Opening of the Kuril Basin deduced from the magmatic history of Central Hokkaido, North Japan

Tectonophysics, 174 (1990) 235-255 Elsevier Science Publishers

235

B.V., Amsterdam

- Printed

in The Netherlands

Research Papers

Opening of the Kurd Basin deduced from the magmatic history of Central Hokkaido, North Japan JIN’ICHIROU

MAEDA

Department of Geology and Mineralogy, Faculty of Science, Hokkaido University, Sapporo 060 (Japan) (Received

September

28.1988;

revised version accepted

June 30, 1989)

Abstract Maeda,

J., 1990. Opening

of the Kuril Basin deduced

from the magmatic

history

of Central

Hokkaido,

North

Japan.

Tectonophysics, 174: 235-255. The Kuril Basin is a fan-shaped arc from the Pacific Ocean

back-arc

to the southeast,

the Late Eocene to Early Miocene The elastic sediments

constituting

Kuril arc, were transported that the Kuril Okhotomorsk

Hokkaido

magmatic

and that the Proto-Kuril

arc in Central beneath

during

arc magmatism

slab covered

Hokkaido

the Eurasian

16-15

arc, which underlies

arc had been located

and by

the modem

in the southern

This suggests margin

of the

that the Kuril Basin was not entrapped

at 17-16

Ma and coevally

an extensional

sedimentary

along

retreat

basins.

arc during

cover on the rugged

along a N-S

trending

was taking

its N-S

trending

of the Proto-Kuril the Middle

place.

by the

the opening

zone where

extension,

Miocene

it is suggested

that the

system.

is inferred

from the occurrence

to subsidence

of the Kuril Basin. The occurrence

of the Kuril Basin is consistent

on the east

Based on the fact that the

arc-trench

This stress field can be attributed

basement

subduction

the Kuril Basin was not yet present

magmatism

stress regime during

sedimentary

Proto-Kuril

was formed

plate. Therefore,

when the Hidaka

and graben-like

by the retreated

and horizontal

by the Kuril

Block to the north

them the Kuril Basin now intervenes.

time. It also indicates

the period

ceased

Hokkaido,

magmatism

to Early Eocene Proto-Kuril

Block; between

Kuril Basin opened just after 16 Ma by the southward In Central

rim. The basin is separated Okhotomorsk

Kuril arc. Hidaka

the Pacific plate was subducting

A-type

Pacific

by the subsided

arc to the west.

the Late Cretaceous

from the Okhotomorsk

Basin had not opened

The Tertiary

Hidaka

magmatic

Block at least until Early Eocene

Proto- or modern

of Central

Hidaka

basin in the northwestern and is bounded

of

of the Pacific

of an undeformed

with rather

rapid opening

at

Ma.

During oceanward

the Middle retreat

the Pacific-North

Miocene,

of an arc-trench American

may have been caused

the Kuril Basin and Japan system.

boundary

by the migration

Opening

for the Kuril

of a hot region

across

Hokkaido Island is located at the junction of the Japanese Islands and the Kuril Islands, and is bounded by the Kuril Basin to the east and by the Japan Basin to the west (Fig. 1). The origin of the Kuril Basin, as well as that of the Japan Basin, has until now been controversial. Savostin et al. (1983) considered that the Kuril 0 1990 Elsevier Science Publishers

basins

by a similar

B.V

the overriding

process,

along different

Basin and the Pacific-Eurasian

Introduction

0040-1951/90/$03.50

Basin were formed

of the two back-arc

boundary

i.e., rotation

plate boundaries, for the Japan

and i.e., Basin,

plate boundary.

Basin was formed by retreat of the back-arc plate during Miocene time. Kimura and Tamaki (1986a) also mentioned that the opening of the Kuril Basin was caused by the northward retreat of the back-arc plate (the Okhotomorsk Block of Parfenov and Natal’in, 1986) related to the India-Eurasia collision. Kimura and Tamaki (1986b) estimated that the opening age of the Kuril Basin was during Late Oligocene to Early

236

J. MAEDA

EURASIAN

CONTINENT

PACIFIC

Fig. 1. Map showing

the island-arc

and back-arc

basin systems

on the pivot of the fan-like

around

the Japanese

Kuril Basin. Dashed

Miocene time, based on the basement depth, sediment thickness, and heat flow values. On the contrary, Niitsuma and Akiba (1986) thought that the Kuril Basin was opened by the southward migration of the Proto-Kuril arc between 15 and 13 Ma. Recently, Jolivet (1987) has demonstrated that the Japanese Islands migrated toward the south, relating to the relative movement between the Eurasian and North American plates. He explained that the Kuril Basin opened due to the southward retreat of the eastern half of Hokkaido during Middle Miocene time.

