Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of southwest Japan

381

Tectonophysics, 177 (1990) 381-399 Elsevier Science Publishers

B.V.. Amsterdam

- Printed

in The Netherlands

Late Mesozoic-Cenozoic strike-slip and block rotation in the inner belt of Southwest Japan YUJI

KANAORI

Department of Geological Sciences, Faculty of General Education, Gifu University, 1 -I Yanagido, Gifu City 501 -I I (Japan) (Received

March 9,1989;

revised version

accepted

October

10, 1989)

Abstract Kanaori,

Y., Late Mesozoic-Cenozoic

strike-slip

and block rotation

in the inner belt of Southwest

Japan.

Tectonoph_ysics,

177: 381-399. The inner belt of Southwest largest

strike-slip

more small blocks approximately

Japan

bounded

to the present.

by active faults

Significant

igneous

Japan

was then placed

this configuration Faulting

the inner

and

igneous

and geological

tectonic

has occurred

strike-slip

in parallel

north

bounding

faults.

occurred

along

Southwest

Japan

Tectonic

Line

underthrusting

system.

Tectonic

location

nearly constant

during

of the blocks

model,

to the position

between

which is the into ten or

are 20 to 80 km in width the long history

occupied

two major

formation. faults;

in such

The inner belt of

in mid-Cretaceous

strike-slip

and since

from the Late Cretaceous

the small blocks were rotated

with each other at the time of the block

to be situated

Line (MTL),

The inner belt is divided

lines. The blocks

along boundaries

Main plutonic block and

activity

terminations

occurred the

during

that

and block

occurred

from

the Late

rotation

model

which

occurs

block

at triangle

boundary

along three block boundaries: Hanaore-Kongo

the rotation

to tilt, even at present. faults also appearing

fault

of the inner

and

Riedel

line,

during

time. With

the MTL

faults

the opening

were generated

and

the

that originated

be geometrically of the two major

Other igneous

inside

Tectonic

Line, the Tsurugawan-Isewan

of the

the block.

activity

fractures

Sea of Japan

of the Sea of Japan along

block

boundary

Bending

Blocks

raised

of by

at about

15 Ma have

faults,

while a few

from the block boundary

faults

to south, as: the Hida terrane consisting of preSilurian gneiss and crystalline schist with Jurassic granitic intrusions, the Ashio-Mino-Tanba-

The Japanese Islands located off the margin of the Eurasian continent (Fig. l), are generally divided into Northeast Japan and Southwest Japan,

Chugoku terranes of Mesozoic to Late Paleozoic non-metamorphosed sedimentary rocks and of Late Paleozoic to Middle Mesozoic gneiss and mica schist (Yamada et al., 1982). These terranes are interpreted to be allochthonous blocks transported by the Pacific plate and jointed in turn to the continental margin as accretion prisms (Kanmera and Matsuda, 1980).

separated by the Tanakura Tectonic Line. Furthermore, Southwest Japan can be subdivided into the inner and the outer belts by the Median Tectonic Line (MTL). Basements in the inner belt are composed of two terranes arranged with nearly E-W orientation and can be identified, from north Science Publishers

can

slip movement

by the block rotation. shear

belt due to the opening

Most earthquake

to the present

left-lateral

the Itoigawa-Shizuoka

along Riedel shear fractures

0 1990 - Elsevier

Cretaceous

induces

gaps generated

faults

Introduction

0040-1951/90/$03.50

of the Median

and block rotation

of its present

belt is found

activity

by the strike-slip

earthquake

north

to an island-arc

fault.

interpreted

continued

in the region belonging

activity

With the use of a geometrical

a way that they are to be rearranged

Sikhote-Alin

Islands

200 km in length. The width and the length have remained

the Late Cretaceous.

Southwest

is located

fault in the Japanese

B.V.

,:!

Japan

: ,i’ $_-

from Kakimi

map of Southwest

of active faults was simplified

Fig. 1. Geological

T’__

the distribution

The distribution

km

af active faults, geological

100

tectonic lines or faults and igneous

rocks is after Yamada

Ryoke -

rocks rocka

granitic older granitic

younger

et al. (1982).

(198S), Sangawa

et al. {1985).

The distribution et al. (1983) and Yamazaki

since the Late Cretaceous.

t ~~~~~ crL~ceov., F Bammt rocks

m

Ryoke

rocks occurring

to Mesozoic

lines and igneous

Paleozoic

et al. (1982, 1985), Kato et al. (19&t), Kato and Sugiyama of tectonic

et al. (1982), Tsukuda

stressing

,..;y -i

0

E

2

LATE MESOZOIC-CENOZOICSTRIKE-SLIPAND

Igneous found

rocks formed greater

numbers

in the

granitic

rocks (Kishimoto,

activity

of the late Cretaceous

that

clockwise 1961)

of Japan

Japan

Japan

Japan

to the

data have

rotated

counter-

(Kawai

rotated

since the time of the opening at about

as

to the Early Paleobelt extended

Southwest

Southwest

belt

and Ito (1988) age data of

region. Paleomagnetic Northeast

against

and

clockwise

inner

exist

1980), that the granitic

in the inner

east Sikhote-Alin revealed

time are

and active faults

opposed to the outer belt. Kinoshita confirmed, from studying K-Ar

gene occurring

ROTATION,SW

since Cretaceous

far more extensively

in much

BLOCK

et al.,

about

50”

of the Sea

15 Ma, while the eastern

margin

383

JAPAN

tude

during

Cretaceous

paleomagnetic

time

evidence

re-alignment

places

as inferred

(Hirooka

the inner

from

et al., 1983). This belt

of Southwest

Japan between two major strike-slip faults which are now parallel to each other. Consequently, the kinematics

of the inner

geometrically tion

belt

can

by a left-lateral

model

(Nur

be interpreted

slip and block rota-

et al., 1986;

