竺竺 =

and Tectonism Fiel.d Stress

Journal of Geosciences，Osaka City University Vol. 21， Art. 4， p. 37-52. March， 1978

P!lioceAe Tbe

Cen.trall' Japan Reg，i.o，n

Akira T AKEUCHI

Shi.n-Etsu the 10

(with 13 Figures)

Introduction 1.

The stress state within the earth' s crust is in general determined by gravitational

stress due to the weight of overburden and tectonic stress related to the straining of the

crust due to external forces (RANALLI， 1975). Regional geologic structures are control-

led by the regional stress field covering the area， thus it is possible to consider the stress

state from the characteristics of the tectonics of that province. The present paper de-

scribes the Pliocene tectonic stress state of the northern part of a large and important

tectonic zone， the Fossa Magna， crossing the central part of Japan， from a geological point

of view (Fig. 1). For this purpose， it is very important to know the relationships between the block movements of the basement rocks directly a:ffected by the crustal tectonic stress

state and the superficial folding structures of the covering layers. A discussion of this

problem is also undertaken.

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Index Map: Enclosed area and dotted parts indicate the Sh.in-Etsu Region and alluvial

plains and basins， respectively. Fig.l

38 Akira T AKEUCHI

The recent tectonic stress field can be determined by various techn同国s; foeal mech-

anism solutions of earthquakes， geodetic surveys of the crustal deformation and in~ situ measurements of roek-stress， etc. 'SJueh methods are tlS'eful in estimating recent

stress conditions， byt it is difficult to kn0w bow d，ld tbese eonditions are. The stress

field during earli引.geologic tirnes s'bou】dbe constructed by the geomorphological' and

geological rnethods. The former is very tlseful i'n }apan foi" Quaternary problems， but is not so for the Tertiary or even the P]iocene because Quaternary crustal movements

have been very severe here. Tu 'discuss the Tertiary stress field， the distribution of dikes， faults and folds are very useful. :Thus， in the study of the history of stress change， it is eonsidered im.portant to draw kinematic p，tctLlres of tbe development process of

tectonlcs.

Summary of the arg叫ments，mentroned abo時 ~s 'Scbematicaity represented as :foUows:

crlltstal

mOl:vement

nb

一一q】

主一

一一

r

O一一一e

十』一

-bu仇

a二r

e--a一一

3

パ叫一

一一R

l

i--

en--e

r--二m

+弘

-t占

s

t

st'Fess field

asd its

itli S te，首Y

tect0nic

elemeRts

tect0nic history

struct'tn:_al dêV~l(;)pment

2. General Trendl of DikesJ and Tectonic Stress Fields

NAKAMURA (t977) proposed a method fQr deωminI，Qg stress state using dike distri~

bution， caUed the “dike method"" which ha:s been appHed in this study. Tbe dike

method I!s based on the recognition of tbe pre，f，erred orientation of radial an:d pata~le1 dike

swarms as a result of large田 scalemagma fr，actlllring.

Cons路si泊deぽrrtmgt仏na拭trdi~泳kes are ~おormed i泊nt“ens幻iLぬ↓οonf合ractuliesol t}仏出heeart白h's crMst， they

have a tendency to develop in a directIJon normal to the minimum principal axis of com四

pressio仏 More@:ver，the' gen.eral trend， of rdlike 'Swarms cam be imterp民 tedto show the ori-

entation of the maximus1l norizontal eompressive axis of the region because dikes intrude

almost vertically.

The olfientation of tectonIJe stres~ which has existed in reoent g~ological times in the

upper crust oi volcan~c regions caR be identified from su，d， volcasic fealtures as the distrib四

ution of sank. craters indicati1ilg the trend of the und釘 g，round，radial， dike swarm (NAKA-MURA et al.， 1977).

The orientation of tectonic stress determined by the dike method is generally consis-

tent to the results obtained! l!sing other weH-kTloWll methods of estimating tbe recent

state of crustal stress， such as the focal mechanism solutiofls ef earthquakes， geodetic measurements of cfuS'tal deform.ation. amd in.-si，tu measurements o{ stress， and studies of active faults and folds. Among techniques for studying the past state of regional stress in

•

Pliocene st:γess field and tectonism in Shin・EtsuRegion 39

the upper crust， analyses of faults and folds of various scale is useful， as well as is the dike

method. However， the dike method is superior to other methods for dating purpose as

weIl.

