COLLISION OF THE IZU BLOCK WITH CENTRAL *JAPAN DURING THE QUATERNARY AND GEOLOGICAL EVOLUTION OF THE ASHIGARA AREA

INTKODUC’TIC)N

Central Japan is the site of a recent collision. Figure 1 gives an outline of the

geodynamical context of this collision. To the west of Izu peninsula. the Nankai

Trough turns toward the north and ends at the bottom of Suruga Bay. To the east of

the peninsula. the Sagami Trough links together the bottom of the Sagami Bay and

the triple junction with the Japan Trench and the Izu--0gasawara Trench. Thr

motion of the Philippill~ Sea plate toward the northwest (Seno, 1977) implies that

IZ,LI peninsula is colliding with central Japan (Matsuda, 197X).

The Ashigara area is tocated in the northern part of the collision zone, between

the Tanzawa mountains to the north and the Hakone volcano to the south. This area

01 D2

El3

Philippine

sea plate

haa been recognized as a possible location of the plate boundary :I\ it i\ the place

v.hcrc a major thrust. the Kannuwa Fault. brings into con(act the Miocene fcmm-

tions of the Tanzaua mountains and the younger formations of the Ashigara ;lrtx.

Using an estimated upper Pliocene (Machida et al.. 1975) or pmGhl> early Quatt’rnar>

(Hasegawa et al.. 1975) age for the Aahigara group. Matsuda ( 197X) considered that

the collision of Izu block with central Japan toc)k place in the early Qualernar\~.

In thih paper. we present new results concerning (1 ) the age and paleoen~iron-

ment of the A\higara Group, based on Foraminiftxa and caIc;mous nannofossils. (7)

the evolution of the tectonic s\trcss field. On the basih of these data. the cvc>lu(ion 01

the plate boundary area is discussed and the age of the beginning of the collision

e4tiniated.

S.1.I~AI’I<;KAI’tiY AND I’AL~OENVIKONMt-N7S

The Ashigara Group is characterized by the deposition of a huge amount of

conglomerate. It is dividccl stratigraphically into four units: the Neishi. Scto. Hata

and Shiozawa formations in ascending order (Fig. 2). All these formations are

overthrust by the older Tanzawa Group along the Kannaw;~ Fault and covered h\,

E

0”

cu

r- 0

the younger volcanic ejecta of Hakonc volcano. The lwvermoxt N&hi Form;ition ih

distributed in the eastern part of the studied artx tsith an E W strike and N dip.

while the Seto. Hata and Shiozawa f~~rnl~lti~~lls outcrop in tltc western part. u.ith :I

NE-SW strike and NW dip (Fig. 2).

The Neishi Formation is composed of alternutin g lapilli tuffx and tuffacwx~

silt~tones, tuffaceous sandstones, pumice tuffs. conglomcratca. silt~tclnea with santl-

stones. and quartz-dinritr-rich conglomerates (Fig. 3). Its ~~l~~~i~lll~~l~ thicknc‘\ ib

1500 m. A pumice tuff layer is well traced in the eastern part (Fig. 2). The

sedimentary facies of this succession i.s that of distal turhiditcs. Thr Seto Formation

consists of alternating sandstones and conglomerates. The conglomeratw arc mc~~tl~

massive or reverse-graded. These sediments wcrt‘ thus redcpoGted in ;l xlhmarine

fan. The relation between this formation and the underlying Ncihi Form~ttion i\ nt>t

2 = fiat .s rnE

E .$ $ depositional

8 go environment

Y) -

S D

clear in the field. It may he an unconformity or a fault. The thickness of the Seto

Formation is 1300 m. The Hata Formation conformably overlies the Seto Formation

and is composed of alternating siltstones. sandstones. and thinly bedded con-

glomerates. Shelf-edge molluscan fossils are abundant in the siltstone part. The

maximum thickness is 600 m. The Shiozawa Formation is composed of alternating

sandstones and conglomerates. Many plant remains and oyster colonies occur in this

formation. An elephant (I’clrtr.vrc~,~o~ot? sp.) tooth has been found in the middle part

of this formation (Matsushima. 19X2). The maximum thickness reaches 2000 m. The

Shiozawa Formation is unconformably overlain by the Suruga gravels. Detailed

stratigraphy will be published in the near future (Kitazato and Ishikavva. in prep.).

