(iue to the colli-
sion-accretion tectonics at the northern
extreme of the PHS plate during the late
Pliocene to Quaternary.
b. The As瞳gara M重s.蹴d亘ts醐jace虚
&rea=The topography to the south of the
Kamawa fault system consists of theAshigara Mts.,Hakone volcano and the hilly
land in the northwestem extreme of the area.
The Ashigara Mts.are composed ofthe Plio-
Pleistocene Ashigara Group. The deepgorges an(1steep slopes in the mountains
indicate that this area has experience(1
intense uplift and denudation process
together with that of the Tanzawa Mts。due
to continuous upheaval during the middle to
late Quatemary. The Ashigara Group is composed of coarse
and thick clastics from the Tanzawa Mts.,
having accumulated in a paleo-trough locat-
ed between Honshu and Izu block associate(1
with the plate subduction.The total thick-
ness amounts to1,000m.
Since the1980ヲs,many much attention has
一624一
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been paid to the stratigraphy,structure and
evolutionary process of the Ashigara Group,
because this group has recorded tぬe bulk of
evi(ience on the c』011ision o:f Izu block an(i the
subsequent upheaval of the Tanzawa Mts.
(Amanoαα1.,19861Huchon&Kitazato,1984,etc).Consequently,the Ashigara Group
was divided into four formations and their
Sedimentary ageS were eStimated tO range
from the late Pliocene to the early Middle
Pleistocene based on the yields of Rz名とz-
s孟6go40%側zo名α6,assemblage of molluscan
shells, foraminiferal biostratigraphy an(i
paleomagnetic stratigraphy.
The Ashigara Group shows a dome-1ike
structure dippingto NWandNE.Moreover,this group has been subjected to large defor-
mation by faulting. For instance,the dips
gra(1ually increase towar(1the northwest by
the drag deformation associated with the
thmsting of Kannawa fault system(Amano6砲1.,1986).
The hilly lan(i to the northwest of the
Ashigara Mts.,where the geology is com-
posed of the Suruga gravels in late Pleis-
tocene and overlying late Pleistocene tephras,
is also well dissected.The accumulation(as
muchas100minthicknessofcobble-richfacies) of the Suruga gravels in(1icate that
this formation was originally fluvial sedi-
ments derived from the Tanzawa Mts.andfilled a relative depression between the Tan-
zawa and Ashigara Mts.
Referring to the origin of the accumulation
of the Sumga gravels,Machida6短1.(1975)
in:ferred that the gravels have been(ieposited
in a relative depression formed by the growth
of volcanic cone of Older Fuji volcano.Kano
α召1.(1984)suggested that some of the lat-
eral slip faults,obliquelyF cut the Kannawa
thrust,have also participate(i in the(ievelop-
ment of that depression.The dissected Sum-
ga gravels indicate that the area has under-
gone intense uplift during Late Quatemary
time.
Thus,the Pliocene to early Pleistocene
sedimentary basin of the Ashigara Group
along the westem part of the southem mar一
gin of the Tanzawa Mts.gradually tumed
into an upheaval area since after the begin-
ning of the Middle Pleistocene(0.7Ma).
Consequently,the upheaval of this area has
promoted extensive denu(iation to form the
steep mountains an(1the hilly land.
(2)・Activ童亡y o£the K継醜awa f躁亘意system
The Kamawa fault system divi(1ing the
Tanzawa Group and the Ashigara Group runs
along the southem margin of the Tanzawa
Mts.Hitherto,this system was thought to be
a north-dipping thmst (Kannawa fault)
because the northem block is upthrown and
the trace is winding (Matsushima &Imanaga,1968).But recently,it is presumed
that the Kamawa fault is a fault system
compounded by E-W trending thrusts and
NW and NE trending strike-slip faults
(Kano6渉α1.,1979).After this,the author
calls the E-Wtrending thrust as the Kan-
nawa thmst,and names the composite fault
system including the thrust and lateral slip
faults as the Kamawa fault system.Machida
αα1.(1975)recognized that the Kannawa
thrust has continued its thrust movement at a
rate of l m/103years since begiming of the
Late Quaternary base(10n the vertical(1is-
placement of Suruga gravels as much as100
mbytworeversefaults(KsandKnfaults)branched from the Kamawa thrust. On the other han(i, Kano α α1. (1979)
pointed out that the Ks fault was not a thrust
but a NE trending strike-slip fault offsetting
the Kamawa thrust.Furthermore,they sug-
gested that the E-W trending Kannawathrust of the Kamawa、fault system has com-
pletely ceased its activity in the late Quater-
nary(ltoαα1.,1986).But the author does
not agree with the above opinions because
there are several south facing steep slopes as
much as100m high,which are likelyto be the
fault scarp representing the recent activity
along the Kannawa thrust.
The above mentioned data derive the fo1-
10wing history on the movement of the Kan-
nawa fault system.During the sedimentation
of the Ashigara Group in Pliocene to early
Pleistocene time,the Kannawa thmst was
一625一
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vigorous as a bom(1ary fault between the
uplifting Tanzawa Mts.and subsiding sedi-
mentary basin. But the Kamawa thrustgradually diminished its activity because the
se(1imentary basin of Ashigara Group tumed
into an uplifting area(iue to the progress of
the Izu collision in the mid(lle Pleistocene.
Ito6オσ1.(1986)pointed out that the NE and
NW trending strike-slip faults offsetting the
Kannawa thrust initiated its activity at that
age.The reason for the commencement ofthe strike-slip faulting oblique to the thrust is
ascribed to the decrease of fault mobility on
the Kannawa thrust,which is cause(1by the
intensi:fication of crustal shortening of the
Ashigara Group and steepening of the fault
plane associated with the Izu collision.
Ito6オα1.(1987)conclu(led that the activity
of the Kamawa fault system has also wea-
kened since the early Late Pleistocene based
on an outcrop data of the NE trending Hira-
yama fault(Amanoαα1.,1984).However,
the author disagrees with this thought
because the faulting (1ata from an outcroP
camot represent the whole tectonic move.
ment on the Kamawa fault system.
5。2 Geo且ogy&恥〔丑cr覗sta丑moveme聾t a亙o血9
意恥e easter醜 憂亘a,且£ 0£ 重he sO眼t地ern
m麗g童盟of臨eT膿zaw我瞭s. Along the southeastem margin of the Tan-
zawa Mts.,there is a strip of the Matsuda
Mts.which is a fault-bounded and flat-
topped block:consists of the Ashigara Group
and Quatemary sediments.The mountainsare in fault contact with the subsidingAshigara Plain on the south and the uplifting
Tanzawa Mts.on the north.As illustrated in
Fig12,the two faults bounding both rims of
the Matsuda Mts.correspond to the north-
westem extensionoftheKozu-Matsuda faultand the eastem part of the Kannawa fault
system respectively.The southem margin of
the Matsuda Mts.is fringed with an area500
mtolkmwideofdissectedfoothills100to300m high.This narrow foothills comprises
mainly middle to late Quatemary sediments.
The geological history of this foothills wi11
describe the intense crustal deformationalong the active faults during the late Pleis-
tocene and Holocene.
(1) Str我tigraphy an{董geological struet覗re
O墨the footh童llS
The geology of the narrow hills on the
southemfoot ofthe Matsuda Mts.consists of
the middle to upper Pleistocene clastics un-
conformably overlying the Pliocene Ashigara
Group.The stratigraphy of the foot hills is as
follows.
a.Ash置gara Gro囎:The Pliocene Neishi
Formation of Ashigara Group unconformably
underlying the Pleistocene sediments consists
mainly of altemating beds of sandstone and
siltstone,andesitic lava and volcano-clastics.
This formation strikes nearly NW-SE and
dips、40 tg 50 northeast・ Consequently,the
younger and gravel rich formations ofAshigara Group appear in the Matsuda Mts.
to the north.These formations were mainsource of the coarse talus deposits in the hills.
め.M量雌eto肱teP旦eistoce盟ede脚S亘ts:
These Pleistocene deposits are composed of
the fluvial sediments of the River Sakawa,
mudflow deposits from Hakone volcano,
talus deposits from Matsuda Mts.and some
pile of tephra layers from Hakone and Fuji
volcano(Fig.13).Table3summarizes thestratigraphy of this area and the facies of
eachlayer.
The Kananzawa gravels (Yamazaki&Machida,1981)consist of pyroclastic flows,
air-fall tephras,mudflow layers from Ha-
kone volcano an(1the fluvial se(iiments of the
River Sakawa in the late Middle Pleistocene.
As shown in Fig.13,the Pleistocene sedi-
ments in the hills has mdergone the strong
shortening to form a foldstructure with NW
trending axial plane.
Kananzawa loam(Yamazaki&Machida,1981),overlying the Kananzawa gravels,is a
pile of tephra layers including some pumice
falls from Hakone volcano in the early Late
Pleistocene(0.12Ma).This loam is overlain
mconformably by the Matsuda gravels.
The Matsuda gravels(Yamazaki&Ma-chida,1981) are a sequence of coarse talus
一626一
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ロ01-1
一1 「
「一1一¥
魯⊥り暢聖!……」」」」一L……i圭i≡i圭i圭i圭i三i≡i≡i≡i≡離i圭i鑑i≡i≡i≡i≡ili≡曲圭i≡i≡i圭ili圭ili圭i≡lli圭i圭i二i圭i圭ili圭i=i鑑i圭i:i!一i」一『 し1⊥1⊥1⊥1.
l l l l l蓼『.凌JJ_IJ_LI」J⊥1-1⊥1一L
ヨ=1’久一1・出=1=1=!・ぐ一”一・、・!・IJ 『 1 0 I l I l l l I LI
I I I I I I I I
l l l l I I l LILLLIロロHIIH1に日IIIHUl811口1凸H8UlrHiIllコ貞r-H-11㍉1-lrl工IrUrロー1
蒙
\愚・ B
500輪 1睡 2麗ヨ
ヘノノ ヤTδmeihighwαy臼∵‘蓮一’一’
0
《《α骨sudαT.
3翻 4匿ヨ
Nort
集MatsudaF
5圏 6園
萎馨萎
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:Matsudagravels,5:Pyroclastic flow(1eposit,6:Fuji tephra Iayers
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
馨
Table3 Stratigraphy of the hilly area in the southem foot of the Matsuda Mts.
Yamazaki6オσ1.,1982).