Islands

lines are Quaternary

OCEAN

and Kuril Islands. volcanic

Star indicates

a cusp

front.

In this paper, the origin of the Kuril Basin is discussed on the basis of the on-land geology of Hokkaido, especially the Tertiary magmatic history in Central Hokkaido. Brief review of the topography and geology of the Sea of Okhotsk region According to the descriptions by Gnibidenko and Khvedchuk (1982) Savostin et al. (1983), and Gnibidenko and Svarichevsky (1984), the Sea of Okhotsk region is topographically and geologically

OPENING

OF THE

Fig. 2. Submarine meters.

N.O.-North

KURII.

237

BASIN

topography Okhotsk

of the Okhotsk Rise,

Sea region

K.t.Z-Kashevarov

Oceanology

modified Linear

Rise, and AX-Academy

divided into three parts, i.e., northern, central, and southern parts. The northern part is on the north of the Kashevarov Linear Zone (Fig. 2) and is

from Gnibidenko Zone,

D-Derngin

and Khvedchuk Deep,

T-Tinro

(1982). Basin,

Depth

contours

Z.O.--Institute

in of

of Science of the USSR Rise.

occupied by the North Okhotsk Rise. The central part is between the Kashevarov Linear Zone and Km-3 Basin, and includes the Academy of Scien-

238

.I MAI-I>A

ces Rise,

the Institute

of Oceanology

Derugin

Deep,

and other

southern

part corresponds

Rise,

rises and troughs.

the The

to the Kuril Basin.

the Kuril Islands ern margins normal and

Northern and central Sea of Okhotsk

regions

faults towards

Svarichevsky,

Kuril

Basin,

continental In the northern depth

part of the Sea of Okhotsk,

of the sea floor is shallower

and in the central

part between

(Fig. 2). The northern is underlain Gnibidenko

by

1983).

Crustal km.

Okhotomorsk

Sea of Okhotsk

a continental-type

thickness A

thickness horizontal,

Block, is assumed

to constitute

the the

central part of the Sea of Okhotsk (Dickinson. 1978; Parfenov et al., 1978). Several scientists have mentioned that the Okhotomorsk Block, which belonged

to the Kula

plate,

migrated

from

the

south and collided against eastern Siberia during Late Cretaceous time (e.g., Kimura and Tamaki, 1986b). After the amalgamation of the Okhotomorsk Block with the North American plate, a new arc-trench system, the Proto-Kuril arc-trench system, began to come into being along the southern margin of the Okhotomorsk Block. The acoustic basement in the northern part is overlain by an undeformed sedimentary cover several kilometers thick which includes accumulations since Late Cretaceous time (Savostin et al., 1983). On the other hand, the sedimentary cover observed in the central part is generally very thin, 200-300 m on average, and deformed. The deposition is estimated to begin in the Late Miocene, accompanied by contemporaneous deformation (Savostin et al., 1983). Until the Late Miocene, the Okhotomorsk Block of the central part was uplifted above sea level, therefore the subsidence of the Academy of Sciences and Institute of Oceanology rises by more than 1000 m probably took place since the Late Miocene (Savostin et al., 1983). Kuril Basin-southern

Sea of Okhotsk

region

The Kuril Basin in the southern Sea of Okhotsk is a fan-shaped back-arc basin of about 3300 m depth and is isolated from the Pacific Ocean by

but

the depression lo-12

oceanic

km type

is not

by

(Gnibidenko beneath

thick, (e.g.,

1982; Savostin

The sedimentary

(e.g.,

is about

and south-

are bounded

the

is not

of

Gnibidenko

et al.. 1983). The

known

from

direct

sampling.