The biostratigraphy of the Ashigara Group haa been investigated by means ot

both calcareous nannofossils and planktonic Foraminifera. Siliceous microfossils are

scarce in these formations. The stratigraphic distribution of significant species is

shown in Fig. 3. Glolwottrlic~ tr~rr~c~~~t~lit~oiLjC’.Y is distributed throughout the lovvermost

Neishi Formation. G‘eo/‘h,,roc,trpscI ctrrihhcw~~ctr is found in the Neishi. Seto and Hata

formations whereas Geoph~~rocupstr ocwr~ic~~ is limited to the Seto and Hata forma-

tions. The occurrence of G. ocrcrr~ic~ and associated flora indicates that both the Seto

and Hata formations belong to the CN-14a nannozonc of Okada and Bukry (1980).

From these results. the Neishi Formation may have been deposited during the lower

Pleistocene and both the Seto and Hata formations in middle Pleistocene.

Paleodepths of the Ashigara Group have been estimated by comparing fossil and

recent benthic foraminiferal assemblages. The fundamental depth distribution of

recent benthic Foraminifera was given by Kitazato (1979. 19X3). Results are shown

in Fig. 3. The paleobathymetry has been changing from middle bathyal (1000--2OOO

m) to littoral during the deposition of the Ashigara group. This is in good agreement

with the studies of molluscan fossils (Matsushima, 1982).

STRUCTURE AND STRESS FIELD

The structure of the Ashigara area has been studied with special attention being

paid to the reconstruction of the stress field. Analysis of fault systems aims at

determining the stress state responsible for synchronous motions (at the geological

scale) along various faults related to the same tectonic event. The orientations of the

three principal stress axes (u, most compressive. u7 intermediate and uj least

compressive) have been computed from the field measurements using methods

developed by Angelier (1979). In some cases where striae could not be observed, we

have used a semi-quantitative analysis of conjugate faults. To carefully separate

successive events in the field, both stratigraphic (sealed faults) and tectonic criteria

(intersections of faults, superposed striae, mechanical consistency) were used.

More than 30 outcrops have been studied in detail (Huchon, 1983) (Fig. 4). The

chronology of faulting and folding has been established by taking into account the

stratigraphic data presented above and field evidence for successive events. Special

Fig. 4. Directions of compression (double arrows) deduced from the analysis of faults. Phase 1 (A) corresponds to the main folding. under a NW-SE ~~lnpr~~si~~n~l stress field. B. At ahout 0.2 Mu. B.P..

the strcx field changed to N-S or NE-SW (phase 2). Diagram\ are stereographic prqcctictns (lower

hemisphere) of faults measured on some selected outcrops. Large open arrows show the direction of

compression. Kannawa thrust. strike-slip faulta and formatmn houndarks a:, 111 Fig. 2.

207

stress tensor haa heen performed. the ratio + = ((T? ~ ox ),‘( 0, - 0: ). that partI>

dcfina the shape of the stress ellipsoid. ih gentxall\ hmall. It mean\ that the

amplitudes of n, and 0; are nearly equal: mc>st of the fault pcjpulationa include both

re\c‘r\e :lnd xtrikc-slip faults (Fig. 4A). Very I‘c’M. tilted (i.e. fc~ldcd) faults ha\,c: htxn

oh~cr\cd. but their geometrv is conG>tent with the folding. As the same ttxtclnic

4tresh cxxurrctl in the wohterii xid caatt’rn axis. L\C concludr that the II W

structur~il trend in the eastern part is not due to later rcfoIdinp but ha5 hccn

gcncrattxl during the same tectonic cvcnt. Morcovcr. the thrusting cjf the TAILCILV~

mount~rin~ prohahlv occurred at the samt‘ time. as hnvn h) the obser\~ation ot

numerous faults and fractured pebbles .iust lxlcnv the thru.st (outcrop <‘If’. see Fig.