Ron

et al., 1984,

1986). By using

this geometrical

interpretations

were

made:

model, about

the following the formation

and movement

of active faults and the position

igneous

that

Southwest

rocks

Japan

occurred

in the inner

of

belt of

since the Late Cretaceous;

also,

of Eurasia, including Southwest Japan, rotated more or less with respect to the eastern margin of

the formation of thrust faults and tilting basins caused by block piling at the time of the rotation

Eurasia during last 100 Ma (Otofuji and Matsuda, 1983, 1984, 1987). Although block rotation of the

of Southwest Japan; and block rotation

Japanese

Islands

formed

their present

shape

and

formation

implications movement

and evolution

of the strike-slip on the structural

of Southwest

Japan

will

present location (Niitsuma et al., 1986), the structural evolution related to the block rotation has not

be discussed.

been sufficiently interpreted. Even though islandarc volcanism caused by oceanic plate subduction

Distribution of active faults, geologic tectonic lines and igneous rocks

is presumed to be responsible for the occurrence of igneous rocks since the Cretaceous (Uyeda and

Inner belt of Southwest

Miyashiro, 1974) it has not been clearly interpreted as to why and where it occurred. Active faults distributed in the inner belt dur-

Figure 1 shows the active faults, tectonic lines and igneous rocks distributed in the inner belt of

ing the Neogene

Southwest

believed

or Early

to be generated

Quaternary

as conjugate

period

are

faults under

triaxial stress states caused by an E-W compression in the same manner as that of the present (Huzita, Neogene

Japan.

tures are described

The

Japan

distribution

of these

fea-

as follows.

Active faults and tectonic lines

1980). However, since time from the to the present is too short and too low in

Features in the sense of direction and displacement of active faults and tectonic lines are

confining pressure to form wide shattered zones of several tens to hundreds of meters in width, active faults had probably moved repeatedly before the Neogene (Kanaori and Satake, 1986; Otsuki,

shown.Based upon these features, the inner belt is distinctly subdivided into four regions, from east to west: the Kanto, the Chubu, the Kinki and the Chugoku regions. The Itoigawa-Shizuoka Tectonic Line defines the boundary between the Kanto

1978). This

paper

first

shows

that

the inner

belt

of

Southwest Japan is definitely divided into ten or more small blocks bounded by active faults and geological discrete lines, based upon their distributions and features. Next, the blocks in the belt (simulated in a geometrical model) were rotated in such a way as to be rearranged in parallel with each other at the time of their formation. Southwest Japan was then relocated to its original lati-

and Chubu regions, the Tsurugawan-Isewan tonic Line between the Chubu and Kinki

Tecregions,

and the Hanaore fault-Kong0 fault line between the Kinki and Chugoku regions (Fig. 2). The features of active faults and tectonic lines in each region are described below. The Kanto region: There are a number of NNE-SSW aligned active faults, most of which are thrusts.

Fig. 2. Block structure

in the inner belt of Southwest

Japan without

hugoku Region

inferred dist~bution

tectonic rocks are denoted

of active faults, of Ryoke older granitic

from the distribution

marks.

lines and igneous as question

rocks, based on Fig. 1. Block terminations

inferred

z

LATE

MESOZOIC-CENOZOIC

The

Chubu

NW-SE

STRIKE-SLIP

region:

alignment

BLOCK

Left-lateral

faults

are predominantly

parallel

to each other.

oriented

There

active faults having

ROTATION,

with

a

left-lateral

lines

oriented

Igneous

of N-S

lateral

WNW-ESE

of these

or WNW-ESE

accompanied

active

faults

by shattered

one hundred

a

with left-

are found

to be

zones of several tens to

meters in width (Ogata

and Honsho,

region

and

those

(Choubert

because

distributed

Japan,

they

it may be inferred

in Southwest

Japan (Kinoshita

lar,

the

granitic

Sikhote-Alin

and

Faure-Muret,

can be correlated

in the inner

Alin region

the

Igneous rocks Various

types and ages of igneous

from

Late

Cretaceous

granitic

rocks rangrocks

belt

that the granitic

extended

with

of Southwest activity

to the east Sikhote-

and Ito, 1988). In particu-

rocks

distributed

along

the

fault to the east are correlatable

with

of the Jurassic

Funatsu-type

granitic

rocks

which make up part of the basement rocks of the Hida terrane. Basement rocks are found to be more widely exposed in the Chubu and Kinki regions, the southern part of the Kanto region and the eastern part of the Chugoku region than in the remaining part of the Chugoku region. Most areas are occupied are spotted

by igneous

rocks and subordinately

with basement

latter

Parfenov

rocks.

region

The model interpret rotation

The distribution and nature of faults and igneous rocks in the east Sikhote-Alin region is of great significance when we attempt to interpret evolution

in the inner

belt of Southwest and block rotation

Japan by using the strike-slip model. Features of faults and

igneous below.

this

rocks

in

region

are

briefly

1972;

1980).

used in this study can geometrically

strike-slip movement and induced block in the region lying between two major

bounding faults (Nur et al., 1986; 1988). When the major bounding

stated

Active faults Satellite images (ERTS) show that faults with an NNE-SSW trend parallel to that of the Sikhote-Alin fault and those with an NE-SW trend dominate in the east Sikhote-Alin region, and that several NW-SE aligned faults also exist