et al.,

parts

about

and Khvedchuk,

crust

microcontinent,

Basin

1984). The crust

age of the basement

1982; Savostin

in both

subsided

the

1000 m,

1000 and 2000 m

and central

and Khvedchuk,

25-30

than

(Fig. 2). The northern

of the Kuril

and

is

except

cover attains completely

topography

sedimentary (1984)

areas

and (Gnibi-

1984). Because the rugged

does not affect the overlying

cover, Gnibidenko

considered

m in

undeformed

in the marginal

denko and Svarichevsky, basement

3000-4000

that

and Svarichevsky

no crustal

extension

oc-

curred during its accumulation. The Kuril Basin is supposed to have been active in the recent past but appear no longer to be active at present, although it is still associated with active subduction (Uyeda and Kanamori. 1979). Outline of the pre-Tertiary geology of Hokkaido Based on the pre-Tertiary divided

into three blocks,

East Hokkaido

geology, Hokkaido i.e., West, Central,

is and

(Fig. 3).

West Hokkaido West Hokkaido corresponds to the N-S trending Oshima Belt, which consists of a Jurassic accretion complex formed along the eastern margin of the Eurasian continent (e.g., Kawamura et al., 1986; Kiminami, 1986). During Cretaceous time, the Jurassic sedimentary rocks were intruded by arc-type granitoids (Tsuchiya et al., 1986). Thus. the Oshima Belt was converted into a magmatic arc in the Cretaceous (e.g., Kiminami, 1986). Central Hokkaido Central Hokkaido trending Sorachi-Yezo ing to Kiminami et al. consists of Cretaceous (Yezo Group) which

corresponds to the N-S and Hidaka belts. Accord(1986), from west to east, it fore-arc basin sediments are underlain by Jurassic

OPENING

OF THE

KURIL

239

BASIN

CENTRAL

HOKKAIDO

141% 0 + 45’N

145% +45”N

EAST HOKKAIDO

WEST HOKKAIDO

q

',.',..~,.~,. :,:::. .. '. 6 6

8

145OE + 42’N

Fig. 3. Distribution Group,

of pre-Tertiary

5 = Yezo Group,

8 = peridotite

and serpentinite, granitoids.

rocks in Hokkaido.

6 = Hidaka

Supergroup

and 9 = Matsumae

2 = Nemuro

(Tertiary Group,

S. H. and N. P. in East Hokkaido

plutonic Kamiiso

Group, Group,

are Shiranuka

oceanic crust (Sorachi Group), serpentinites and the Kamuikotan high-pressure-type metamorphic rocks, and Cretaceous accretion complex (Hidaka Supergroup). All of them constitute a N-S trending westward subduction system related to the Cretaceous arc magmatism of West Hokkaido. This arc-trench system is called the Yezo arc-trench system in this paper. On the east of the Yezo arc-trench system, the Late Cretaceous terrigenous sediments (Hidaka Supergroup) were intruded by MORB-type greenstones (Fig. 4; Mariko, 1984; Miyashita and Katsushima, 1986). On the basis of such observations, Miyashita and Katsushima (1986) suggested

2 = Saroma

Group,

3 = Yuubetsu

rocks

are included),

and metamorphic Rebun

Group,

Hill and Nemuro

Kumaneshiri Peninsula,

Group,

4 = Nikoro

7 = Sorachi

Group,

Group,

and Cretaceous

respectively.

that the MORB-type greenstones were generated at an oceanic ridge near the continent. This oceanic crust is likely to have belonged to the Kula plate, because the Kula-Pacific boundary passed this region at about 60-55 Ma (Kimura and Tamaki, 1986b). It is, therefore, concluded that, during Late Cretaceous time, the boundary between the Eurasian running meridionally

and Pacific Basin plates was through Central Hokkaido.

East Hokkaido East Hokkaido comprises the Tokoro Belt and Nemuro Belt. According to Kiminami et al. (1986)

240

J. MAtDA

50km

11.2 Ma

Kamishiyubetsu

Magmatism

w2

Pipairo-Toyokoro

I +

Magmatism

* f.j Hidaka

I&K-\\.

Western

Chain

A Eastern

Chain

19.6 Ma 17.1 Ma

xw7.3

Ma

V-R

___..

is.1

Hidaka

Ma

Magmatism

Metamorphism

\t ’ \t

_ .

\’ /

Hidaka

Peridotite

Late Cretaceous(?)