(after
Facies Features Age
Hol ene Scoria ash fall Stratified魁1 A丘er30,000yrago
o罵
F両iTephras
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-beddedAWtypetephras鵬intercalated
o Hakoneyounger Black-graycolored TP飢1(1minthic㎞ess) c&50,000yrB。P.qの
pyroclas丘cflow Pumice flow underlies the flow
QO
o(Tpnow)
一の 宅お霞
℃●自
LowerMatsuda gravelS
Talusdeposits(Blecciaandloam)
B1㏄cia derived丘om A曲匝gara gmup
ca.50,000-80,000yrB,P.
o ill theMatsuda Mts.一d
■ Kananzawaloam A廿剣ltephra Pumice咄s(KIP6-14) ca。120,000yrB。P。
銚一鵠
mainly加mHakoneVolcano 飢e血tercalated
Last血ter-
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,aεhinMiddlePleistocene
andPm丘ceflow丘Dm H欲one Volcano
蒸
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deposits derived from the conglomerate of
Ashigara Group in the Matsuda Mts.The
gravels contain several intercalated key te-
phras such as Pm-I pumice of O.08Ma from
central Japan and the Younger Pyroclastic
Flow of Hakone volcano(TP flow)of O.05
Ma,and underlie the scoria ash from Fuji
volcano.The age of the gravels is estimated
to be middle to late Late Pleistocene time.
(2)Fa磁movementa亘o聾gthesouthem foot of Ma重s題〔丑a、Mts.
The sedimentary sequence of the southem
foothills of Matsuda Mts.shows two major
turning Points on the tectonics in this area.
One is the begiming of the accumulation of
Matsuda gravels in the early LatePleistocene.This rapid increase of the sedi-
mentary supply implies the accelerated up-
heaval of the Matsuda Mts.,associated with
the activation of the fault(Matsuda-sanroku
fault l Yamazaki&Machida,1981)alongthe
southem margin of the Matsuda Mts.
The other is the end of the accumulation of
the Matsuda gravels.Since then,the south-
em flank of the Matsuda Mts.has been ter-
raced to form the foothills.This also sug-
gests a weakening of the activity of the
Matsuda-sanroku fault and the onset of the
new faulting(North Matsuda fault:Yama-
zaki&Machida,1981)along the presentmargin of the foothills. Accor(iingly,the
fault scarp rapidly developed during the late
Late Pleistocene to Holocene at the southem
margin of the foothills.The eastem exten-
sion of this fault scarp seems to continue to
thenorthemtrace oftheKozu-Matsudafaultmentioned later.
Figures13and14illustrate that the foot.
hills area has undergone a short wave-1ength
folding with NW axis associated with the
activity of the North Matsuda fault.This
folding accompanies several reverse faults
with same strike to the fold axis. The
arrangement of these structures suggeststhat
め
A 。,帆,.,,三二∫.㌧1緩1:擁
暴撃・\Ma,Sud、
、graveIS\
典 Kananzawa
[oam\
B ρ く〉 ク むゆゆ Nor†h 。・。23
〆糠パ
・…、4嚢≧
二護、撚罪 ・蓼一ρ㌧z桜蘇・,
弩’㌘、鴛薦織1’汐’
.認編・三・、㍗・マ1:1’1マ・9・ギ弼灘
簑無,熱戯≧講隅
。 懸唱 .鑓 tワ ロロ つ ま ッDOP ρρ’σ一G
100
麟鷲誤欝
リ ロ くン ひ ゅ ロゆ
・D。 Kananzawa
B’
Ashigara150 Group,川1川川IIl
200m
Tc Ioam
一25・ノ
稀
Matsuda fauI
無
O
grave!S
c TP flow
,診.9・’!.9≦←’~』o,’r 150
200
300m
郷色ノ~ソ≧砲マぐζ一蔑考!
ノ’)狸’多
、秩肺《
100
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
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the North Matsuda faulthas a leftlateralslip
component as well as N upthrown.From this
sense of the fault movement and E-W strike
of the fault,the stress field in this area is
estimated to be NE compression during Late
Pleistocene to Holocene time. Northeast
compression is also pointed out from the
micro stmctures developed in the Ashigara
Group.That is,Huchon&Kitazato(1983)inferred that the stress field in the Matsuda
Mts.area shiftedfrom a NWcompressionto
a NNE at about O.3Ma,based on the minor
faults analysis of the Ashigara Group.
However,the orientation of stress field
derived from the geological evidence(1iscords
withthe expected NWcompressionfrom theplate motion of the PHS.This fact suggests
that the complicated local cmstal movement
including the rotation of large block has
occurre(1along the plate convergence bound-
ary during the late Quatemary.
5.3 Character亘s髄cs o蛋tぬe crust&l move-
menta互0醜gt恥esO鵬he騰marg量聾0£
蝕e T癩zawa Mts.
Along the southem margin of thepersistently uplifting Tanzawa Mts.,the
style of cmstal movement shows a spatiaI
(1ifference between the eastem and westem
parts.In the westem part,the se(1imentary
basin of the Ashigara Group has been exten-
sively uplifted since the Middle Pleistocene.
No subsi(ience has occurred except on the
westem extreme where the fluvial deposits
accumulated in the small basin formed by
local tectonics or environmental change.
During the Middle Pleistocene,the activity
of the E-W trending Kamawa thrust dimini-
shed,andNEandNWtrendingfaultsconcur-rently initiated the strike-slip movement.
The indistinctness of the topographical
boundary between the Tanzawa and Ashiga-
raMts.is ascribedtothisdiminishingvertical
movement since the Middle Pleistocene.
On the other hand,the E-W trending fault
has created a salient topographical boundary
between the Tanzawa Mts.and the Ashigara
Plain in the eastem part.The characteristics
of faulting in this area is the migration of
fault movement toward the south with time
and the associated uplift in the backward
area of the active fault.These characteris-
tics are very similar to the ones in the south-
westem foot of Mt.Fuji described in chapter
4,though the time scale and magnitude of the
movement in this area are a much lesser
(1egree than in Fuji area.
The spatia1(east-west)difference of the
tectonic features along the southern margin
of Tanzawa Mts.might be ascribed to the
difference of relative distances from the
northem tip of Izu block where the PHS
co皿des with the EUR.That is,the westem
part of the Tanzawa margin,10cated on the
north to northwest of Izu Peninsula,is
thought to be the plate collision area.The
extensive upheaval of the Tanzawa and the
Ashigara Mts.might be caused by the inten-
sive crustal shortening and uplift,1ike a pres-
sure ridge,due to the collision of Izu block
which camot subduct under Honshu.
In the eastem part,there are some tectonic
features such as the salient topographical
boundary between the Tanzawa Mts.andAshigara Plain,and the migration of fault
movement. The existence of a subsiding
depression,the Ashigara Plain,perhaps indi-
cates that the crustal shortening of the east-
ern part is less a(ivanced than that of the
westem part due to the endurance of plate
subduction despite the apProach of Izu block.
Although the migration of fault movement
shows a similar pattem to that of the south-
westem foot area of Mt.Fuji,there are con-
spicuous differences between two areas for
the occurrence interval of migration events.
As previously mentioned,the Tanzawa and
Ashigara Mts.areas are thought to be the
plate collision zone during the Quaternary
period.
Therefore,the shorter interval for the fault
migration in the eastern Tanzawa area as
compared to that of Mt.Fuji area suggests
that the interval relates to the rate of crustal
shortening and the convergent style between
twO Plates.
一629一
慧
馨
鷺懸鰯
Bz61」6ガn (~ブ 凌h6 G召oJo9♂αzl Sz67z76ッ (ゾノ41)α%, Vlol.43,1〉o。10
6.ACT亘VE FAULTS AND TECTONICS IN T翻E ASH亙GARA P:LAIN-OISO H亘LLS AREAS
6.1 0盟t亘量聡e
In the northem extension of Sagamitrough,which is generally believed to be the
plate convergence boundary between thePHS and EUR,there are several topographi-
cal provinces such as the Oiso Hills,the
Ashigara Plain,Hakone volcano and theTanzawa Mts.in northem extreme(Figs.2
an(i12). The Oiso Hills,an uplifte(1block
surroun(ied by several active faults,is located
in the northwestem extension of Okinoyama
banks,which extended aIong the northeast-
ern side of Sagami trough,and is thought to
be a part of a tectonic zone cause(i by the
vigorous crustal movement associated with
the plate convergence(Kaneko,1971). At
the northwestem extension of trough axis,
there is the subsiding Ashigara Plain(Ashigara depression)on the west of the
Oiso Hills,which is thought to have been the
extension of Sagami trough filled with the
thick deposits(1erived from the Tanzawa
Mts.The NW tending Kozu-Matsuda fault
rms between the Oiso Hills and the Ashigara
Plain forming the distinct fault scarp of300
m high.
6.2 Geology and cr腿sta旦 moveme盟t 畳n
O量so Hills蹴αAs聴g躍a Pl&i聖
(1)Oiso照1且S
The geology of the Oiso Hills consists of
marine and fluvial sediments and air fall
tephras,which were (1eposited since the
beginning of Middle Pleistocene underlain by
the Neogene-Tertiary.The geological struc-
ture of the hills is characterized by a basin
stmcture with an anticlinal axis along the
westem margin of the hills and a depression
at the central part(Uesugi,1986).With
reference to the tectonic evolution of the
hills,the Association of Kanto QuatemaryResearch(A.K.Q.R.)(1980)pointe(i out that
the tectonic movement change(i from subsi-
dence to upheaval during Middle Pleistocene
time.
a. Pre-Quatern鑓y rocks:The pre
-Quatemary rocks whose outcrops areboun(ie(1by faults are distribute(i in several
parts,especially in the northwestem part of
the hills(Fig.15). These rocks consist of
altemating Miocene volcanics,mudstone and
Pliocene conglomerate(Otuka,192910taαα1.,198211to,1985).
蝕.Q舩te膿麗y sed匡me艶ts:The Quatemary
sediments in the Oiso Hills consist mainly
of the middle Pleistocene Ninomiya Forma-
tion,middle Pleistocene to Holocene marine
and fluvial sediments and an overlying thick
tephra sequence from several volcanoes to
the west.With reference to these tephras,
Machida α α1.(1974)established the
chronology of tephra,marine an(1fluvial
sediments since the Middle Pleistocene(Table4).
The underlying Ninomiya Formationwhich outcrops in the southem part of the
hills consists mainly of sand an(i mud layers,
total thickness of which amounts to200m.
The age of this group is estimated to be the
early Middle Pleistocene because theMatuyama-Brunhes magnetic boundary(0.7
Ma)is fomd at the lowest part of the
Ninomiya Formation(Koyamaαgl.,19861Honnla (3渉ごzl.,1985). On the basis of ben-
thonic foraminifera,Yano(1986)estimated a
shallow sea(less than a few hundred meters
in depth)was the sedimentary environment
of the Ninomiya Formation.The relatively
small thickness of the Ninomiya Group indi-
cates that this group is a non-basin fill sedi-
ment accumulated in the exteriors of the
subsi(iing area.