Martel faults

et al., move,

they generate parallel faults (block boundary faults) in the region lying between them and subsequent strike-slip movement on the faults causes each block to rotate each adjoining block. This blocks

and igneous

Volchanskaya,

et al., 1978; Kishimoto,

produces termination

The phenomena

the structural

and

The strike-slip and block rotation model

rotation East Sikhote-Alin

(Baskina

to

Quaternary volcanic rocks are found to be extensively distributed in the inner belt, with the exception

1976),

the Ryoke older granitic rocks, as may be ascertained from the K-Ar age of around 90 Ma for

1981).

ing

et al., 1980). Slip sense of

rocks of Late Cretaceous to Early age are widely distributed in the east

Sikhote-Alin

with

slips.

Some

(Ichikawa

rocks

Granitic Paleogene

slip and a few active faults lying ENE-

SWS with right-lateral

in the region

slip.

are a number

active faults, most of which are thrusts.

tectonic

385

are a few NE-SW

The Chugoku region: There are some characteristic

SW JAPAN

the faults has not yet been reported.

distributed

a right-lateral

The Kinki region: There aligned

AND

triangular

gaps

and the major

between

bounding

that have occurred

the fault.

in these gaps

have not been explained (for example, al., 1986 and Martel et al., 1988).

see Nur et

When this geometrical inner belt of Southwest

model Japan,

faults, blocks

gaps and the two major

termination

bounding faults that interact detail in what follows. Block lines

boundary

faults-active

is applied to the block boundary

will be interpreted

faults

in

and tectonic

Active faults, which can be traced as linking tectonic relief and fault outcrops to others, appear intermittent because alluvial deposits cover the outcrops and reliefs while geomorphological mod-

Y KANAORI

386

TABLE

1

Ages and types

of igneous

rocks

created

since the Late

Cretaceous

time (based

on Yamada

et al., 1982) and location

of main

occurrence Types of igneous

Ages Late Pleistocene

Volcanic

to Holocene

Location

rocks

of main occurrence

Riedel shear fractures

rocks

block termination Volcanic

rocks

Block terminations

to Middle Miocene

Granitic

rocks

Block boundary

of the Japan

Volcanic

rocks

Block terminations

Middle Miocene

to Early Pleistocene

(Post-opening

of the Japan

Late Paleogene (Pre-opening Late Cretaceous

granitic

Sea)

Ryoke younger Rhyolitic

activity)

Granitic

granitic

and dacitic and granite

Block boundary

porphyrytic

Major bounding faults

the boundary

faults.

20 to 80 km wide.

Due

The

blocks

are

to the extensive

Quaternary volcanic cover, the boundary faults are not always clear in the Kanto region, but the positions and

of Quaternary

seismicity

and locations NW-SE

volcanoes,

in the area

suggest

of the block boundary

aligned

faults

in the east

tectonic

line

the existence faults.

Some

Sikhote-Alin

region can be regarded as block boundary faults corresponding to those in the inner belt of Southwest Japan. Block rocks

termination

distribution

and block boundary

faults

gaps

ceous,

the

Each

block

following

of

granitic

of igneous

rocks

is closely

related to the MTL and block boundary faults (see Table I). The block termination are inferred from the northern limit of the Ryoke older granitic rocks in the inner belt (Fig. 3). The angle between the MTL and the northern limit line is estimated about 20” in the Kanto, Chubu and Kinki regions and about 35 o in the Chugoku region. The Ryoke younger granitic rocks are mainly distributed in the area south of the inferred block terminations in the Chugoku region, while they widely spread to the area north of the block terminations in the

The blocks

region

were

procedure

in the Kanto,

gions was rotated

20”

rotated

are about

was

Chubu

and

performed. Kinki

re-

and those in the Chugoku 35”

resets the block rotation

counterclockwise.

angles and removes

This block

gaps that were inferred from the distribution of the Ryoke older granitic rocks mentioned above. All blocks in the inner belt are rearranged parallel to each other,

gap-distribution

regions.

To restore the inner belt of Southwest Japan to the position it occupied before the Late Creta-

the Kanto

region being rotated 65” to the Chubu region Tectonic Line. The

counterclockwise with respect along the Itoigawa-Shizuoka Kinki

The

gaps

activity)

fault or a tectonic line to an other one in an approximate straight line (Fig. 2). The parallel are

faults

faults.

Block termination

Kinki and Chubu 200 km in length.

faults

and block boundary

Block terminations

rocks

rocks

ifications take place due to weathering. The inner belt can be subdivided into small blocks bounded on both sides by parallel faults that link an active

from

faults

Block termination

rocks

rocks

Ryoke older granitic

Late Cretaceous (Ryoke older granitic

Kyushu

Sea)

to Early Paleogene

(Ryoke younger

inside the blocks and gaps in central

region

was then

wise with respect Tsurugawan-Isewan

rotated

35” counterclock-

to the Chubu region along the Tectonic Line. The Chugoku

region was also rotated

20 o counterclockwise

with

respect to the Kinki region along the Hanaore fault-Kongo fault line. Southwest Japan was restored to a position north of its present location, according to paleomagnetic inclinations during Cretaceous time (Hirooka et al., 1983). In order to subtract the contribution of volcanic rock occurring in central Kyushu, the western part of the MTL was rotated 15 o to 20 o clockwise around a pivotal point on the MTL in the central Shikoku