Compilations of tectonic stress orientation in J apan based on the available data using

various techniques have been presented by ANDO et al. (1973)， NAKAMURA & UI (1975)

and HUZITA & OTA (1977). Fig. 2 is a stress trajectory map showing a horizontal stress

field， which has been compiled with the addition of new data (SHIONO， 1977). With the

exception of the southern part of Central J apan， the Kanto region， stress trajectories of

maximum compression are slightly curved but linear and， as a whole， in the direction of

east-west trend. Such stress states suggesting east-west horizontal compression is attrib-

utable to the subduction of the Pacific Plate along the Japan Trench (MATSUDA， 1977).

一一2---3

- 4

13S0E

400N

張、二;岬¥・¥ 、/'::0...... .:-J-

35・

、

1'40・

Fig.2Stress trajectory map of the recent stress aeldhJapan，compiled from ANDo et al. (1973)，NAKAMURA&UI(1975)，HUZITA&OTA(1977)and SmONO(1977)・ Inthe southern part of Central Japan， these trajectories showa radial pattern radiating from the northern part of the Izu Peninsula. 1; trajectories of maximum horizontal stress (compressive)， deduced from seismic waves (2)， active faults (3) and dikes (4)・ Arrows:directions of oceanic plate movements.

3. Pliocene Stress Field Inferred from Dikes

An attempt was made to determine the Pliocene stress aeid in the shin-EtSU Region

in the central part of Honshu Island，using Pliocene dikes，the intrusion of which have close connection with the volcanism of the NishiYama stage mainly distributed in the

central part of the sedimentary basin (CHIHARA， 1974; SHIMAZU et al.， 1977)・ TheKa北kuト •

dasan (ο101)り)，Yo∞ne悶e句yar町na(10明2勾)and A釘ra山kura江I

(Figs. 3 and 4). Brief descriptions of these three areas are as follows:

40 Akira T AKEUCHI

General trend of Dikes Phocene dlkes

Pleistocene dtkes

③ V伽 初叶 附十101

102

-T

Fig. 3 Dominant dike-directions of the Plio~ and Pleistocene in the northern part of Central Japan.

101， 102 and 103 are the symbol numbers of the Pliocene dikes， and correspond to those in Fig. 4. Pleistocene dikes are referred to by NAKAMURA & UI (1975).

101. Kakudasan

N N

102. Yoneyama 103. Arakurayama

Fig.4 Rose-diagrams showing azimusal distribution of Pliocene andesite dikes in the Shin-

Etsu Region. The total number (and dominant direction) of each dike swarm is 30 (N850E)， 33 (N75 OW) and 20 (N 60PV¥ワ， respectivelγ

101.. Kakudasan Area

The Kakuda Mountainland composed uf the Kakuda Formation (SHIRAI et al.， 1976) is rich in various kinds of e:ffusive and intrusive andesite rocks which partially have pillow

structures. The volcaniclastic rocks are interfingered with mudstone and sandstone

alternations of the latest Miocene (later Shiiya stage) and the Pliocene (Nishiyama stage).

The e:ffusives are distributed in the core of an anticlinal axis trending north to south.

Dikes of andesite are developed in the west side of the Kakuda Mountainland and show

a dominan.l direction of N8soE. The sumber of diJkes is 30 in all'.

102. Y oneyama Area

1"'he Pliocene volcaniclastic rocks of the Y oneyama Formation lie unconformably on

•

一

Pliocene stress field and tectonism in Shin瞳 EtsuRegion 41

the Middle Miocene mudstones which are folded with the axes trending north to south

(YONEYAMA RESEARCH GROUP， 1976). The volcanism of this district started at the

end of the Late Miocene and continued unti1 Pleistoncene， and products composed of olivin田， pyroxine・ andhornblend-andesite have cyclically' accumulated (CHIHARA， 1974). Intrusive rocks show dotty distribution. There is a total of 33 dikes in this

province， all of which have been examined and found to have an average strike of N750W.

103. Arakurayama Area

Pyroclastics and lavas called Togakushi or Arakurayama Volcanics are intercalated

with the mudstones and sandstones of the Plio四 andPleistocene formations (Tは ESHITA，1974). They are part of the folded structures of the Hikage and Orihashi Synclines.