The Quatemary sediments inthe Oiso Hills
contain8units of marine and fluvial terrace
sequence,namely the T-f,T-e,T-d,T-c,
T-b,T-a,K and H formation in order of
decreasing age(Table4),and basin fill sedi-
ments of the Sogayama formation in the
Middle Pleistocene.The age of each terrace
deposit is estimatedtobe O.5Ma,0.4Ma,0.35
Ma,0.28Ma,0.23Ma,0.18Ma,0.13Ma and
O.06Marespectivelybasedonthestratigraphic
一630一
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備簸 Hadano Basin
5 2
蝿:蟻
麟翻’Fe
’Fd
31
2
34
ゑ5下b
_,総瞬 6
下a 7
K 8
が日ノ9
・・………iii籔ゑ
Ashigara
Plain
下蝋”驚転
1霞K。
0
H
護. ’
灘,
Ni
Sagami Bay
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
α1.,1974,Machida,1975,A.K.Q.R.,1987).
The Oiso Hills can be(1ivided into the
following four areas,due to the inequality of
crustal movement in the Oiso Hills.
In the northern area of the Oiso H:ills,the
T-f to K fQrmations of terrace(1eposits,
composed of sand beds and gravels less than
a few tens of meters in thickness,show accu-
mulative tilting toward the east.Accordingly,the older formations are we11
exposed in thewest part ofthe area.Most・of
the formations are intercalated with the air
fall tephras towar(i the east.The altemated
sedimentary an(i eolian(1eposits in this area
indicate that the eastwar(1tilting occurred
since after the accumulation of T-D tephra
formation(0.4Ma).
The westem area of the Oiso Hi11s is under一
1ain by the middle Pleistocene:Ninomiya and
Sogayama Formations and terrace deposits
younger than T-c formation.The Sogayama
Formation which constitutes the Sogayama
upheaval block,occurs along the westem and
southwestem margin of the Oiso Hills.This
formation consists of mud layers inthe lower-
most part and pebble to cobble size uncon-
solidatedgravelsabout500minthickness.Hitherto,the correlation of the Sogayama
formation in the stratigraphic sequence were
not confirmed because of various opinions on
its age(Kojima,19541Kikuchi&A.K.Q.R.,
19771Yano,1986).
Recently,a glassy ash layer which can
presumably be correlated to the Handa tuff
in the upper most Osaka Group,in south west
Japan was(1iscovered at the lower part of the
Sogayama formation(Mizuno&Kikkawa,
一631一
B%ll6蕗n (ゾ オh6 (360109ぎo召l S%7∂6:y (Zプノ41)α%, Vlo乙 43,No。10
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).
Age 羅辞transgr regr
←一炉Surface 「k)rrace depo. Mqrine sed。
×104y
10
20
30『
40
50
Tc軒の9 一坐Ω一Φ一〇」
「:r皿 r I H
M2 一
S Ksurface
Φ⊆Φoo彰の■ 隔Φ一ユΦ一でP:Σ
Φ一〇陶Φ
u
T-Am I
盈 「臨tion K 幽下aI 聴uchiya
T-B π下b Akisawa uT-C l 一 T・CU
一下cl
Sogayamq
uT-D l
下du 「颪「一
一下dI
丁一E下e
uP∈
丁一F
応F一
1991).As the Handa tuff has already been
recognized in the middle part of the T-D
tephra formation of the Oiso Hills(Uesugi,α
α1.,1985),the age of Sogayama Formation is
estimated to be the age of T-E to T-D tephra
formation.
The similarity of lithofacies and thickness
of the Sogayama Formation to that of the
Bessho gravels in the southwestem foot of
Mt.Fuji indicates that this formation was a
subma血e basin fill sediment accumulated in
the subsided trough close to the coast.The
deposition of Sogayama Formation implies
the occurrence of the NW trending subsided
trough in the westem part of present Oiso
Hills during middle Middle Pleistocene time.
As the deformed Sogayama Formation isunconformably covered by the less(1eformed
T-c terrace deposits,the subsiding movement
in this area is estimated to have tumed into
upheaval before the accumulation of T-c
formation(about O.3Ma).
The Quatemary geology of the southem
area contains the Kuzukawa(Hamkawa6!α1.,1977),Akisawa,Tsuchiya(Kikuchi&A.
K.Q.R.,1977),K and H formations overlying
the Ninomiya Formation.The first three
formations correspond to the Sogayama,T-c
to T-b,and T-a formations respectively.
Each of these formations consists of marine
sand and mu(11ayers intercalating with thin
gravel strata. The thickness of each
formation is less than100m.The K(Kis-
sawa)formation is the transgressive sedi-
ments of the last inter-glacia1(0.13Ma).
The H formation is the marine terrace sedi-
ments formed by the post glacial transgres-
sion.The depositional surface of this forma-
tion is called the Nakamurahara surface
(Yonekura6齢1.,1968),emerged6,300years
ago.The top of marine sediments is at an
altitudeofabout20malongthesouthemcoast of the Oiso Hills.The mean uplift rate
in the southern area of the Oiso Hi11s is
一632一
T60孟o吻sα1・%8云h6%07渉h67n耀顧銘oゾ伽P6伽s%」α侭y魏膨切
calculated to be about3m/103years.
Therefore the gentle subsidence continued
through the Middle Pleistocene age and this
area was frequently inundated with sea water
at high sea-1evel stages.The change from
subsidence to upheaval might have occurre(i
since after the last interglacial period ih the
southem area.Machida&Moriyama(1968)pointed out that the upheaval of the Oiso
Hills became active in the.recent geological
time,based on the elevation ratio of K to H
terraces.The recent increase of uplifting is
in goo(1agreement with the above estimation.
In the eastern area,the last interglacial K
formation composed of sand and pebblelayers extensively distribute(i to fringe the
protruding Pre-Quatemary rocks.As mostof Quaternary se(iiments un(ierlie the K for-
mation(Oka6渉α1.,1979),it seems that the
subsiding in this area had exceed before the
accumulation of the K formation.This fact
also suggests that the tectonic inversion oc-
curred from subsidence to upheaval in the
beginning of Late Pleistocene.
The following tectonic evolution of the
Oiso Hills is deduced from the geological
data mentioned above.
The Oiso Hills was subjecte(1to slight
subsidence during the early MiddlePleistocene(0.7to O.5Ma).About O。5Ma,
the NE tren(iing(iepression occurred in the
northem and westem areas of the hills to
deposit the thick Sogayama Formation.Butthe stable tectonic condition was still continu-
ing in the southem and eastem areas.The
Sogayama Formation accumulate(1mainly in
the westem area of the hills. The crustal
movement of the northem area,where subsi-
dence was originally not intense,gradually
changed into upheaval an(i cause(i the east-
ward tilting.
About O.3Ma,the crustal movement in the
westem area converted from the intense sub-
sidence to upheaval and consequently the
Oiso Hills tilted towar(i east.Although the
slight subsidence in the southern and eastern
areas still contimed after O.3Ma,the whole
Oiso Hills changed into active uplifting area
after the Last Interglacial Stage(about O.13
Ma).
(2) A曲量gara P互a量聡
The Ashigara Plain,10cated in the north-
westem extension of Sagami trough,iscomposed of extensive alluvial fan develo-
ping in the lower reach of the River Sakawa
and the Holocene terrace with small relative
height.This plain,bounded on the eastem
and northem margin by the active faults,is a
kind of fault angle depression developed in
the eastem foot of Hakone volcano. As
shown in Fig.16,the geomorphic features of
the Ashigara Plain are characterized by
extensive development of the HoIoceneKamonomiya terrace and a lack of the last
interglacial terrace (K terrace). The
Kamonomiya terrace is the depositional
surface of the Gotemba mudflow(GMF:Machida,1964)from Fuji volcano at2β00
years agO。
The gravity map of South Kanto and
Tokai district(Komazawa,1982)shows the
continuous low anomaly zone which stretches
from the Ashigara Plainto theAshigara Mts.
This anomaly pattem suggests that the thick
accumulation of the Plio-PleistoceneAshigara Group fills the(iepression basin
beneath the Ashigara Plain.After this the
author calls this subsiding depression the
“Ashigara depressionヲ’.
According to the drilling data(Yamazaki
6砂1.,1982,Fig.17),the Holocene sediments
in this plain are composed of the alluvial fan
gravels distribute(i in the northern part and
along the river course,and marine fine sedi-
ments of the Kamonomiya terrace. TheKamonomiya terrace deposit is intercalated
with several key be(is such as peat layers,
volcanic ash,and GMF etc.,which are pos-
sible to yield the age of the layers。Figure18
shows the relationship between the ages of
these layers an(1their(iepth.
The younger Hakone pyroclastic flow(1eposits(TP flow:50,000years o1(1)an(i the
M2terrace deposits(60,000years old)under-
1ie the alluvium ofthe Ashigara Plain(Yama-
zakiαα1.,1982).These deposits are under一
一633一
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國V・ICaniCS 囲硲rtiary
懸 Dril畦ng site
ダBeachridge4Bars
騨Oawal隻::ミ
饗/ 0
Site
2km
Fig。16 Geomorphological map of the Ashigara plain,southwest Kanto.
1ain by the thick gravels inclu(iing cobbles
derived from the lava of Old Somma stage2
(OS2) (Kuno,1950,1951)of Hakone volcano
up to500m in depth。
As the beginning of the OS2stage proned
to be about O.3Ma by tephrochronology
(Machidaαα1,1974),the age of the sedi-
ments filling the(1epression is assumed to be
yomger than O.3Ma.Therefor,the calcu-
1ated long-term subsidence rate of the
Ashigara depression is more than1.7m/103
years,While,the mean subsidence rate ofthe
younger sediments has a smaller value than
that of the older ones.For instance,the M2
terrace deposit,which appears at-45.3m in
depth beneath the eastern Ashigara Plain,
shows39mofsubsidenceatmostifthealtitude of the original sea-1evel of 60,000
yearsagoisassumedtobelowerthan-6m
(Machida,1975).Accordingly,mean subsi-
dence rate of the M2terrace deposit is esti-
mated to be O.65m/103years.Further,the Ah
ash,deposited in the brackish water environ-
ment near the coast,apPears at-1.8to-2.3m
in depth(Matsushima,1982).This depthindicates4m of subsidence an(i O.6m/103
years of mean subsidence rate during last
6,300years ifthe origin&1sea-level is assumed
tobe2mabovepresent. The difference of the mean subsidence rate
between the older and younger markers indi-
cates that the subsidence movement in the
Ashigara depression gradually became weak
through the Late Pleistocene.The depth of
Ah ash below the present sea-level means
that the subsidence of the Ashigara depres-
sion has still continue(i in the Hlolocene,
though the rate has decreased. On the other
一634一
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12RADIOCARBON
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2
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// ◎ i
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>
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欝/ (MArSUSHIMA,
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1982)
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Fig.18
30
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300 婁
円
田
50
Comparison of the sample heights and their
ages of the H610cene sediments between the
both sides of the Kozu-Matsuda fault.The
dated samples of Odawara were obtained
from the Kamonomiya500m drilling.Ahshows the Akahoya ash layer.