LATE

MESOZOIC-CENOZOIC

STRIKE-SLIP

AND

BLOCK

ROTATION,

SW JAPAN

(Fig. 3~). The subtraction of this volcanic rock contribution occurring in central Kyushu results in the blocks having a constant length of about 200 km. When Southwest Japan was restored to its original position in the manner mentioned above, two major bounding faults generating strike-slip and block rotation are found. The southern bounding fault is apparently the MTL. The MTL is parallel to the Sikhote-Alin fault which passed

387

through the Sea of Japan and possibly extends to the Tsushima fault between the Korea Peninsula and the Tsushima Islands (Otsuki and Ehiro, 1978). The MTL resembles the Sikhote-Alin fault in fault dimension, slip sense and generation age (Ichikawa, 1980; Otsuki and Ehiro, 1978) as will be discussed later in detail Both of these faults are accompanied by granitic rocks of Late Cretaceous to Early Paleogene age; these rocks are in the southeast area of the Sikhote-Alin fault and in

a

Fig 3. Three magnified RYE>ke younger

gmnitic

areas from Fig. 2. Block termination rocks

extend

to the north. Central

a. Chubu,

Kyushu

region.

is inferred Kinki

from northern

and eastern

limit of the Ryoke

Chugoku

Block f. = block termination.

regions.

older granitic

b. Central

Chugoku

rocks. The regior 1. c.

388

Y. KANAORI

LATE

MESOZOIC-CENOZOIC

a

e

BLOCK

ROTATION,

Ryoke younger

b

granitic

389

SW JAPAN

Late Cretaceous to Ea;ly Paleogene (Ryoke younger granitic activity)

& Rhyolitic /

AND

Mid-Cretaceous

C A

STRIKE-SLIP

Late Cretaceous (Ryoke older granitic activity)

Paleogene to Middle Miocene (Pre-opening of the Sea~of Japan)

Late

4/

rocks and

Granitic rocks and granite porphyritic

Middle Miocene’to Early Pleistocene (Post-opening of the Sea of Japan)

f Volcanic

rocks

: Earthquake Thrust

00 Fig. 4. Structural Thin arrows

and igneous

in (e) indicate

(2) Hirooka

evolution

paleomagnetic

fault

/

“0 of the inner belt in Southwest declinations

(1986), (3) Tosha and Tsunakawa

of Miocene

Japan

from the mid-Cretaceous

rocks before 15 Ma and references

(1981), (4) Hayashida

and Ito (1984) and (5) Otofuji

(a) to Holocene

are shown as numbers; and Matsuda

period

(r).

(I ) and

(1983, 1984).

390

Y. KANAORI

the northwest

area of the MTL.

Sikhote-Alin

fault

is regarded

Accordingly,

the

as counterpart

of

the MTL.

agreement

with a kinematic

the region

which

is one of two kinematic

zone evolution

lying

model

across

of contraction

between

proposed

the two faults, models

by Martel

for fault

et al. (1988).

Structural and igneous evolution Late Cretaceous Using (Nur

a strike-slip

et al.,

connection

1986; with

curred

in

igneous

evolution

and Ron

the

Japan,

during

to the present

the

rotation

movement

are distributed

1984, rotation

the

the period

Cretaceous lateral

rotation

et al.,

regional

Southwest

block

block

model

1986)

in

that

oc-

structural

and

from the Late

could be interpreted

movement of parallel

induced faults.

by

by

These

in the region lying between

leftfaults

the two

major bounding faults, i.e. the MTL and Sikhote-Alin fault. Figure 4 schematically lustrates probably Japan

the il-

the structural and igneous evolution that occurred in the inner belt of Southwest from

mid-Cretaceous

until

the

Holocene

period.

The Japanese Islands were located at the margin of the Eurasian continent. The MTL and the Sikhote-Alin fault began to move left-laterally. aligned

and become

parallel

which

grow

faults in a later stage,

in the region lying between

major bounding

the two

faults.

Late Cretaceous (Ryoke Initiation

fractures,

block boundary

are generated

Since faults

the

left-lateral

increasingly

slip of the two major gressed, all

extensive

regions.

granitic

filled

along

boundary

and the left-lateral faults

rotation a

also pro-

was induced

great

gaps formed

quantity

in of

by the rotation

mainly in the Chugoku region and extended to wide areas north of the gaps mainly in the Chubu and Kinki. The angles between the MTL and inferred block terminations are estimated about 35 o in the Chugoku other

regions,

may

be related

region

respectively.

and about The

to the block

angle

rotation

20 o in the difference angle

which in turn reflected

be-

in the

Ryoke younger granitic activity. This may cause the MTL to bend around the Kinki region and move about 80 km northwestward west of the Kinki region. Additionally,

a great quantity

such as the Nohi rhyolitic uted

around

along

block

in the region of rhyolitic

rocks,

rocks which are distrib-

the Atera

fault,

erupted

probably

boundary

faults,

while

afterward

rocks intruded

into various

tures such as block termination boundary faults in all regions.

older granitic activity) movement

of block

bounding

block

Accordingly,

rocks

granitic

of left-lateral

slip

progressed

tween both regions,

Mid- Cretaceous

NW-SE

(Ryoke younger granitic activity) to

Early Paleogene

types of fracand

block

the

two major bounding faults generated the same sense of movement in the NW-SE oriented fractures in the region lying between the major faults,

Late Paleogene

rotating each block clockwise. Plutonic activity occurs in triangle gaps formed by the rotation and

Although fault activity and block rotation were generally weak, granitic activity occurred locally

results

along

in an intrusion

rocks. Strike-slip

of Ryoke

movement

older

granitic

of the block bounding

faults generated Riedel shear fractures inside the block which were derived from the fault line (Tchalenko and Ambraseys, 1970; Wilcox et al., 1973). The left-lateral slip of block boundary faults and clockwise rotation of blocks by left-lateral slip of the two major bounding faults are in good

to Middle

Miocene

(pre-opening

of

the Sea of Japan)

the block

Chugoku

boundary

fault

in the western

region. At the end of this period, volcanic

activity occurred along block termination and block boundary faults in all regions. Paleomagnetic declinations measured from Miocene rocks that were created before 15 Ma are also shown in Fig. 4e. The direction of the declinations point out the same orientation, independent of their position.