4).

A wcond population of faults has hwn observed and mtxa~rcd. It Itxds to ;I

direction of compression drasticall\ different from the pre\ ioiis one. ranging from

N S to N t: SW. In several outcrops where both e\‘ents \\ere decipherable. the N S

ccmprtasic~n clearly took place after the foldin, L 0 ,111d associated faulting. These faults

II;ILC’ hew obser\,ed csprciall\ near the recent or active fault> which cut the

Kmn~tw~r thrust (e.g., outcropa 111 ad :M,l’ in Fig. 4) and alao in a large quart-!

(outcrop /Cl ) where a measurement of residual stress has been performed b\,

Iloshino et al. (1979). giving ;I N-S compression identical to that computed from

fault measurements. The large strike-slip faults \vhich cut the Kannau~ fault (Kmo

et al.. 1979) have heen activated recently. ;I> they affect the Suruga gravels in the

Mestcrn part of the studied area. The lat movement along the Hira\,ama Fault

(outcrop 111. Fig. 4) has been dated at 24.000 yrs. B.P. (Ito et al.. 19X2). In the

eaatcrn part of the Ashigara area. the strike-slip faults cut uppc‘r Pleistocene to

Holocene terrace deposits and volcanic ejecta (Sate. 1976: Uesugi et al.. 19x1 ).

Finally. age determination of fault movements by ESR method indicates ages

ranging from 58.000 to 510.000 yr.s. B.P. (Ito et al.. 1983). The direction of

compression is also consistent with the present pattern of deformation as deduced

from geodetic measurements (likawa. 1981). We thus conclude that this tectonic

regime is the present one.

C‘ONC‘LUSION AND DISCUSSION

III early Pleistocene. the Izu peninsula was located more than 50 km to the south

or southeast of its present position. On the northern coast of the Izu peninsula. the

Yokoyama siltstone (Koyama. 1982) indicates upper slope conditions (2OOG800 m

depth) while south of the Tanzawa mountains, the Neishi Formation corresponds to

a bathyal environment (1000-2000 m depth) with paleo-currents from northeast to

southwest (Ito. 1982). This area may thus represent the lower slope or the abyssal

plain.

About 1 Ma. B.P.. the conglomeratic sedimentation (Seto Formation) that oc-

curred in this area was probably related to the very strong uplift in the Tanzawa

mountains. This event also correlates with an irnportmt cha11gC of the p:llW-

hathymetry. from 1000-2000 m in the lower part of Neishi Formation to 200 h0 111

in the lowermost part of Seto Formation. Actually. the boundarv between these two

formations is not clear and may he a fault contact or an unconformity. In the first

case. the eastern part of the Ashigara mountains mav have an origin different from

that of the western part: in the second case. a modification of the ahapc of the

submarine fan may explain the origin of the unconformity. The sedimcntar~ facies

of the Seto congfomerate indicates that it is a resedimented conglomerate: sediments

of the shelf were redeposited on the slope. Then. the Huta Formation may rcprtxnt

the top of the submarine fan. The siltstone part is autochthonous but aandstonca and

intercalated conglomerates may have been transported from a shallow water area. ax

they contain shallow water molluscs.

After the deposition of Hata F(~rrnat~~~n, the sedimentary facies changes to that of

a shallow fan (Shiozawa Formation). The presence of oyster colonies (biocenose)

indicates a tidal to suhtidal environment (O-30 m). The sedimentary facies of the

upper part of the Shiozawa Formation are upper deltaic or fluviatile.

The sedimentary evolution of the Ashigara Group is therefore clearly controlled

by (1) the approaching and then colliding Izu peninsula. (2) the very strong uplift

and erosion in the Tanzawa mountains. the sediments being accumulated on the

northern margin of an embayment lyin g between the lzu peninsula and Tanzawa

mountains.