Dikes distributed in this area petrographical1y consist mostly of calc-alkalic andesites and

high-alumina tholeiitic rocks， so-called basalts (T AlαSHITA， 1975). 20 dikes were examined， and their dominant direction was found to be N60oW.

Considering that dikes are dilated cracks or fractures :filled by magma， the stress field

existing during the main volcanism in each province was found to be as follows: The

directions of the maximum horizontal compression was N850E in the Kakuda Area

(101)， N750W ia the Yoneyama Area (102) and N600W in the Arakurayama Area (103).

As for the northern part of the Fossa Magna region， the Pliocene stress :field shown by dikes are for the most part consistent with that deduced from the distribution of folds

(KOMATSU， 1967) and faults (TAKEUCHI， 1977). Moreover， it is signi:ficant that in the

obtalloted from

'14ぴE138・EPleistocene ----Recent Stress Fiel d d • .で子、、

. ..・・4"・・・・・・・・ . -・・・・ ..

ー四圃圃圃・selsrnlcwaves

_，ー-ーIn-Sltu stress

1111111111111111 actlve faults & folds

.1> ，_" ， dlkes(flank volcanoes)

むacedback by means of

=二二==Pllocene dlkes

+ 38・.

• • • • • • • • • • • • • • • • • • • • • • • • -

E

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i

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i

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1

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•• 01

・.. .

.1ttt1111f I •• • 1.' m. • ・・・

• • • • • ・

• • • • • • • • • • • • • • • ・

• • • • • • ・

• • • • • • • • • • • • • • ・ . . .・r!'..， .

-・・・・・"........・・・・・・・・・... ...

. . F ・.

T ra;ectones of Maxlmum Compression

(SHtnax) -・-_ιーーー・-

. . o. . .. .

• • • • • • • • • ・ • • • • •

・

• • • • • • • • • • • • • • • • • • • • • ・

+、ミ;. 36・N寸

Fig. 5 Stress trajectory map of Pleistocene-" Recent s仕essfield in the Shin-Etsu Region.

Stress directions obtained from seismic waves and active faults are quoted from A.Noo

et al. (1973) and HUZITA & OTA (1977). Pleistocene-.， Recent s位essfield in this region can be traced back to the Pliocene by

means of the preferred orientation of dikes.

Akira T AKEUCHI

Region the directions of maximum (SHmax) and the minimum horizontal

compression (SHmin) are nearly the same as those of the recent stress field (Fig. 5).

Such results suggest that the recent regional tectonic stress state which has principal

axes of SHmax in the east-west trend and SHmin in the north-south trend had already

42

Shin-Etsu

appeared at the end of Late 1¥在ioceneand has continued through the Plio-and Pleistocene.

Stress History and Structural Development 4.

Neogene Stress History 4-1.

The orientation of the tectonic stress field during the Miocene in the southern part

of Northeast Honshu has already been examined on the basis of the dike method. Ac-

cording to the analytical results of HORI & TAKEUCHI (1977)， the axes of the “principal" stress SHmax and SHmin have been found to be the north to south and east to west

trends， respectively， in the Inner Belt of Northeast Honshu， including the Shin-Etsu Region， through the Miocene (Fig. 6). However， the dhrection of SHmax and SHmin changed abllupt~y in the ]atest Miocene. The change ()f tectonic stress field from the

E・W tensional state to the E-W compresωnal one is wel1 expressed in tectonism (T AKEU-

CHI， 1977).

Tectonic Development in. the Neogene

The tectonic history of the Shin四 EtsuRegion is summarized below， making reference

4-2.

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問問問MM

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デ

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・

Miocene Stress Field

estimated from dikes

川地•.

， a . -. • 380N

r'O !

。

-

J

• • • •

• •

• 360N

1400E 1380E

、.

Orientation of the Miocene stress field estimated from dikes (HORl & TAKEUCHI， 1977; with the addition of new data by courtesy of Mr. K. MIZUGUCHl)

Fig.6

Pliocene stress field and tectonz"sm in Shin-Etsu Region 43

to the results of IKEBE et al. (1972)， MORITA et al. (1973)， TAKESHITA (1974)， TAKEUCHI (1977) and SHlMAZU et ，al. (1977).

The Shin四 EtsuRegion had been a sedimentary basin in the southwestern part of the

Inner Belt of Northeast Japan， where the thick Neogene sediments have moderately

folded to form one of the most productive oil-fields in Japan. Since intensive deep

drilling and geophysical surveys have been carried out for the purpose of oil prospecting，

the subsurface structures have recently become remarkably clear.