LAW

MESOZOIC-CENOZOIC

STRIKE-SLIP

AND

BLOCK

ROTATION,

391

SW JAPAN

IOlJUm -

Fig. 5. Geological

Middle Miocene

cross section of the Nobi plain after Kuwahara

(post-opening

of the Sea of Japan)

and

their

(1968), see Fig. 3a for location.

secondary

faults

such

as Riedel

shear

to Early Pleistocene

fractures

According to the study of paleomagnetic data by Otofuji and Matsuda (1983, 1984) the opening

Discussion

of the Sea of Japan, was believed to cause a rotation of the Japanese Islands at about 15 Ma.

Strike-slip ring between

This also caused regional rotation along blocks which set the Japanese Islands

tioned above are found in abundance with various scales from the outcrop to the regional scale (Freund, 1974; MacDonald, 1980; Mandl, 1987;

present

location

and formed

their present

two thin in their shape.

(Fig. 6).

and block rotation movement occurtwo major bounding faults as men-

In addition, the rotation at this time caused one block to thrust up an adjacent block resulting in

Martel et al., 1988). For example, the strike-slip and block rotation movement was clarified by

tilting

paleomagnetic

of the raised

block.

In the southwestern

part of the Chugoku region (central Kyushu), volcanic activity occurred with the block rotation, possibly

causing

the MTL south of this region

rotate 15-20” counterclockwise the MTL in the central Shikoku

to

about a pivot on region.

tion

evidence

mechanisms

to Holocene

al., 1986) in North Zealand

Chugoku

region

and in the area that was rotated

in the Early Pleistocene (Huzita, 1980). The activity developed in a very wide area and has continued to be active even at present. This is prob-

America,

plate-boundary

this movement dence

Volcanic activity occurred locally along Riedel shear fractures in the Chubu region, at the end of the block boundary fault in the central-north

earthquake around

generathe San

Andreas fault (Luyendyk et al., 1980; Nicholson et al., 1986), the Lake Mead fault system (Ron et

well as in northern Middle Pleistocene

and

in the region

found

in a part of the New

zone

was identified in

(Lamb,

1988),

as

Israel (Ron et al., 1984). Also, southern

Iran

by geological (Freund,

evi1970).

However, implications of gaps formed by the block rotation and phenomena associated with gaps have not yet been clarified. Evidence of strike-slip and block rotation movement implying the evolution of geological structural and igneous rock formation are discussed

below.

ably related to the initiation of a right-lateral slip movement along the MTL (Ichikawa, 1980) which

Major bounding faults and the movements

causes the same slip sense along block boundary faults in the Chugoku region producing subsequent block rotation. Tilting of the raised blocks continues even to present times (Fig. 5). Recent earthquake faults have occurred along the block boundary faults

The formation of the two faults working as major bounding faults, the MTL and the SikhoteAlin fault, and their left-lateral movement are thought to be originated by the oblique subduction of an oceanic plate (Karig, 1980). The MTL was known to be generated during Early Creta-

Y. KANAORI

392

1948 F;&i

Earthquake

1

Nobi Earthquake -

Upheaval (in meter) Subsidence Earthquake

fault

0.05

LATE

MESOZOIC-CENOZOIC

STRIKE-SLIP

AND

BLOCK

ROTATION,

trast, this paper interprets that these active faults are initiated due to large-scale strike-slip faulting during the mid-Cretaceous. ENE-WSW oriented faults such as the Atotsugawa fault have been widely accepted to be conjugate companions for the NW-SE aligned faults in the Chubu region. Observations of microstructures found in Jurassic Funatsu-type granitic rocks (Kanaori, 1986) revealed that the Atotsugawa fault was already formed before the Early Cretaceous (Kanaori et al., 1988). Accordingly, the Atotsugawa fault existed no later than the initiation of the major bounding faults. Almost all microearthquake activity around the active faults in the Chubu region has occurred at a depth no less than 15 km (Wada et al., 1979) suggesting that block boundary faults exist up to this depth and that a brittle-ductile transition zone also exists at this shallow depth because of a high geothermal gradient in a mobile belt such as the Japanese Islands (T. Takeshita, pers. commun., 1988). Blocks eventually seem to be buoyed on the ductile lower crust. Mesozoic (Early Cretaceous) to Paleozoic basement rocks constituting blocks are parts of a continental crust which was separated from the eastem margin of the Eurasia at the opening of the Sea of Japan (Faure and Lalevee, 1987). Gravity anomalies measured in the Chubu region by Yamamoto et al. (1982) clarified that the Atera fault sharply changed the Bouger anomaly pattern. This is also evidence that the Atera fault is a crustal fault extending to a deeper level.