Fig.17 Geologicallogsofthe500mdrillingcarried
out by GSJ at Kamonomiya in the Ashigara
plain.
GMF:Gotemba mudflow deposit of2,300yr
B.P.from Mt.Fuli.Ah:Akahoya glassy ash
layer of6,300yr B.P.from south Kyushu.
hand,the Ashigara Plain,together with the
OisoHills,wereuplifted1.8to1.2mbytheco-
seismic crustal movement associated with
the1923Great Kanto Earthquake(Omura,1925).
Furthermore,the Kamonomiya terracewhich is a part of the uplifted flood plain of
2,300yearsagohas3to4mrelativeheightfrom the present flood plain.These facts
suggest that the formation of the
Kamonomiya terrace was closely relatedwith the co-seismic cmstal movement of the
great earthquakes cause(i by the plate sub一
(1uction along the Sagami trough.After this
author calls this type of earthquake the
“Kanto-type earthquake”,The relationship
between the long term accumulative subsi-
dence of the Ashigara depression and the
recent uplift associated with the Kanto-type
earthquake will be discussed in Chapter8.
6。3 Koz腿一Ma蕊s腿dla量a朋亘t
In the westem Oiso Hills area where the
Sogayama Formation thickly accumulatedduring T-D tephra stage,the subsidence
weakene(l and partly changed into a slight
upheaval in the T-C tephra stage(ca.0,3to
O.25Ma).Then,the Sogayama block hasemerged to form the hilly Iand.This tectonic
change was presumably caused by the activa-
tion ofthe vertical slip onthe:Kozu-Matsuda
fault with east side upthrow.Consequently,
the origination of the Kozu-Matsuda fault is
一635一
嚢
Bz61」6!勿z(~ブ 孟h6 G60Jo9∫oαl S%7∂(3:y (ゾノ4冨)ごzフ¢, Vbl,43,〈石o,10
inferred to be at the T-D tephra stage(0。3to
O.4Ma).
Inthe laterhalf ofMiddle Pleistocene(025
to O.13Ma),the T-a marine terrace of O.15
Ma locally extend over the southem part of
the Kozu-Matsuda fault.This means that
despite the vigorous activity of the Kozu-
Matsuda,・the fault scarp did not have such a
large relative height at that time compared
with present.Accordingly,the scarp hもight
of the present Kozu-Matsuda fault is thought
to have mostly grown(1uring the late Pleis-
tocene to Holocene.
Withreferencetotheverticaldisplacementof the Kozu-Matsuda fault,no precise esti.
mation can be derived from the DRH formed
before60,000years ago because of the lack of
detaile(i stratigraphical information in the
Ashigara depression.But if the author broa(i-
1y estimates the Iong term slip rate of the
Kozu-Matsuda fault,it can be said to have a
value of more than2m/103years since O.3
Ma. That is,the sediment beneath the
Ashigara Plain at500m in depth is possibly
correlated with the T-c to T-a(0.3to O.15
Ma)terrace deposits which lie in the Oiso
Hi11sabove100minaltitude.Accordingly,the displacement of the T-c terrace deposit
bythe:Koz比Matsuda fault is estimatedtobe
more than600m and the mean slip-rate is
calculated to be more than2m/103years.
As mentione(1in the previous section,the
M2marine terrace deposits in the Ashigara
Plain and Oiso Hills apPears-45.3m and十90
m in altitude respectively.Meanslip-rate for
the Iast60,000years is calculated to be2.3m/
103years.With reference to the Holocene
sediment,Matsushima(1982)showed22m ofvertical fault offset during last 6,300 years
and calculated at3.5m/103years of slip-rate.
Thus the mean slip・rate on the Kozu-
Matsuda fault,which is obtained from the
various fault DRH,indicates that this fault
has recurred to the movement at the slip-rate
of2to3.5m/103years since the middleMiddle Pleistocene. But,as previouslymentioned,the appearance of fault scarp on
the Kozu-Matsuda fault fellbehindthe initia一
tion of fault movement for a few hmdred
thousand years.This delay suggests that the
mean-slip rate which represents the relative
movement of the two blocks divided by the
fault is not always adequate to reconstruct
the fault evolution and related geological
history in detai1.The author thinks the most
important factor to indicate the degree of
fault evolution and to affect the growth of the
fault scarp is not the mean slip-rate of the
fault but the absolute movement of each
faulted block。
Table2shows the absolute displacementan(1(iisplacement-rate of each faulte(1block
based on the present altitude and original
height of the M2terrace and Holocene sedi-
ments.The data on the table reveal that the
displacement of the Kozu-Matsuda fault dur-
ing mid(ile Pleistocene time was mostly
compensated by the subsi(1ence of the
Ashigara depression an(1the upheaval of the
Oiso Hills was relatively smal1.This means
that the faulting of the Kozu-Matsuda fault
hardly contributed to the growth of the fault
scarpinthemiddlePleistocene.Whileduringlate Pleistocene to Holocene,although the
faulting on the Kozu-Matsuda faultcontinued at a constant relative slip rate,the
absolute movement of each block varied
widely.That is,the subsidence rate of the
Ashigara depression(1ecreasedwithtime an(i,
on the contrary,the uplift rate of the Oiso
Hills increased to form the conspicuous fault
scarp・
In spite of the recent activation of the
movement that formed the fault scarp,nofault scarplet displaying the recent faulting is
found along the major scarp.The lack of a
fault scarplet can presumably be interpreted
as a result of flexure appearing as the surface
expression of the reverse faulting l this may
be (iue to the (iistribution of thick uncon-
solidated deposits along the Kozu-Matsudafault.
Along the northem trace of the Kozu-
Matsuda fault,the obvious fault scarp with
50mofrelativeheightappearsatthewestemmargin of the fluvial M3terrace of50,000
一636一
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
the east by a graben younger than the M3
terrace.These facts indicate that the west-
ward migration of faulting has occurred in
the northem part of the Kozu-Matsuda fault
during the later half of Late Pleistocene to
Holoce纂e。This feature(therecent migration
of the fault movement)is well similar to that
of the North Matsuda fault in the southem
margin of the Matsuda Mts.,10cated to the
northwest of the Kozu-Matsuda fault.This
tectonic similarity between two faults indi-
cates that the northern part of the Kozu-
Matsuda fault and the North Matsuda fault
have behaved as a single fault in the recent
geological time.
Nakamura6オ召1.(1984)supposed that the
WNW moving PHS caused the eductionalong the westem part of Sagami trough and
consequently the Kozu-Matsuda fault was
forme(1as a normal one.But,the develop-
ment of numerous thrusts with NW-SEstrike and the anticlinal structure with N-S
axis in the westem Oiso Hills area do not
supPort the above hypothesis and, to the
contrary,it indicates that the Kozu-Matsuda
fault is a thrust fault with some left lateral
slip component.
6.4 Tec窟o聾亘c evo且盟t量on
The crustal movement took place towar(i
the middle Middle Pleistocene in this area.
The lithofacies and sedimentary structure of
the Ninomiya formation which is the lowest
Middle Pleistocene underlying the terrace
deposits of the Oiso Hills(io not contain any
evidence of the vigorous tectonic movement
during the accumulation of this formation.
As the plate boundary connecting southem
Tanzawa region and the Japan Trench ought
to have existed in the early stage of the
middle Pleistocene,the calm crustal move-
ment at that age indicates that the style of
the tectonic movement along the plate bound-
ary was possibly not plate subduction but
transform faulting or eduction without verti-
cal displacement.
Although the reason is uncertain,the sub一
siding(1epression forme(10n the westem Oiso
Hills area about O.5Ma where the thick
Sogayama formation accumulate(1.One plau-
sible reason for this tectonic change is the
transition of the convergent style of plate
boundary from transform faulting to sub(iuc-
tion along the Sagami trough.
Circa O.4to O.3Ma,in the middle Middle
Pleistocene,the Kozu-Matsuda fault com-
menced its activity in this subsiding
depression or westem margin of it. Thewestem side of this fault subsided at a rate of
more than2m/103years to form the Ashiga-
ra depression.In spite of the vigorous activity
of the Kozu-Matsuda fault,the fault scarp
dividing the Oiso Hills an(1the Ashigara
Plain was not overt because of the upheaval
of the hills was still small at that time.
In the late Middle Pleistocene(0.2to O.15
Ma),the upheaval rate of the Oiso Hills
gradually increased and the Sogayama block
composed of the Sogayama Formation in the
westem Oiso Hills area emerged. On the
other hand,the subsi(1ence of the Ashigara
(iepression slowly weakened.Since after the
last interglacial period(0.12Ma),the uplift
of the Oiso Hills accelerated according as
subsi(1ence in the Ashigara Plain has de-
creased.As a result of this tectonic change,
the scarp of the Kozu-Matsuda fault has
rapi(ily developed,though the relative slip
rate of the fault remained unchanged.
7.SUMM:ARY OF THE QUATERNARY SYSTEM ANI)THE ACT亙VE STRUC. TURES AL(》NG THE亙ZU BORDER- LANゆ
7.l Q聡舗e職蹴ysyst艦s The Quatemary chrono-stratigraphy and
the evolution of fault systems shown in Fig.
19reveal that the Quatemary system of the
Izu borderland is classified into two cate-
gories:the basin-fill type and the non-basin-
fill type.The former was deposited in sub-
siding tectonic basins and accumulated to
more than several hundred meters in thick-
ness.The latter was deposited outside the
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量一 11 12
o =㊤ ㌧
仁¢ ’
==し
o>
『 一
∈ 16(o
o 、『 ⊆
奪1:o 自
1 -
Z 18ヱ ,・
① 、;
邸 ・
Σ、、
20
?マむ ド ロ じ
翻麟
14
¶3
15
17
1す一
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
deposit is shown in Fig.20
The regional evolution of crustal move-
ment brought about two groups of sedimen-
tary basins. One is un(ler continuous subsi-
dence up to present inclu(iing the fill deposits
in Fuji-Fujinomiya lowlands and Ashigaraplain.Thus the deposits in the basins of this
group,namely l a type,are foun(i along the
continuation of trough axes on land. The
crustal movement in the other group(1b
type basin)was converted from subsidence
to uplift,and consequently the se(iiments in
these basins are now expose(1and eroded.