The basement complex of the basin consists of Paleozoic and Mesozoic sediments and

p代田Tertiarygranitic rocks. The Neogene “Green Tuff Region" originated as grabens

bounded by faults， which obliquely cut the old structures formed in the Cretaceous.

“Green ITu鉦"was abundantly erupted in these grabens of the northern and southern

parts of the Shin-Etsu Region during the Early Miocene.

Almost whole areas of the region were covered by sea and thick mudstone deposited

during the maximum transgression. Submarine eruptions of alkali basalts took place

along the margin of the sedimentary basin. Besides， volcanic activity was high in the

Tsugawa-Aizu province and in the eastern part of the region， being characterized by bimodal volcanism of rhyolites and basalts. In the northern Fossa Magna， volcanic activity was vigorous， and quartz-diorites intruded within the “Central Belt." IJ.l the

Late Miocene， the activity of dacite occurred along the margin of the Central Belt of Upheava1.

Accompanied hy differential subsidence due to block faulting of the geological base-

ment， di:fferentiation of the sedimentary basin which became shal10wer and narrower started in the late Middle Miocene.

At the end of the Late Miocene， a significant change in sedimentary structures and

volcanic activity occurred. Starting in the Pliocene (Nishiyama stage)， the paleocurrent system began to be controlled by the structural trend called to “Niigata trend." In other

words， folding of the oiトTertiaryis considered to have begun to increase after the latest

Miocene (Shiiya stage). Since the Pliocene， volcanic activity has been localized in space少

but thick pi1es of calc-alkaline andesites were formed in the Y oneyama， Arakurayama and other areas. In the Early Pleistocene， volcanic activity of andesite took place in up叫

heaved land areas along the margin of the Shin-Etsu Region.

Most of the deformations of the sedimentary covers were completed by severe tec-

tonic movements which took place at the end of the Late Miocene and proceeded during

and after the Pliocene. This tectonism was caused by block movements associated with

reverse dip-slip faulting， which is due to the compressive stress resulting from a force

acting oblique to the long axis of the “Green Tu:ff Region".

4-3. Folding Process from the Viewpoint of Tectonic Stress

The characteristics of structural features and folding patterns developed in the

Shin-Etsu oil-field are wel1 shown in the structural profiles of KATAHIRA (1974) based on

the detailed surveys and numerous drilling data for oil prospecting. These are summa-

44 Akira T AKEUCHI

rized briefly as follows:

One of the characteristics of the folding patterns in this oil field is the development

of asymmetrical anticlines associated with thrusts along overturned limbs and wide syn-

clinal structures among these anticlines. ln most cases， the folding of covering N，eogene

layers is considered to be strongly affected by the westward tilting movements of the

basement fault blocks， and most of the main anticlines oecurred along the boundaries between tilted blocks. These basement blocks are inferred to have cylindrical1y curved， not simply vertical surfaces because they moved not only vertical1y but also rotational1y.

Fig. 7 shows schematic models of the movements of basement fault blocks in two

different states of tectonic stress (after KIT釧 URA，1963; MATSUDA， 1977). ln a tensional

state， tilting blocks occur in the basement devided by normal faults. Considering the

sedimentation of the overlying layers under the influences of tilting block movements of

the basement， wedge-shaped strata might be formed， becoming thick tilting downwards of

the hanging wall blocks (Fig. 7a). ln an E圃 W compressional state， on the contrary， cylindrical faults of reverse dip-slip type are inferred to develop in the basement， so

overlying sediments might become thicker in the direction of the downward tilt of the

foot waU side of each fault block (Fig. 7b).

Some tectonic features such as followings should be noticed for successful configura~

tion of the folding process: Typical examples of buckling folds can be found in the

Matsudai district of the Niigata area (UEMURA & SHIMOHATA， 1972) and in midway up the Saigawa River in the Nagano area (T政 EUCHI& SAKAMOTO， 1976). Such folds are

formed directly in the horizontal compressional states. sesides， in the Niitsu district， one of the main oil fields of the Niigata area， a typical bending fold can be seen as Niitsu

Anticline， w'hich is considered due to the pushing 'Up of the tilting basement blocks (Co・

LLAB. RESEARCH GROUP FOR NIITSU A.NTICLINE， 1976). Various styles of anticlinal struc-

tures developed in this region can be attributed! to the differences in tbIJckness of N eogene

strata andfor depth of the basement blocks. The overlying layers having different thick-

ness are expected to have behaved somewhat differently responding to block movements

of the basement.