ceous time and showed a predominant left-lateral movement (Ichikawa, 1980; Hara et al., 1980), except for a right-lateral movement in part of the Shikoku region during the Quaternary period (Ichikawa, 1980; Okada, 1980). During the rightlateral period, blocks in the Shikoku region probably rotated counterclockwise, in contrast with the clockwise rotation before this time. The width of the shattered zone of the MTL measures 100 m or more (Kanaori et al., 1980; Kakuta et al., 1980). The Sikhote-Alin fault having a 2-km wide shattered zone has also moved dominantly in a left-lateral sense since the Early Cretaceous (Otsuki and Ehiro, 1978). Because both faults have common features such as generation in the Early Cretaceous age, the predominant left-lateral movement and wide shattered zones, they may be considered as the major bounding faults. The Tsushima fault is probably linked to the SikhoteAlin fault through the Sea of Japan (Otsuki and Ehiro, 1978). Block boundary fault and block termination gap Since the faults bounding blocks were originated as strike-slip faults, most have continued such displacement even to the present time. Some faults such as the Neodani, Atera, Yanagase, Hanaore and Sakai-toge faults (Fig. 6a), which were investigated in detail, are accompanied by wide shattered zones of several tens to one hundred meters (Ogata and Honsho, 1981; Muto et al., 1981; Yoshioka, 1986; Kano and Sato, 1988). The existence of a wide shattered zone means that the faults have experienced repeated movement while remaining in the same location since the Late Cretaceous. Active faults have previously been accepted as conjugate shear faults which were formed by an E-W compression during the Early Quatemary period or Neogene time (Huzita, 1980). In con-

Fig. 6. Fault distributions

Igneous activity in block termination gaps Blocks composed of basement rock maintained a constant length and width during the period from the Late Cretaceous to the present. Block rotation which was induced by strike-slip produced igneous activity at the block termination

in central Japan and surface displacements

central Japan. MIX = Median Tectonic 3 = Atera fault, 4 = Yanagase

Line, ISTL = Itoigawa

fault, 5 = Hanaore

faults shown in Table 2. b. Surface displacements

393

SW JAPAN

around earthquake

Shizuoka Tectonic

fault, 6 = Kongo fault, 7 = Yamazaki of the 1948 Fukui Earthquake,

faults. a. Local and main active faults in

Line. I = Atotsugawa

fault, 2 = Sakaitoge fault,

fault. Roman numerals denote earthquake

the 1891 Nobi Earthquake

the 1945 Mikawa Earthquake (Iida, 1978).

(Muramatsu,

1976) and

394

Y. KANAORI

gaps and through son

et al.,

existing

the block boundary

1975;

Sibson,

in block

quence

1987).

termination

of block rotations.

is not necessary the northern Ryoke.

However,

inferred Ryoke

termination

corresponding

radially

gap (Hara

from

block termination,

with an increasing

including

Southwest respect

rotated

more

can be

of the

parallel triangle

the eastern

to

than margin

gap

in the central

to

was rotated

of Eurasia

Chubu

region

part

Japan

of was to

since 20 Ma. Itoh rocks found

region had the same declinarocks of the Late Creta-

from those found

considered

line

of

with respect

Miocene

that

which could not be identified tectonic

rocks of the Late Cretaceous

Japan,

40” clockwise

tion as the Nohi rhyolitic He

margin

to the main

that Lower

ceous age differing regions.

ages of Miocene

the eastern

last 100 Ma, Southwest

(1988) revealed

rocks of

that while

or less with

rocks of

area caused by block rotation. Ages of igneous

Eurasia,

during

the vertex

to have been formed

with the absolute

or rocks,

Eurasia

et al., 1980). The schistosity

can be considered

strata more

limits in all regions. in older granitic

in connection

with

many block terminations

(S,) found

develops

triangle

rocks

are a conse-

of the older granitic

from the northern

Schistosity

gaps

The block

to be exactly

limits

faults (Sib-

Igneous

existed

west

and that the block

in both side

a block

boundary

as an active fault or of the central

rotation

Chubu

angle changed

Early Paleogene become younger from south to north (Kanmera and Matsuda, 1980). This is also evidence of gradual gap spreading to the north.

spect to West Kyushu

Similarly, the rocks become younger in age from west to east (Matsumoto, 1977) suggesting that

be fixed to Eurasia of Japan.

the block movement

The Itoigawa-Shizuoka Tectonic Line and the Tsurugawan-Isewan Tectonic Line are considered

Plutonic Paleogene termination during block

progressed

in this direction.

rocks from Late Cretaceous to Early age extend to the north from block gaps. Also, volcanic

the same period extend boundary

fault.

rocks that erupted to both sides of the

These igneous

rocks broke

and separated both the side blocks, and rotated the blocks that surround them. Because igneous activity in Late Cretaceous to Early Paleogene times occurred mainly in the region lying between the MTL and the Sikhote-Alin fault (Kinoshita and Ito, 1988) triangle gaps should have originated in the area southeast of the Sikhote-Alin fault and its extension, symmetrical to the area north of the MTL. The area southeast

of the Sikhote-Alin

has not yet been investigated

in detail,

fault

since most

of the area is submerged beneath the Sea of Japan and active fault distributions in the Eurasian continent have not been sufficiently examined, compared with those in the Japanese Islands. Japanese

as

Islands rotation and block rotation

Kawai et al. (1961) concluded from the paleomagnetic study that Northeast Japan rotated counterclockwise to its present position. On the other hand, Otofuji and Matsuda (1983, 1984, 1987) found evidence from the paleomagnetic data,