The Kambara conglomerate,.Iwabuchi an-
desite and Bessho gravels in the]Kambara,
Habuna,and Hoshiyama Hills,the Ashigara
Group in the southem margin of Tanzawa
Mts.and the Sogayama Formation in theOiso Hills are deposits of the l b type basins.
The deposits of the l a and l b type,except
for volcanic rocks,are mostly composed of
monotonous conglomerate intercalating with
thin sand or mud beds. The sedimentary
environment of such sediments is considered
to be a rapidly subsiding area around amouth
of river which has transported a large
amount of coarse materials.
On the other hand,the non-basin-fill type
deposits are subdivided into two groupsaccording to their thickness an(i(iuration of
deposition.The thicker deposits of the2a
type,200to300minthickness,havebeenaccumulated continuously for a long time at
a low sedimentary rate,without affecting the
intense subsidence.The thinner deposits(2b
type),1essthan100mthickareterracedeposits,that covered the l b type deposits
unconformably during the uplifting period.
The Ninomiya Formation,the Akisawa-Tsu-
chiya Formations in the Oiso Hills belong to
the2a type,an(i the Saginota gravels in the
一638一
T6・渉・耽sσ」・%9孟h齪・πh翻耀顧πげ伽P6痂s吻侃.}物耀2剃
㌧.
〆
\1)’》
い\
いl!l
ノ\11汰\\
/
ノ
フ)驕.“・㌃』聾r1欝e
2
1 戒聾, 511ノ
.甲1蹴既.甲
.細二.。1隔.ρ二・合
圏猛騒團
Present subsiding
depression (1a)
Depression convertedto the upheavaI(1b)
Non basin fi[I type
deposits(2a,2b)
Quatemary volcanic rocks
Active fauIt
ノ/ノノ・/
>5m/103y.
〉1m/103y,
く1m/103y
non aCtive fault
漫』\\i・0 10km
、 ’
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
in the Hoshiyama and Habma Hills,Suruga
gravels in the southem margin of Tanzawa
Mts.and from the T-e to H formations in
the Oiso Hills belong to the2b type.
7.2 Ch鍵acter亘st童cs o£Aetive Fa腿且ts
The main active faults in the studied area
are compiled in Table5.Figure20shows the
distribution of major active faults in the
South Fossa Magna around Izu Peninsula.The thickness of each fault line in the figure
indicates the long term slip-rate of the fault.
The vertical components of slip rate of the
faults in Table5are so large as to be classi-
fied as the most active faults in Japan.Their
net slip will be Iarger if strike-slip compo-
nent is taken into account.Notwithstan(iing
their high activity,the length of each fault is
extraordinarily short in comparison with
other major active faults in Japan,such as
the Atera Fault(70km)and the MedianTectonic Line(300km).
Active faults except for the fault system in
Izu Peninsula are intensely developed along
the northern border of Izu block(Figs.2and
23). They comect Suruga and Sagamitroughs surrounding Izu Block from the
north.This fault zone consists of many short
faults.The geometric and kinematic pattem
of these faults,such as the strike of the faults
an(i the sense of fault movement,differs from
place to place systematically.
In the western part,to the north of Suruga
Bay,a number of N-S trending faults have
(1eveloped inthe Neogene(1eposits since early
Pliocene.Among these faults,the active ones
are found in the eastem side. This active
fault system is expresse(i as zigzag lines
running from south to north,where N-S
一639一
B%Jl6あ刀 ρプ 孟h6 σ60109乞oαl S%7び8ッ (ゾノ41)α刀, Vbl.43,ノ〉;o.10
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-Swa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-Smafaultsys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aultF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-Wsystem 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&Machidafault (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
en echelon configuration are connected by
NW-SEtrendingnormalfaults.The activityof individual faulting increases from west to
east.
In the central part,to the south of Tan-
zawa Mts。,a major reverse fault trending E-
W(:Kamawathmst)anditsconjugatestrike-slip faults trending NE-SW and NW-
SE compose an active fault system(Kan-
nawa fault system).The strike-slip faults
stretch into the Tanzawa and Ashigara Mts.
across the Kannawa thrust.Several active
faults run parallel to the Kannawa thmst in
its southern si(ie along the border zone
between Ashigara Plain.The southem most
one,extending to theKozu-Matsuda fault is
the most active among them.
The Kozu-Matsuda fault has approximate-
1y straight NW-SE trending fault trace with
a slight undulation.Reversal dip-slip disloca-
tion seems to be predominant on this fault.
Many minor faults rm in the Oiso Hills
parallel or diagonally to the Kozu-Matsuda
fault,but most of them are not evident in the
fault topography. Therefore,these minor
faults can be regar(ie(i as insignificant
subsidiary faults,such as branching or super-
ficial ones,induced by the movements of the
Kozu-Matsuda fault.
In all these three regions,the active fault
systems are located exactly on the bomdary
between subsi(1ing areas on Izu side and
uplifting areas fringed with older basin-fill
type(1eposits.
一640一
T80渉07zづos ごzJo%g 渉h(3η07孟h8ηzη¢ごz堰勿z(ゾ12z6P6%勿zsz〆α 〔E.yヒz%zごz2ご漉の
7.3 丁蝕e evo互眠tio醜o{cr腿st&且moveme喪ts
estimated£rom t塵e c血&醜ges o£fa躍lt
&ctiv童ty
The change of fault activity in time was
reconstructe(1 0n the basis of the relative
dislocation of two blocks bounded by each
fault.Figure21shows the track of vertical
movement rate in each tectonic block.The
Quatemary se(iiments can evidence the crus-
tal movement,subsidipg,stable,or uplifting,
0f the covered areas(1uring their depositional
ages.The tectonic evolution ofboth sedimen-
tary basins and fault systems in time an(1
space was revealed as in Fig.21.
The relation between sedimentation and
crustal movement is summarized as follows・ フ
a)Basin-fill sediments(type la and lb)
in(iicate the predo卑inancy of subsi(1ence over
the basin,or the area of deposition.b)Deve1-
opment of terrace(1eposits(type2b)or’ove rall erosion evi(1ences the predominancy
of uplifting in the area.c)Accumulation of
non・basin-fill type deposits (type 2a) indi-
cates the transitional period between subsi-
dence and uplifting. In all three cases above,
the duration of each crustal movement is
indicated by the(1epositional age of the sedi-
ments.The affects of the eustatic sea level
changes are not taken into account・
The screened spaces between two lines in
Fig.21indicate the rate of relative displace-
ment,of adjoining blocks.For instance,the
change of the space between the Kambara
Hills and the mountainous area to the west
represents the decline of the activity of the
Iriyama-Shibakawa faults l the westem
0 50 70 x104yBP
1. 1Hamaishidake TbnshuMts,
十
,1~2πゾ103チ
1lRl㌔AMA,SHIBAKA陥
弘U口S2KanbaraHiUs
一 2Iwabuchi肱Kanbaragr
皿SaginotagraveI _甲』一一pp IP彫謙一燃』{『『
HabunaHoshiyama
十 i
【ニコOldFujimudflow
3
1. 3 Hi11s lRl漁MASE,AG(フYAMA
皿 Saginotagr.
_爾塵-.一蹴灘燃一羅聯 Besshogr lwabuchi鴨lca. Kanbaragr
酬U口S4Fuli Fulinomiya
一・i瀧灘§ll∫
OldFujimudrbw
2.
5拍nzawa M姶.十
5
KANN触A FAU口國 6
6AshigaraMts一
⊂ゴSurugagraveiAshigaraGoup
7Mt、Matsuda area
9
7
皿囲礪罵撒3.
SANROKU訊UlT
寧.9あ十
Ashigara
Gゆup
MA「SUDAKl「際
8MatsudaHi”s一十li
Matsudagr跳8灘 8
黙㌔Ashigaraplainfillsediments Sediment
弘ULr 9AshigaraPlain一 9『 、煮弊 plainfi”sediments 囮
羅〔
¶ransiti㎝period
Subsidenceperiod
Others
KOZU MA『SUDA 一 9 10
弘UL『100isoHiIls
十1 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 Quatemary.So the track remains always
the plus side.On the other hand,the track of
the Kambara Hills stays on the negative side
in the early Pleistocene ju(1ging from the
deposition of basin・fill type Kambara
conglomerate,which evidences subsidence of
the region.Then the movement of the Kam-
bara Hills was converted to uplift in the
middle Middle Pleistocene.The alluvial fan
deposits of the Saginota gravels in the Kam-
bara H皿s marks the transitional epoch.The
Saginotagravelsisasthickas200mtosuggest gentle subsiding during the de-
positional age.However,it was emerged just
after the deposition and have never been
covered with yomger se(iiments mtil pres-
ent.The Holocene marine terraces in thehills also indicate that active uplifting has
been continuing from the mi(1dle Pleistocene.
The track of the:Kambara Hills,therefore,
shifts from minus to plus in the middle Pleis-
tocene as shown in Fig.211t is thus concluded
that the activity of the Iriyama-Shibakawa
fault had culminated in the Early Pleistocene
and have declined since the Middle Pleis-
tocene.The Kamawa thrust and the Matsu-
da-sanroku fault have also declined in the
Pleistocene.On the contrary,the Iriyamase-
Agoyama faults and North Matsuda fault
have become more active.Though any dras-
tic change has not occurre(i to the relative
movement alongthe Kozu-Matsuda fault,the
absolute movements of the Ashigara Plain
and the Oiso Hills have changed in the late
Pleistocene. In the middle Pleistocene,the
Ashigara Plain ha(1subsided rapidly beside
the Oiso district which had been stable rela-
tive to sea leve1,but in the late Pleistocene,
the Oiso Hills starte(1to uplift accor(iing as
the decline of subsidence in the Ashigara
Plain.
The growth of Kozu-Matsuda fault scarp
also in(1icates this change.The fault scarp
ha(i not appeared in the mi(idle Pleistocene
due to the one-sided subsidence of the
Ashigara Plain,as in画e case shown in Fig.3.
Then it emerge(1in the late Pleistocene with
the uplift of the Oiso Hills.This change can
explain the discrepancy between the constant
activity of the fault and the topographic
evi(1ence.
The above-mentioned examples show the
migration of the front of active faulting
which is commonly observed in Izu Border-
1and.Ayoungerfaultdevelopsalongthelzuside of an older fault which becomes inactive
or less active after the emergence of younger
one.Such migration of active faults induces
the shift of tectonic basins,as the active
faults are usually located between the uplift-
ed basin-fill deposits on the Honshu si(1e and
the active subsiding basin on the Izu si(ie.