Above facts suggest followings. States of tectonic stress are not always continuous

nor uniform between. th.e b31sement and the sedimentary'cover. It is the basement rocks， not the Neogene sedimentary cover to respond directly to the regional crustal tectonic

stress. Within the sed加lentaryCOVier， there exist both regional旬ctonicstress and local

one resulted from block movements of the basement.

By the way， it is very significant that the folding of the Neogene strata bas started when the regional crustal stress field abruptly changed nearly 90 degrees from the E-W ten ...

sional state with SHmin of E圃 W trend into the E-W compressional one with SHmax of

E圃 W trend.

Such right-angled change in arrangement of stress-axes could occur the foundamental

ehange to reacti.vate or reju刊 natethe former normal faults as reverse faults.

In the Matsushiro earthquake swarm area， AsANO et al. (1973) found a clear example

•

Plioceγze st:γess fieldα:nd tectonism in Shi-nトEtsuRegion

(a)

Cコ

Tensionat

Tectonism

(b)

亡〉

Compressional

Tectonl'sm

push

/ /

/ /

多

CI)

/ /

/ /

一

夕、J4こ-.-、、、』

/ ノ

亡=>SHmln

〈コSHmax

Fig. 7 Neogene tectonism and regional tectonic stress field in the Inner Belt of the Northeast ]apan Arc. (a) Tensional tectonism: In the upper crust of the earth， normal faulting proceeds in responce to the tensional state of regional stress， which is characterized by the direction of SHmin. (b) Compressional tectonism: Reverse faulting is likely to appear in a compressional stress field characterized by the direction of SHmax. In both cases， preferred orientations of stress indicators (earthquakes， faults and dikes) are shown in the upper parts. The schematic profile showing block faulting with cylindrical surfaces is also given in each lower part.

45

of“reactive" fault along the western boundary of the Nagano intermountain basin. The

crustal structure of this basin was revealed by seismic prospecting crossing the boundary

in the direction of northwest-southeast. The sedimentaty layers with a velocity of 4.0

kmJsec become thick discontinuously with a head of 3000 m in the west of the boundary.

vVhile， the younger sediments with velocities less than 4.0 k.mJsec are rather thick in the Nagano Basin just east of such underground structural discontinuity.

When the quite opposite stress field occurred， it is very impotant to the newer folding whether the initial deflection and wavelength of folding， which is essential to the buckling

46 Akira T AKEUCHI

process， has been prepared or not. The antic】inalfl.exures (step圃 likemonoclinal struc-

tures) formed in the former tensional tectonism played a role of illitial deflection and de圃

cided a dominant wavelength of buck1ing folds in the newer compressional tectonism.

So， it is obvious that the embryos of the folds in this region had been constructed in the former tectonics during the Miocene times.

Figs. 8 show 9 are schematic profiles and Fig. 10 gives a relation diagram， summariz圃

ing the folding process of the Neogene strata in the Shin圃 EtsuRegion.

The Dおolld

imIn1gピ，"， butiおsr悶at出he釘rcombined process of b加otぬhill the complicated tectonic history 0ぱfa sed必i輔

men此ta紅rybasin. Such process can be eXplained as fol1ows:

From the Early to Late Miocene， this region was in a tensional state in the E・wdirection， and tensional tectonics had been formed. A vast amount of volcaniclastics of

the early Miocene was closely assosiated with fissure eruptions in the trend of north to

south.

In and after the maximum transgression of the Middle Miocene， one by one normal

faults of the basement trending north to south became so active that differentiation of

sedimentary basins ，due to diffe~ential subsidence proceeded. At the same time， gentle anticlinal fl.exures due to“bending" is considered to have developed within the sedimen-

tary cover alQng the boundaries of each basement block.