along the boundary. Faure et al. (1988) confirm that Southwest Japan rotated clockwise with re-

to be important (Kuwahara, rotation west

and assumed

during

tectonic

1968; Okada, as discussed

to

of the Sea

lines in Southwest

Japan

1980). Although

block

often occurs in numerous

Japan,

the latter

the opening

places in South-

above,

the

geological

features of the locations have not yet been evaluated. The present study showed that further rotation in Southwest Japan occurred along the Hanaore fault-Kong0 fault line, because recent tilting basins such as the Osaka plain and Lake Biwa exist at the respective sides of the fault line. A number the tectonic rotation structures

of active

thrusts

lines can be found

and

construction

(Yokota,

especially

around

to be related

of complex

1981; Huzita,

to the

geological

1980; Ichikawa,

1980). To summarize, regional rotation occurred along the three main tectonic lines and also along each block bounded by active faults and tectonic lines. When paleomagnetic declinations measured at one point are compared with those at other points, the declinations will be found to be related to the regional structure. Bending of the entire Japanese Islands formed the main deformation structures of geologic terranes (Faure and Lalevee, 1987). Yanai and Otoh (1990) reported that megakinks were commonly

LATE

MESOZOIC-CENOZOIC

STRIKE-SLIP

AND

BLOCK

ROTATION,

SW JAPAN

found in metasedimentary and sedimentary rocks that developed in the outer belt of Southwest Japan and that these structures were actually formed by a NE-SW compression, which was already pointed out by Takeshita (1982). The bending and mega~nking structures and the NESW compression (which differs from the present NW-SE compression) does not contradict the mechanism causing the regional rotation in Southwest Japan. The collision of the Izu peninsula and central Japan during the Quatema~ period (Matsuda, 1978) can also be responsible for local block rotation in the southeastern part of the Chubu region. Since the NNE-SSW alignment of the Ryukyu arc was parallel to that of the MTL before the rotation of the Japanese Islands of 15 Ma, flextural structures found in the outer belt of southwestern Kyushu (Takeshita, 1982; Murata, 1987) did not exist before this time. When the Japanese Islands were restored to their original position before the Cretaceous, some geological terranes in Southwest Japan were possibly related to other terranes found in the Eurasia continent. The marginal zone of the Hida terrane correlated to the Okcheon belt in the Korean Peninsula (Hiroi, 1980) and the Ashio-MinoTanba-Chugoku terrane was continuous to the Sikhote-Alin zone in the Eurasian continent (Mizutani, 1987). Major strike-slip faults in Northeast Japan such as the Tanakura, Hatagawa, Futaba and HizumeKesennuma faults (Otsuki and Ehiro, 1978; Koshiya, 1986), are all NNW-SSE trending, left-lateral ductile faults. Recently, the Abukuma metamorphic rocks were known to be formed by a

TABLE

395

left-lateral ductile shear movement during the Early Cretaceous (Faure et al., 1986). The MTL is considered to extend to the Tanakura Tectonic Line in Northeast Japan (Niitsuma et al., 1986). The Tanakura Tectonic Line became parallel to the Sikhote-Alin fault when Northeast Japan rotated 40“ clockwise (Kawai et al., 1961). Biock tilting basins The Nobi plain is located on the east side along the Tsurugawan-Isewan Tectonic Line while the Osaka plain is found on the west side along the Hanaore fault-Kong0 fault Line. Lake Biwa is located between the lines. These plains are tilting basins generated after the regional rotation at the opening of the Sea of Japan during the middle Neogene. This shows that thrust faults were formed by regional rotation and the underthrusted or raised blocks began to tilt to form active sedimentary basins. Figure 5 shows a geological section of the Nobi Plain as an example of an active tilting basin. Sedimentary covers younger than the Miocene age have been deposited with the tilting. This is also evidence that regional rotation produced a block piling. Earthquake

activity

All earthquakes with surface deformation features occurred along block boundary faults and the Riedel shear fractures derived from the faults (Table 2). For example, as shown in Fig. 6, surface deformation induced by the activity of three large

2

Earthquakes

with surface

faults in the inner belt of southwest

Number

Date

Earthquake

I

Oct. 28, 1891

Nobi

name

Japan

since 1891 (after

M *

Surface

fault name

8.0

Neodani

fault etc.

Usami,

1975) Location

of main occurrence

Block boundary associated

II

March I, I927

Northern

Tango

7.5

Gomura

and Yamada

fauits

Block bound* associated

III

Sept. 10,1943

Tottori

7.4

Shikano

fault

IV

Jan. 13,1945

Mikawa

7.1

Fukozu

and Yokosuka

V

June 28,1948

Fukui

7.3

no name

* Japan

Meteorological

Agency

Scale.

fault and

Riedel shear fractures fault and Riedel shear fracture

F&de1 shear fracture faults

Block boundary

fault

Block boundary

fault

396

earthquake faults reveals one single block boundary fault. The Neodani fault of the 1981 Nobi Earthquake (M = 8.0, Japan Meteorological Agency Scale), the Fukozu and the Yokosuka faults of the 1945 Mikawa Earthquake (M = 7.1) and a non-named fault of the 1948 Fukui Earthquake (M = 7.3) exist on a single straight line (Iida and Sakabe, 1972). This line distinctly reveals one block boundary. This suggests that the entire block boundary fault does not move simultaneously, but moves in different places along a particular length. Summary and conclusions