8.:DISCUSS亙ON
8.1 P丑就e tecto脈ic豊mBl亘catio簸o£a,ct量ve
歪a,魍亘ts量n the IZUl Bord!er且a,鷺dヒ
(1)A model of tectonic evolution
Case stu(iies of active faults and crustal
movements in the Izu border land led to a
common feature of tectonic evolution in this
region.It is the Izu-oriented migration of
active faults and sedimentary basins. The
typical field of this migration is the area to
the north of Suruga Bay. In this area,the
following processes of tectonic evolution
were recognize(1.1)Thick sediments of the
Kambara-Bessho gravels were deposited in
basin developed on the(iownthrown Izu si(ie
ofafaultsystem.2)Anewfaultsystem(lriyamase fault)was forme(i inside or along
the Izu.ward margin of the basin.3)A new
subsiding area(Fuji lowland)has developed
in front of the Iriyamase fault replacing the
older basin of the Kambara Hills,where
fluvial Saginota gravels were deposited un-
conformably to the former basin・fill sedi・
ments.Yamazaki(1984)proposed a mo(1elof the tectonic evolution in Izu Bor(ier lan(i
accor(1ing to the three processes above.
Here,the author proposes a new model con-
sisting of four stages in a cycle and two
substages,improving the previous model in
order to make it applicable to the entire Izu
Bor(1erlan(i(Fig.22).
一642一
7セ6オo%Jos σlo%9 孟hε %07渉h6ηz 解ごz碧づn (ゾ ノ2z6・P6%づ%sz〆ごz O7. yとz彫ごzβごzを∂
G
1 2 3 4
//
A B
薪,欝一
形「『ノ↓
鞭
A A B
づ夕
奮一獅且
A B
嘗. s、.
欝 ◎ ◎ い 幽 幽
sぬ且搾騨s蝋奮 A A
藷
嚢
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:
Sねge1:The upthrown side of a thrust
faultAinFig.22upheavesconstantlyandthedownthrown side subsides,where the terms
upthrow and downthrow denote relativemotions along the faults and‘‘upheava1”and
“subsidence”are measured relatively to the
sea-1eve1. The upheaval and subsidence
share the net displacement of the fault A.
The active faults systems from Surugatrough to the southwestem foot of Mt.Fuji
are now on this stage l l the Iriyamase fault
an(1Agoyama fault correspond to the fault A
in Fig.22.The subsiding portion of the axis
of Suruga trough,Ukishimagahara,and Fuji
Fujinomiya lowlands are the tectonic depres-
sions of the downthrown side of the active
faults.The thick basin-fill sediments such as
the Kambara conglomerate,Bessho gravels,
Ashigara Groups,Sogayama gravels,and so
on,which now compose hills or mountains,
were accumulated in the previously subsiding
areas at stage1.
Sねge2:The subsi(1ing area in front of the
fault A is divided into two blocks by a new
fault occurred in the basin.The upthrown
sideofthefaultAcontinuestorise.Bothblocksboun(1edbythefaultBcontinuetosubside. The subsiding rate of the strip
between faults A and B decreases,while the
dov財nthrown side of the fault B subsides very
rapidly.As to the faults scarps,this differen-
tial movements of three blocks bring about a
sharp contrast between fault A and B.The
scarpofthefaultAcontinuestogrowasaboundary between the uplifting and the sub-
si(1ing area. On the other hand,no scarp
emerges along the fault B,as both sides are
subsi(1ing to bury the fault topography.
The present Kozu-Matsuda faultcorrespondstothefaultAinthestage2,judging from the rapid growth of the fault
scarp in the late Pleistocene and from the
decreasing subsiding rate in the Ashigara
Plain.A concealed fault,namely B,is expect-
ed in the Ashigara Plain or on the eastem
foot of Hakone volcano to the west of the
:Kozu-Matsuda fault.This fault,which has
一643一
翫 麟欄灘騰齢酬購灘騨糊灘欄燃葦撚糠膿欝鞭
B%lJ(財勿¢ (~プ !h8 0601091αzl S%7∂(3:y (ゾ’ノ4多)ごz銘, Vlol。43,〈b。10
not yet been recognized,grows possibly far-
ther west of the River Sakawa.Ota6渉召1.
(1982) supPosed an active fault along the
westem flank of Chiyo Upland (Fig.16),
which is composed of pyroclastic flow
deposits,in the eastem part of Ashigara
Plain.This fault might be regarded as fault
B.However,the altitude of K-Ah ash layer
laid down in blackish water(10es not change
across the cliff assumed to be a fault scarp.
So it seems that this linear feature is not a
fault scarp but the erosional topography of a
fluvial terrace cliff,and no concealed fault
can be expected between the River Sakawa
and the Kozu-Matsuda fault.
Stage3:The block between faults A and
Bisbeinguplifted(inanabsolutesense)bythe activity of fault B.This makes the fault
Aafaultbetweentwoupliftingblockanditsslip rate decreases. The uplifting block
between faults A and B is coveredmconformably with thin deposits from the
upthrown side of the fault A,due to the slow
rate of rising.
The Iriyamase fault near the mouth of
River Fuji is regarded as the fault B in the
stage3.Alluvial fan type deposits,such as
the Saginota gravels in the:Kambara Hills,
the Older Fuji Mudflow deposits in the Ho-
shiyama and Habuna Hills,.fluvial terrace
deposits of T-e to T-a Stages in the Oiso
Hills,were deposite(1in the gently subsiding
or uplifting areas between faults A and B in
this stage3.
Stage4:The uplifting rate of the block
between the faults A an(1B is accelerate(1and
consequently the sedimentary surfaces on
this block・are emerged to be dissected.The
activity of fault B becomes more intense with
the growth of a fault scarp,(iifferentiating
the former subsi(ling area intQ terraces an(1
hills in upthrown side.Meanwhile,fault A
becomes less active,.as the (iifference of
uplifting rate decreases between thedownthrown and the upthrown side. The
present cmstal movement on the westemfoot of Mt,Fuji exactly corresponds to this
stage.The Iriyama-Shibakawa faults and
the Iriyamase-Agoyama fault system in the
eastfallunderthefaultAandthefaultBinthe model respectively.While the three topo-
graphic units bounded by these two fault
systems correspond to thethree zones divided
by faults A and B。、They are the Hamaishida-
ke Mts.,the Kambara-Hoshiyama-HabmaHills,and the lowlands of Ukishimagahara
and of Fuji-Fulinomiya.
The fault B in the stage4is the equivalent
of fault A in the stage1,with the same
crustal movement onboth sidesl so,a cycle of
tectonic、evolution is complete(i in these four
stages. The next cycle is marked by the
occurrence of new fault A derived from the
fault B in the previous cycle.That is to say,
the most active fault zone migrates from
Honshu side to Izu side accompanying a
sedimentary basin in downthrown side and a
uplifted area in upthrown side.
Some cases of deviation from this cycle led
to the information of the following sub-
stages:substage l follows the stage l with-
out the formation of a fault B or a new
subsiding area in the(10wnthrown side of the
faultAlitshangingwallcontinuestouplift.The recent cmstal movement alongthe:Kozu-
Matsuda fault may correspond to this sub-
stage,supPosing the concealed faults to the
west of Sakawa River do not exist.In this
region,the fault scarp of the Kozu-Matsuda
fault grows continuously without any older
fault A behind it.Meanwhile,the rate of
subsidence in the Ashigara Plain decreases.
Such evidence may be interpreted as devia-
tion from stage l to substage1.
Substage2is characterize(1by the extinc-
tionofthefaultAwithoutmigration.Ascompressive stress increases,the down-thrown side of the fault A(thrust fault)is
defomedseverely,andfinallyrisesfromthesubsi(1ing basin.This deformation results in
weakening the activity of the fault A.This is
a consequence of the augmentation in the
rising of the downthrownblock,which makes
the rate of displacement along the fault sma1-
1er and the dip of the fault plane steeper.
Excess shortening of a region induced by
一644一
Tθ0孟0競Sα10箆9孟hε盟07オん㈱彫α響伽げ伽P翻盟S%Zα侭.y4耀禰の
very large horizontal compression explains
the phenomena described above. Recent
deformation in the Tanzawa Mts.and the
Ashigara Mts.fall under this substage2,
where the Kannawa thrust corresponds to the
fault A.At last the fault A becomes extinct
and both adjoining blocks uplift miformly.
(2)P亘説etecto髄量c童卸亘i蝋量o聾of我ct亘ve 亙蹴且ts
In the Izu Borderland,fault systems and
tectonic basin have migrated continually
towar(11zu Peninsular during mid(ile to late
Quatemary.This migration resembles to the
evolution observed at thrust fronts in mar-
gins of inlan(1 basins (lkeda, 1983,0ta &
Sangawa,1984).However,the migration in
the Izu Borderland is quite different from
that in inland basins because of the lack of
the MBF(main bomdary fault),which at
depth is singularly expressed and its low-
angled branch appears as a migrated frontal
thrust in inlan(1basins.Inthe Izu Borderland,
the direction of the migration always ori-
ente(1to the Izu Peninsula.Rapidly subsiding
zones which extend to the axes of the troughs
develop in front of the most active faults.
The basin fill sediments in these zones will be
differentiate(1by new faults,certain parts of
which will emerge as upthrown blocks on the
Honshu side being folded or tilted toward
Honshu。
The Izu-ward migration of tectonic depres-
sions an(1the(ieformation of uplifte(101(1er
basin fill sediments are the characteristics of
the tectonic evolution in the Izu Borderlan(1.
This evolution is the same as the growth of
accretionary prism which takes place along a
trench axis at the foot of a continental slope,
where trench fill sediments are continuously
accrete(10n continental slope and deformed
by imbricated thrusts。The thrust develops
successively into yomger accreted mass.In
the Izu Bor(ierland,this accretion process
occur not in deep sea bottom but in shallow
sea or on land,while the process of sedimen-
tation and tectonics are quite similar to nor-
mal accretion.This exceptional occurrenceof accretion on land or shallow sea is possibly
ascribed to the form of plate convergence in
this area.Plate dynamics will be discussed in
the section8.2.(2).
The trough fill se(iiments on the northem
extension of Sumga and Sagami troughs,therefore,are considered to have been accret-
ed to Honshu and(1eformed along with the
subduction of the PHS during at Ieast these
hundreds of thousands years.And the defor-
mation of the ac6retionary prism along Nan-
kai trough continues through Suruga trough
to the Izu Bor(1erlan(i on land.Consequently,
the crustal deformation in this region can not
be regarded as interplate deformation,that is
to say,none of the active faults is a superfi-
cial expression of a plate boun(iary,but it is
(1eformation of accretiorary prism,where
imbricated thrusts system develop in the
hanging wall over the subducting plate
(Kagamiαα1.,1983)。
Seely6砲1.(1974)propose(10n the mecha-
nism of migration that the underthrusting of
wedge-shaped slices composed of youngertrench fill sediment ma(1e the dip ofthe thrust
steeper finally to immobilize it,and then new
thrust develope(i in front of the older ones.