At the end of the Late Miocene， the regional tectonic stress field changed into an E-W compressional state， which resulted in the start of compressional tectonism. The

tilting block movements of the basement associated with reverse faults began to take

place， and most of these faults are inferred to be “reactivated" ones which had originated

at the time of the tensional state.

west Latest Miocene __， Pleistocene east

+ + + COrTl>ress ional十 +

+ + + Tectonism + +

LEGEND

o 5km

口刊川l

届 M蜘ゆ…C…en附e郎S

west Earlyへ_，Late Miocene

east

巴Ba尚…sby Mlooene 9悶nI拘tcls

zz we巾仰sedlmentary rocks

圏 Sheared Pre-Te巾仰

「τ1Pre-Te巾Jary1+' I gralilltlc rocks

、

Tensional Tectonism

+，/

+

+

十

十

+

+

x

y_ v v :t .::t.羊一

色m

o 5km 骨

I 1~ I Cylindrical fault μこJ Wlth dlp-shp

Fig. 8 Interpretative schematic profite for the folding process. of the Neogene strata in the nor-thern Fossa Magna region. See text.

•

Pliocene stress fieZd and tectonism in Shin-Etsu Region 47

Laiest Miocene ---Pleistocene Compressional w

Tectonism LEGEND

同幽

Early-Late 2

Miocene

Tensional W

Tectonism

E一・

+h+

日 3

図 4

Fig. 9 Idealized geologic section across the Niigata sedimentary basin from east to ¥vest.

See text.

1: latest Miocene"" Pleistocene sediments， 2: Early""Late Miocene sediments， 3: basement rocks， 4: boundary between basement blocks

円ODEOF TECTONISM TENSIONAL TECTONISM COMPRESSIONAL TECTONISM

Anticlinal flexures I nitialdeflections > An ticlines

IBuckl i ng process!

Bending process Bending process

Asymmetrical

folds SEDIMENTARY COVER

BASEMENT Normal block・faulting .:::... IReverse block-faulting

TENSIONAL STATE QlJllE OP問SITE

CHANGE

pu

----M円n

unU

T--L

C

E

FE'A

Tl「「

『』

C叫

AMncd

N

E

nυpn

vιT'

Gnb

p巳

nHH COMPRESSIONAL STATE

Fig. 10 Schematic diagram for explanation of the folding process with respect to the relation-

ship between the block movements of the basement and its influences on the oyerlying

strata.

The overlying layers are also atfected by the lateral compressive stress as ，yell as the basement and tend to be “buck1ed" into a more or less regular sinusoidal form. It is sig-

nificant in this case that such an initial wavelength had already been resected as anticlinal

flexures formed during former tensional tectonism.

Along with the buckling process， there is bending moment due to reverse block fault-ing in the basement， so that the initial desections are amplified and modified into asym・

48 Akira T AKEUCHI

metrical folds. “Green Tuff" and Miocene granitic rocks would behave as rigid bodies the same as pre-Tertiary basement rocks.

5. A Proposal -Chikumagawa Tectonic Lineー

After UTADA (1973)， th.ree parallel zones of hydrothermal alteration activity can be recognized in the Central Belt of the northern Fossa Magna (see Fig. 11). ThIls hydro-

thermal activity occurred during the latest phase of Miocene volcano圃 plutonicactivity

within the Central Belt and ceased just before the earliest of Pliocene volcanism. During

the Late Miocene， the northern half of the Central Belt was more uplifted than the south-

ern half， and the houlldary hetween them is very clear in the summit .level 'map (Fig. 12). Therefore， it is not unreasonable to assume that there existed a fault zone trending north-

west to southeast along the present-day Chikumagawa River during the Late Miocene.

Paying attention to the distribution of the hydrothermal alteration zones showing discorト

tinuity between the two blocks separated by this zone， it Can be also assumed that the nor-

N

ITOIGAWA

ID臥LlZEDSTRUCTURAL MAP OF THE NOR111E附 FOSSAMAGNA REG削

LEGEtサo

回 NISHIKU印KIBelt of up阿

国 SAIG肌 FoldedBelt

ん/ aXlal trace of a州 ne/円←=dlrectlonof overtuming

困 CentralBe山 pheaval

~ M locene granltold

会 f〆 Hydrothermalalteratlon zone

¥例KUMAGAWATectonic Line

臼 h恥町-T花e巾削a叫吋 Baお…S

;伝Z |汀TOα|陥GAWA一 SHl吃ZUωOKA手会 T ectonic Line

o 10 20附n

Fig. 11 Idealized structural map of the northern Fossa Magna region. The “Chikumagawa Tectonic Line" is assumed on the basis of the distribution of the hydrothermal altera-

tion zones. This line suggests left-lateral displacement of the basement blocks in response to the Plioc~ne stress field. (complied from MASATANI & ICHIMURA， 1970; UTADA， 1973; AKAHANE， 1975)