Igneous rocks from the Late Cretaceous to the present are widely distributed in basement rocks composed of sedimentary, metasedimentary and granitic rocks, crystalline schist and gneiss of Mesozoic (Early Cretaceous) to Paleozoic age in the inner belt of Southwest Japan, which is located in a region north of the MTL. Based upon the type, the orientation and distribution of active faults and tectonic lines, the inner belt can be divided into four regions; the Kanto, the Chubu, the Kinki and the Chugoku. They are bounded by the Itoigawa-Shizuoka Tectonic Line, the Tsurugawan-Isewan Tectonic Line and the Hanaore fault-Kong0 fault line. Active thrusts aligned NNE-SSW exist in the Kanto region; left-lateral active faults aligned NW-SE exist in the Chubu region; active thrusts with a N-S orientation dominate in the Kinki region, while a few strike-slip active faults of an ENE-WSW or a WNW-ESE alignment are found in the Chubu region. Each region can be subdivided into small blocks bounded by active faults and tectonic lines. Assuming that blocks were parallel to each other when the inner belt is restored to its original position before the Late Cretaceous, the resulting MTL is oriented in the NE-SW direction and parallel with the Sikhote-Alin fault in the Eurasian continent. The inner belt is found between the two faults. These faults originated before the Late Cretaceous having a left-lateral slip. When the strike-slip and block rotation model is applied to the region lying between the two faults, the model can geometrically interpret the

Y. KANAORI

igneous and structural evolution of the inner belt of Southwest Japan. This evolution can be traced as follows. (1) Mid-Cretuceourr: The Japanese Islands were located at the margin of the Eurasian continent. The MTL and the Sikhote-Alin fault started to move, forming NW-SE oriented parallel fractures. (2) Lute Cretaceous (Ryoke older granitic activity): The initiation of left-lateral movement along the two major bounding faults generated a movement of the same sense on the NW-SE fractures in the region lying between the major faults, i.e.., a clockwise rotation of each block. Plutonic activity occurred in triangle gaps formed by the rotation and resulted in emplacement of Ryoke older granitic rocks. (3) Lute Cretaceous (Ryoke younger granitic activity) to Early Paleogene: Since the left-lateral slip of block boundary faults progressed with the ongoing left-lateral slip of the two major bounding faults, a large quantity of granitic rocks filled the gaps and extended to a wide area north of the gaps. This may cause a difference in the angle of rotation between the Chubu and the other regions. The rotation angle difference possibly moved the MTL about 80 km northwestward in the region west of the Kinki region. Additionally, a large quantity of rhyolitic rocks erupted possibly along block boundary faults, followed by granitic rocks intruding into various types of paths such as block terminations and block boundary faults in all regions. (4) Lute Paleogene to Middle Miocene (preopening of the Sea of Japan): Although the fault activity and block rotation were generally weak, the granitic activity occurred locally along the block boundary fault in the western Chugoku region. At the end of this period volcanic activity also occurred along block terminations and block boundary faults in all regions. (5) Middle Miocene (post-opening of the Japan Sea) to Early Pleistocene: The opening of the Sea of Japan, probably at about 15 Ma, caused regional rotation along three block boundary faults which resulted in the setting of the Japanese Islands in their the present location and formed their present shape. Due to the rotation, one block thrusted up an adjacent block to cause tilting of

LATE

MESOZOIC-CENOZOIC

STRIKE-SLIP

AND

BLOCK

ROTATION.

the raised block. In the southwestern part of the Chugoku region (central Kyushu), volcanic activity occurred when the small block was rotated causing the MTL to move southward 15-20” clockwise around the pivotal point located on the MTL in the central Shikoku. (6) Middle Pleistocene to Holocene: Volcanic activity occurred locally along the Riedel shear fractures in the Chubu region, at the end of the block boundary fault in the north-central Chugoku region and in the area rotated in the former period. The volcanic activity in the central Kyushu develops over a very wide area and continues to be active even at the present time. Recent earthquake faults occur along block boundary faults and their secondary faults. Presently, tilting still continues in the blocks raised by underthrusting. Fault and igneous activity occurring in the inner belt of Southwest Japan has been discussed. Since a small-scale strike-slip and block rotation can be expected to occur locally, the geometrical model will be able to interpret local sedimentary basins and igneous activity that originated since the Late Cretaceous.

SW JAPAN

391

Faure,

M.,

Cadet,

framework Faure,

M., Fabbri, constraints

Earth

R.,

Freund,

I., Shyoji,

argon

age of igneous

DokI. Akad. Choubert, Faure,

Unesco,

M. and

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I.K., 1972. The Potassium-

rocks

Sci., U.S.S.R.,

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F., 1987. Geology,

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15: 49-52.

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S. and Hide,

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References

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paleomagnetic

The author would like to express his sincere appreciation to Dr. M. Ada&i of Nagoya University for critical reviews of the manuscript, to Dr. T. Takeshita of Ehime University for a critical reading of an earlier manuscript and helpful comments, to Prof. K. Yairi and Dr. S. Kawakami of Gifu University for very useful discussions, and to Mr. Y. Endo for supplying data for the Neodani fault. A part of this work was supported by a Grant-in-Aid from the Ministry of Education, Science and Culture of Japan (No. 63540608).

F., Gusokujima,

of the Abukuma

and

Acknowledgment

Lalevee,

J.P., 1986. The pre-Cretaceous

with

the

Mikawa

of the Fukozu

Earthquake

in 1945.

(in Japanese).

Differential

Japan

ceous and Neogene

inferred

rotation

of the eastern

from paleomagnetism

rocks. J. Geophys.

part

of

of Creta-

Res., 93: 3401-3411.

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