Besi(ies this mechanism,the author supposes
that in case of buoyant subduction,strong
compressional stress in accretionary prism
tends to immobilize older thrusts.The stress
can only be released by generating new
thrusts in front. Tsuchi(1984)concluded
that Neogene series had successively accret-
e(1to the west at the Suruga trough in the
South Fossa Magna region west of SurugaBay,because the tectonic units divided by N-
S trendihg faults got younger from west to
east.Accordingly,the author thought that
the formation of accretionary prisms has
continued since the Miocene until present
repeating the migration and regeneration of
fault systems and sedimentary basins.
8.2Reg童o賊a亙c血&餓cterist置cso釜c臓sta丑
moveme虚s躍繊臨e量ror旦9量醜
(1)Reg置0総亘置蝕e配e o聲臨e e職sta亘 move贈e盟t
The active tectonics in the Izu Borderlan(1
一645一
鑛
選羅
慧
き
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-
mentary prism in an accretion process,as it
was conclu(ied in the previous section.
Nevertheless,the crustal movements here are
not uniform in time and space fortheir extent
or duration and they sometime deviate from
the mode1.As to the extent and duration of
accretion process,the one on land to the
north of Sumga Bay has more closely-spaced
imbricated thrusts(as much as a few kilo-
meters)and much shorter time perio(10f
reactivation(about several hundred thousan(i
years)than that of Nankai trough and south-
em Suruga trough,where it is5to10km(Kato(3渉α1.,1983)an(i several million years,
respectively(Shimamura,1986).
Typical deviation from the circular mode1
(Fig.22)was observed in the area of the
southem margin of Tanzawa Mts.,where the
tectonic evolution has developed from the
stage l to the substages l and2.Notwith-
standing the conversion of the cmstal move-
ment from the subsidence to upheaval in the
sedimentary depression of Ashigara Group,
which had occurred during middle Quatemar-
y time,neither new fault system nor basin
have developed in front of the depression.
C6nsequently,the uplift of the Ashigara
Group has continue(1to convert it to a dissect-
ed mountainous area (A§higara Mts。)。
Accordingly,the fault between the two uplift-
ing blocks,here the Tanzawa Mts.andAshigara Mts.,became less active andfinally
ceased to move.This transition of the fault
movement presumably was induced by the
decrease of relative slip (iue to the rapi(i
uplift of the Ashigara Mts.The dip of the
reverse faults has also become steeper
because of the shortening of the block with
increasing compressive stress.
The crustal movement between Yamakita
and Matsuda along the southem foot of the
Matsuda Mts.agreeswiththecircularmode1.But the width of each block between two
faults is very short,up to l km and the
(1uration of one cycle is also very short up to
one hundred thousan(1years.So,the horizon-
tal spacing and the time interval for the fault
migration in this region are the smallest in
the studied area.
The tectonic evolution inthe Oiso Hills an(l
the Ashigara Plain is characterized by the
lack of an o1(1er cycle of accretionary prism.
No imbricated structure is found behin(i the
present active zone。 The author considers
that an accretion process begun in the middle
Quatemary in this region an(i a(1ifferent
tectonic regime had been predominant before
the initiation of the accretion.
(2)黙eorig量聡o£臨ereg童o聾a亙d榔ere配e The regional characteristics of the crustal
movements in the Izu Border land can be
ascribed to the difference in the form of plate
convergence. In this sense,the Izu Bor(1er-
1and is no longer an uniform collision zone as
has been supposed by Sugimura(1972)or
Matsuda(1978)and others. This widelyaccepted collision has never been studie(i in
detailto substantiate.Forexample,Matsuda(1984)define(i the beginning of collision mere-
1y by the (1isapPearance of the open sea
between Izu an(1Honshu due to the north-
westward shift of Izu Block.Only Ishibashi
(1896a)discussed the evolution of the form of
convergence in time. He conclu(1e(i that
subduction,buoyant subduction and collision
have been repeate(1in this order since the
Middle Miocene in the South Fossa Magna.
Now,the author proposes to apply Ishiba-
shi’s three variation of the convergence form
not only to the historic development but also
to the regional difference of crustal move-
ments.On the solid basis of active tectonics,
the so-called collision zone of the Izu Border-
1and can be sub(iivide(i into collision zone
sensu stricto,sub(iuction zone an(1buoyanガ
sub(iuction zone.
In the area to the north of Suruga Bay,the
crustal movement has been subjected to the
sub(1uction or buoyant subduction of the
PHS.The migration of fault system and the
formation of sedimentary basin are common
tectonic results of both subduction styles.
With reference to the these two subduction
style,Ishibashi(1986a)took the cumulative
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
area to the north of Suruga Bay is supposed
to be buoyant because the distinctive and
extensive uplift(4mm/yr)is recognized in
the Akaishi Mts.,the hinterland of this area
(Danbara,1971). This feature is distin-
guished from normal subduction such as
along Nankai trough,where coseismic uplift
has been repeated without conspicuous cumu-
1ative upheaval of the accretionary prism an(i
frontal arc.Along the southem margin of
Tanzawa Mts.,both sides of the Kamawa
fault system have been uplifted without
generating younger basin,and the fault activ-
ity itse1:f is not so high. Intense horizontal
compressive stress raises all this area,and
the former main fault is no longer a boundary
fault between blocks,being truncated by a
number of strike-slip faults.This may indi-
cate that brittle disruption of the blocks is
occurring,or about to occur in this region。
This style of cmsta1(1eformation exactlycorrespon(i to the collision s6nsz6 s渉万o孟o
(1efined by Ishibashi(1986a).Therefore,Izu
Block colli(1es against Honshu only along the
southem margin of the Tanzawa Mts.in the
Izu Bor(ierland.
The active accretion process around the
Matsuda Mts.,the Oiso Hills and the Ashiga-
ra Plain indicate the underthrusting of the
PHS beneath these areas.The rapid uplift of
the Tanzawa Mts.area behind the Matsuda
Mts.canbe ascribedto buoyant subductionin
this part.However,the spatial intervals of
active faults here are much shorter than
those to the north of Suruga Bay an(i also the
migration of active fault front has occurre(i
more frequently.Therefore,the transition
from buoyant sub(1uction to collision may be
under way in the Matsuda Mts.area.These
regions lack older accreted block in Honshu
side behind recent active zone,and now we
can observe the first an(1the latest cycle of
accretionary evolution. This cycle com-
menced in the middle Pleistocenebecauseofa
new plate convergence system that had oc-
curred at that time.But this convergence as
well as the possible transition inthe Matsuda
Mts.areahasnotbeenevidencedgeologicallyto discuss its cause.These regional charac-
teristics above are summarized in Table6.
The arrangement of plate convergence
forms seems to be systematic in the Izu
Borderland. That is,the collision s6%s%
s翻o孟o occurs between the Tanzawa Mts.and
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,marginofofF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㎞+ 0thmsts
Age ofthe Mi㏄ene to Lower to Middle MiddletoUpPeryoungest LowerPli㏄ene4) PlOistocene pleistocene 一
a㏄re豆onawprism
T㎞e illterval
ofmゆrfault afewMa less than l Ma less than O.1Ma 0mig【ation
0血er fbatures Upheavalofplateboundalylegion
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
and in the east of the collision zone,north of
Sumga Bay and the Matsuda Mts.arearespectively,an(i then normal subductionoccurs in both extensions of the Izu Bor(1er-
1an(1.This symmetrical arrangement of three
convergence forms must be originated from
the steepening of the subducting slab.Figure
23is an isobath map of the upper surface of
the subducting slab of PHS,with submarine
active faults(R.G.A.F.J.,1980)overlaid to
show the intervals of imbricated thrusts.The
(iip of the subducting slab becomes steeper
from Nankai trough to Suruga trough(Kato
6渉α1.,1983).The absence of isobath further
north of central Suruga trough is because the
dip is too steep to recognize the slab surface
by seismic reflection profiles.The relation-
ship between the forms of convergence and
the dip of slab is supposed as follows:as the
(iip becomes steeper,the form varies from
normal subduction to buoyant subduction,an(i finally to collision an(i in the meantime
the intervals of imbricated thrusts get nar-
rower.The steepening of the slab dip toward
the collision bomdary should mainly be
originated from the buoyancy of Izu Block.
Besi(1e,the steeper subduction results in tight-
er coupling of the blocks on both sides,where
the tectonic stress in(1uce(1 by the plate
_ぢD一錠09 ε
』」
趨
2
4勇
ぢ多、健▽ !一/一/一 し6・8ノ / - / / !、 / / ( 、57 / ご_ノ
/ 6/
30
20
ゆ
ρ 勿 .祠《亀、
いM、 輩 .齢
{
名レ乏
50km
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
bring about intense crustal movement in the
hinterlands.
8.3 Seismotecto恥ics o歪t翫e Iz盟Bo「〔翌e「且a醜dl
Large vertical slip rate disproportionate to
the length distinguishes the active faults in
the Izu Borderland from the other majorinlan(i active faults in Japan.Frequent occur-
rence of large earthquakes is usually-expect-
ed from faults in order to maintain this high
activity. Nevertheless,no earthquake has
been recor(1e(1in this region except for the
Ansei-Tokai earthquake of1854,when the
Iriyamase fault moved around the mouth of
the River Fuji.
The mode of seismic activity,i.e.recur-
rence interval of earthquakes from a fault
and their magnitude or amount of slip,has
been estimated only for the:Kozu-Matsuda
fault in the Izu Borderland.Matsudaα召1.
(1978)identified the:Kozu-Matsuda fault to
the source fault of the Oiso-type earthquake.
Furthermore Matsuda(1985)estimated itsrecurrence interval at l70years and the verti-
cal slip per event at O.5to1.Om,based on the
assumption that the magnitude is M=7con-
si(iering that the fault length(10to20km)is
not large.
On the other hand,Yamazaki(1985)proposed a quite different estimation that the
interval is as long as 2,000 years and the
vertical slip more than7m.These values
were drawn on the basis of geological an(1
geomorphological evidences.They are:1)
The Ashigara Plain has been subsiding con-stantly at a・rate of O.65m/103years since
middle Late Pleistocene.2)Both the Oiso
Hills and the Ashigara Pain were uplifted by
the great Kanto earthquake of1923.3)Three
levels of the Holocene marine terrace sur-
faces are recognized along the Oiso Hflls.