Soon after the deposition of the Ashigara Group this area was very strongly

deformed under a NW-SE compressional stress field. parallel to the direction of

relative motion of the Philippine Sea plate with respect to Eurasia. This deformation

clearly occurred after the deposition of the upper Ashigarn Group. whose maximum

age is 0.7-0.8 Ma because of the presence of Purastegodot~ sp., that exists in the Boso

peninsula in this time range. Folding also occurred before the deposition of the

Suruga gravels (0.4 Ma). that unconformably overlie the Ashigara formations.

Nevertheless, recent observations of NE-SW trending reverse faults have been made

in the old ejecta of Hakone volcano and the analysis of dykes leads to a direction of

compression N30”W to N40”W (Amano et al., 1983) showing that the NW-SE

compressional stress field may have existed during the deposition of the lowest part

of Suruga beds. The change of the direction of compression from NW-SE to

NNE-SSW may thus have occurred some 0.3 Ma ago. From the dip of bedding

planes, the amount of shortening may be estimated very roughly at 30%. If the

duration of the folding event is about 0.3 Ma (from 0.7 to 0.4 Ma. B.P.), it should

correspond to a strain rate of 7.4 X lo-l4 s-‘. According to this estimation, and as

the base of the Seto Formation is dated at 0.9 Ma. (boundary between CN 13 and

CN 14a nannozones), the duration of the sedimentation of the Seto, Hata and

Shiozawa formations would have been 0.2 Ma. As their total thickness reaches 4 km.

it corresponds to a sedimentation rate of 20 mm/yr. In the Nobi plain (near

Nagoya), the rate of sedimentation of delta deposits is 10 mm/yr. during the

Thih htudy thus shows that very rapid changes in both sedimentation and htt-tx

field occur during the colli.\ion or ;I vc)lcanic island. As noted by Nakamura and

Shimazaki (1983). the fresh suture zone hctwcrn the colliding body and rht: host

mates i> marked at the surface by 21 thick pile of mainlv conglomrratic sedimenls

5hoMing upward coarsening and shallowing. The leading edge of the overriding plate

(here the Tanzania mount~tins) is uplifted bv the deformation occurring during the

ct,llision. At depth. the suture zont‘ prohahl~ follo\-\h the SO” inclincd seismic plant

revcalcd hv microearthquake studies. which ma\’ correspond to the presence of the

Philippine SW plate beneath the Tanza~a mountains (Nakamura and Shimazi~ki.

1983). The palcodepth change that accompanies the collixion. in a p~leogeographical

sense. precedes the main deformation and the drastic change of the stress field due

tcl the increasing coupling between the colliding IZLI peninsula and central Japan.

From the present example of “mini-collision” gtxlogy. it is likely that man\ other

geological structures might be interpreted in term.5 of such collision process.

/\C’KNOWLEDGEMENTS

The first author thanks Prof. H. Okada for providin, 0 facilities during a stav at

Shizuoka University under 3 grant of French Foreign Affairs Ministry. CNEX07and

C’NRS provided financial support. Many Japanese collragues are acknowledged for

helpful discussions. T. Ishikawa did some of the work under the guidance of the

second author. H. Okada kindly identified the calcareous nannofossils. J. Angel&

took part in the field work and lent his computer programs.

REFERENC‘ES

Amano. K.. Yokoyama. K.. Yokota. C. and Txhikawa. T.. 19X3. Features of deformation 111 the Ashigxr;h

group. Ahstr. Annu. Meet. Geol. Sot. Jpn.. 1X0.

Angelier. J., 1979. Determination of the mean principal directions of strcsseh for il given fault popul,ltlon.

Tectonophysics. 56: T17-T2h.

FLIJI. N.. Matsushima. Y.. Fuji. S.. Kitaz.ato. H . ;md Mori. S.. 19X2. Paleclenvironment of the Holocene

deduced from paleontological evidences in Nagoya Harbor. central Japan. Daivonki Kenkvu

(Quaternary Res.). 21: 153-167 (in Japanese with Engl. abstr.).

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869-X72.

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t ht‘

(Quatern an Kc.). 21): x-47 (ill