•

Pliocene stress field and tectonism in Shi14トEtsuRegion

N

4一一・・・・・・圃・一一ァ

中2

3 。 50 100km

Fig. 12 A map showing the distribution of the main faults which were formed or rejuvenated in the Quaternary period and their close relationship to the geomorphology of Central ]apan. The contours show summit圃 levelwith 100 m interva1. 1: strike-slip fault， 2: thrust fault， 3:“Chikumagawa Tectonic Line." This figure is quoted from HUZITA et aZ. (1973， Fig. 1) with slight addition.

49

thern block of the Belt dislocated toward the northwest against the southern block. As

it is， at the front of the western edge of the northern block of upheaval， the folded structure

of the Neogene strata is thrown into confusion. According to the topographic map illus-

trated by UKAYAl¥仏 (1968)，this line is significant as the geomorphological boundary

between Central and Northern Japan (Fig. 13). However， there is no topographical

evidence that this fault zone has been active during the Quaternary period. Consequently，

reamrkable lateral block movement with a sinistral displacement is concluded to have

occurred after the hydrothermal activity ceased， or possibly during the Pliocene. In the

Pliocene， regional， stress field discussed here， left-lateral strike-slip faulting with a north-

west to southeast trend is reasonably expected. The name of “Chikumagawa Tectonic

Line" is proposed for the assumed fault zone mentioned above. Further studies are expec-

ted to add clarity to this problem.

50 Akira T AKEUCHI'

N

⑨ ⑨

畠圏図

J 9

。 50 100km

Fig. 13 A map showing the distribution of the main tectonic lines occurring along the boun-daries between large geologic bodies (OKAYAMA， 1968; with slight revision). ML: Median Line， Sr: Suruga・BayLine， Sg: Sagami・BayLine. B: boundary be-tween Central and Northern Japan=“Chikumagawa Tectonic Line." 1: alluvial plains and basins， 2: Quaternary volcanoes， 3: lines of topographic discontinuity.

6. Summary and Conclusions

(1) The dike method for determining the regional stress field was applied in the Shin-

Etsu Region. The recent E圃 W compressional state can be followed up to the end of the

Late Miocene through the Pliocene. During the Miocene， however， this region had been subjected to an E-W tensional state.

(2) The tectonic development of the region can be understood based on the above men圃

tioned stress history， by which the following process can also be constructed. In the

tensional stress field du，ring the Miocene， normal faulting of the hasement of the regionally subsiding sedimentary basin resulted in blocky differential subsidente， which formed some initial heterogeneities in the sediments deposited in the basin. Ls' the fol1owing

compressional tectonism， such pre-existing sedimentary structures as embryos have

'0

Pliocene stress field and tectonism in Shin-Etsu Region 51

developed into asymmetrical anticlines.

(3) Such a kinematic picture is presented showing the normal faults rejuvenated as

reverse faults owing to the nearly right-angled change in the arrangement of the axes of

tectonic stresses (SHmax and SHmin). The possible mechanism of folding should be

examined in relation to the change in the regional stress field.

(4) The abrupt change of the tectonic stress state at the end of the Late Miocene ob-

tained by dike method is expected to be recognized in the “Green Tuff Region" because the studied area is a representative part of the Inner Belt of the Northeast Japan Arc.

(5) The existence of the “Chikumagawa Tectonic Line" is assumed according to Plio圃

cene tectonic movements. This line suggests left-lateral block movements of the base-

ment blocks， which resulted in the severe folding of the sedimentary oover in the nor帽

thern part of the Fossa Magna.

Acknowledgements

This study was carried out with encouragement of Assistant Prof. K. NAKAMURA of

Tokyo Univ.， Dr. Y. KOBAYASHI of Kobe Univ. and Prof. Y. KASENO of Kanazawa Uni-versity. Helpful discussions and suggestions for improvements were provided by Prof.

K. HUZITA， Assistant Prof. K. WADATSUMI and Dr. K. SHIONO of Osaka City Univ.， and Prof. T. UEMURA of Niigata University. The paper was critically reviewed by Prof. K.

HUZITA.

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