Accordingly,two modes of coseismic uplift,
the first being the Oiso-type earthquake with
long recurrence interval and the secon(i the
Kanto-type earthquake with shorter interval
(about800years),were considered to overlap
in the Ashigara Plain and the Oiso Hills.The
model of the cmstal movement in Fig.24
explains well this overlapping an(1the three
pieces of evidence above.The step-like lines
in Fig.24show the vertical movements of the
Oiso Hills(upper,uplifting)and the Ashigara
Plain(10wer,subsiding).This demonstrates
that the latest Oiso-type earthquake is dated
ca.2,300yr B.P.and there after this region
has been uplifted only by Kanto-type earth-
quakes.
The other active faults seems to have simi-
1ar characteristics of seismicity to the Kozu-
Matsuda fault ludging from their large slip
rate an(i absence o:f earthquake record in the
last1,000years. These active faults are
imbricatedthrusts in accretionaryprisms and
can be regar(ied as subsidiary faults branch-
ing from the main fault.The main fault itself
is the plate boundary between the PHS and
EUR and only this fault acts as earthquake
source.Therefore,the recurrence intervals
of earthquakes cannot be proportional to the
activity and the length of each surface active
faults.The seismicity along the plate bomd-
ary in t五e Izu Bor(ierland is not so active as
39
恥申島=》
08器罹蓋
3
/!
/①
糀、二擁//
//
//
4/
/
回α6錦3y
議
Fig.24
A△A蝿Ev.。農② AS糊GARA PLAlN
EARTHQUAKE
△OISOfypeぬKANTO骨ype
『『T 6Ky
Diagram showing the relationship between
the cmstal movements on the Oiso Hills and
Ashigara plain and the occurrence of the
Oiso-type earthquakes.
L Uplift rate of the Oiso Hills. 2:
Subsidence rate of the Ashigara Plain.Filled
triangles: Occurrence of the Kanto・type
eαrthquakes. Open triangles:Occurrence of
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 southernSuruga trough.The difference of the form of
plate convergence again well explains the
longer recurrence interval in the Izu region.
That is to say,the buoyant sub(1uction and
the collision in the Izu Borderland consume
much less energy in faulting along the plate
bomdary than a normal subduction.So,slow
rates o:f stress accumulation along the plate
boundary faults causes the recurrence inter-
val to be longer than that of the boundary
along Nankai trough.
Ishibashi(1988a,b)proposed the N-S tren-
ding fracture zone,West Sagami Bay frac-
ture zone,splitting the PHS onthe east of Izu
PeninsulaforthesourceofM=7sizeearth-quakes which have caused serious damage on
Odawara city with about73years interval
during historical times. This proposal is
mainly base(i on the analysis o:f the crustal
movement at great Kanto earthquake of1923,0f tsmami sources and of damage dis.
tribution of several historical earthquakes.
In spite of the obvious structure o:f the
proposed model of the fracture zone,no evi-
dence supPorting the proposal is found except
the uplift of the Hatsushima Island off Atami
coast and submarine scarps in WestemSagami Bay.Although the fracture zone is
inferred beneath the westem margin of the
Ashigara Plain,any supporting evidence can
not recognized in this area at present time.
Accordingly,it is difficult to comment on this
fracture zone from the viewpoint of theQuatemary tectonibs on land..
9.CONCLUSIONS
The Izu Bor(1erland is a convergent plate
boundary on land that combines Suruga
trough with Sagami trough. The tectonic
evolution and recent cmstal movement along
this boundary are discusse(i on the basis of
geological and geomorphological research
from the viewpoint of plate tectonics.
Through this research,the author revealed
the significance of the regional Quatemary
faulting in the setting of plate tectonics in the
Izu Bor(ierland an(1 interprete(1 that the
active faults in the region are characteristi-
cally the imbricated thrusts with shortlength and high slip-rafe occurred in the
accretionary prism.This study also revealed
that the Izu Bor(1erland,previously believed
to be a uniform collision zone between the
PHS and EUR,can be(1ivided into two zones,
the zone of collision s6%sz6 sかづ6オo an(i the
buoyant subduction zones,on the basis of the
regional features of Quaternary tectonics.
Besi(1es,the author discussed.the style and
recent activity of an active fault in this area
with respect to the coseismic cmstal move-
ment.Consequently,this stud.y may be useful
in the advancement of the knowledge on the
seismotectonics in the Izu Borderland which
is in(1ispensable for the earthquake pre(iiction
inJapan. The main results are as follows.
1) The study area to the north of Izu
Block is divi(ie(l into three areas accor(iing to
their tectonic characteristics. In each area,
the Quatemary chrono-stratigraphy, geo-
10gic structures and active faults,especially
with respect to their history of activity and
the crustal movement in both sides of the
fault,were described.
2)Distinctive active fault systems have
developed in the Izu Borderland surrounding
the northem margin of Izu Peninsula.Thrust
type dip slip faults are predominant in this
region with the highest class slip-rate in
Japan. In the area to the north of Suruga
Bay,there are two N-S trending fault sys-
tems which consist of several short faults
arranged en echelon.The eastem,Izu side
system is younger and more active than the
westem system in the Quatemary.This indi-
cates that the place o:f intense fault activity
migrates Izu・ward from west to east.The
same phenomena wasrecognize(1inthe south-
em foot of Tanzawa Mts.The length ofeach
fault,1ess than20km,is very short dispropor-
tionately to its high activity(slip-rate more
than l m/103years)as compared with thesame type of active faults in other regions of
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 studyarea is classifie(1into basin fill type and non
basin fi11-type deposit on the basis of their
lithofacies,thicknesses and tectonic features.
The active faults develop on the boundary
between older uplifted deposits in the Honshu
side and present sedi血entary basin in the Izu
side.Subsiding areas have migrated towards
Izu Peninsula along with the migration of
fault systems.
4)The Quatemary tectonics inmost Pa「ts
of the Izu Bor(ierland are controlled by buoy-
ant subduction of the Philippine Sea Plate
except for a small portion where exact colli-
sion occurs.That is,the Izu Borderland is
not a uni:form collision zone as has been
supposedl widely.The buoyant subduction is
evi(ienced by the formation of accretionary
prism on lan(1in this region.Accordingly,the
active faults in the Izu Bor(1erland are im-
bricated thrust faults caused in accretion
process,as branche(i fault from the concealed
boundary fault of plates.This is one of the
reasons why the active faults in the Izu
Borderland have higher slip-rate than those
in other region of Japan independent of their
fault length.
5)It is impossible to interpret all of the
active faults and crustal movement from the
viewpoint of the accretion process.The three
regions divided in the conclusion1)show
distinct regional tectonic difference. This
difference is ascribed to the variety of the
form of plate convergence. The formchanges from collision s6nsz6 s渉万660 in the
southem margin of Tanzawa Mts.,to buoy-
ant subduction in both eastem and westem
flanks of the Borderland,and finally normal
subduction along the Suruga and Sagami
troughs.Besi(ie this difference in space,the
transition of convergence form were reco9-
nized:in the southem margin of Tanzawa
Mts.,buoyant subduction was converted to
collision at the en(l of Early Pleistocene.In
the Oiso Hills and the Ashigara Plain,buoy-
ant subduction commenced in early Middle
Pleistocene.
6) The active faults in the Izu Borderland
have a recurrence interval of several thou.
sand years and the displacement by an event
should be more than several meters.They
are essentially imbricated thrusts in the ac-
cretionary prisms:therefore,it is themegathrust along the upper surface of the
subducting slab which is the controlling force
of the activity of the above mentioned fahlts.
The seismic activity represente(1by the
active fault movement along the northem tip
of the PHS(iecreases from the normal sub-
duction zone of Suruga and Sagami troughs
to the collision area ofthe southemmargin of
Tanzawa Mts.due to the steepening and the
buoyancy of the subducting slab. This
decrease accords with the low frequency of
earthquakes in the Izu Borderland.
Ack盟ow亙e〔墨geme麺ts
The author expresses his gratitude to Mr.
Yoshihiro Kinugasa,the chief of the Seis-
motectonic Research Section,GeologicalSurvey of Japan,and Professor emeritus
Sohei Kaizuka and Professor Hiroshi Ma-
chida of Tokyo Metropolitan University,for
their kin(1 supPorts and guidances of this
work.An(1he is also in(1ebted to Professor
Tokihiko Matsuda of the EarthquakeResearch Institute,University of Tokyo and
Dr.Katsuhiko Ishibashi,Building Research
Institute,Ministry of Construction,and co1-
1eagues of Seismotectonics Research Section,
Geological Survey of Japan for their discus-
sion and suggestion on the Quaternarytectonics in the Japanese Islands.
Refere亙亙ces
Amano,K.(1986)South Fossa Magna as multi -collision zone. E6zπhノ臨o%云h砂,vol.8,P.
581-585.*
,Yokoyama,K.and Tachikawa,T.(1984)
Hirayama fault which cuts an older
somma of Hakone Volcano.10蹴G601.
Soo.ノ41)召%,vol.90,P.849-852.*
一一一r Takahashi, H。, Tachikawa, T.,
Yokoyama,K.,Yokota,C.and Kikuchi,J.
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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-
on tectonics of Izu micro-continent with
Eurasian plate.Kitamura Commem。E3s砂s
G601.,p.7-29.**
Association for Kanto Quatemary Research(1987)
Stratigraphy an(i geologic structure of
Oiso hills.Bz61」且ssoo.Kごz%孟o Qz6ごzオ.ノ~6s.,
vol.13,p.3-46.*
Danbara,T.(1971)Synthetic vertical movements in
Japan during the recent 70 years.ノ;oz6名
0604.Soo.ノ4)召%,vol.17,P.100-108.**
Ikeda,Y.(1983)Thrust-front migration and its
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thrust fault systemr Bz611.五)ゆ渉.Gθ097.
U痂∂.To勧o,vol.15,p.125-159.
Harukawa,M.,Iso,N.,Uesugi,Y.,Mori,S.and
Nagasaki,T.(1977)On the stratigraphic
classification and correlation of so-calle(1
Ninomiya formation (2nd rep.). β%ll.
/15sooκα盟孟o Qz6ごzオ.1~6s.,vo1.4,p.18-32.*
Hatano,S.,Tsuzawa,M.andMatsushima,Y.(1979)
Vertical movement in Holocene age and
geodetic time along the northem coast of
Suruga Bay.R勿, Coo名ゴ劾σガng Coηz吻.
Eごz7オh(7.P名召4.,vol.21,P.101-106.*
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(*in JaPanese,**in Japanese with English
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。
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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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