.: 6 17.7Ma \

Fault and Thrust /

/c

MORB

OPENING

OF THE

it consists

KURIL

241

BASIN

of a Late Cretaceous

(Yuubetsu

and

high-pressure

Nikoro

accretion

Hokkaido

were reviewed

the

Tokoro

et al. (1986). Major-element

rocks (Nikoro

Group;

igneous

groups),

metamorphic

from Central

complex

rocks

collected

analyses from

in Maeda

of about 400

Central

Hokkaido

Sakakibara, 1986), and Late Cretaceous to Early Eocene fore-arc basin sediments and arc-volcanic

were obtained from fused-glass discs by X-ray fluorescence at Hokkaido National Agricultural

rocks

Experiment

Station

University

(Philips

University

(JEOL

(Nemuro

and Kontani,

and

can be interpreted The kaido

general

arc-trench

system.

geologic

structure

to Early

changes

Kushiro

Eocene

its trend

(Shiranuka

of Kushiro

groups;

Kiminami constitution

in terms of a Late Cretaceous

to Early Eocene Cretaceous

Saroma

1983). Such a geologic

dures of

rocks

from N-S

the

Late

of East

Hok-

Peninsula

Tsuchiya

association. Boundary of Tertiary volcanic

Mapped Thrust, plutonic front

granitoids

area in (A) matches

the rectangle

and KTZ-Kamishiyubetsu

Tectonic

rocks in Central

and back-arc Sakhalin.

basins, JT-

Hokkaido

trench,

MORB-like

are not divided

greenstones

El,

Western

E2, and WZ-IV5

Sakhalin.

Stars

indicate

KT-

Kurt1 trench,

Dashed

the location JB-

in Central

from meta-sedimentary

in (B). HWT-Hidaka Zone.

and southern

respectively. Japan

high K,O and medium Na *O + K,O, and represent a talc-alkaline nature (Figs. 5 and 6).

rocks and Cretaceous

S-type

rocks range

K,O and medium Na ,O + K,O contents (Fig. 5) and they show a talc-alkaline nature (Fig. 6). The ages of the Western Chain rocks range from 36 to 17-16 Ma. Field evidence shows that in general the magmatism varied with time from basic to acidic. The basic-intermediate rocks are characterized by low to medium K,O and Na,O + K,O contents (Fig. 5). They include tholeiitic and talc-alkalic series rocks (Fig. 6). The acidic-intermediate rocks are characterized by medium to

ages reported

plutonic

paper.

from 43 to 41 Ma. These plutons are acidic in general and are characterized by medium to high

Many Tertiary plutons are exposed in Central Hokkaido. Based on geological relationships and isotopic ages, these plutons can be divided into three groups as follows and shown in Fig. 4 (Maeda

of Tertiary

of the results

et al. (1986) and the complete

Eastern and Western chains (Fig. 4). Isotopic ages of the Eastern Chain

Magmatic history in Central Hokkaido during the Tertiary

are also shown.

et al. (1989). A discussion

and

derlain by the oceanic crust of the Kula plate. The extent of the Hidaka magmatism is subdivided into two parallel N-S trending chains, i.e., the

Late Cretaceous to Early Eocene, the E-W trending boundary between the North American and Pacific Basin plates was running near this region.

plutons

proce-

et al. (1980)

Hidaka magmatism began a 43 Ma and continued until 17-16 Ma. The Hidaka Magmatic Zone extends 300 km from north to south in Central Hokkaido. The Hidaka plutons intrude into the Late-Cretaceous Hidaka Supergroup, un-

the southern margin of the Okhotomorsk Block, which had amalgamated with the North American plate in Late Cretaceous time. Thus, during the

representative

Analytical

in Yamasaki

Hidaka arc magmatism

for the rotation of the E-W trending part (Nemuro Peninsula) (Y. Hamano, pers. commun., 1987). Therefore, the general trend of the Late Cretaceous to Early Eocene arc system is considered to be primarily E-W. As will be discussed later, this arc system was the Proto-Kuril arc formed along

Fig. 4. (A) Distribution

JSX-6OS7).

Hokkaido Yamaguchi

in the east

area) (see Fig. 3).

of the isotopic

and

data will be shown in a separate

Paleomagnetic data revealed that the N-S trending part (Shiranuka Hill) rotated clockwise by 70 ’ after the Cretaceous-Tertiary boundary (Hamano et al., 1986). On the contrary, there is no evidence

et al., 1986). Details

AFV777),

PW1404),

was given by Maeda

in the west of

Hill area) to E-W

(Nemuro

are given

(Toshiba

Boundary

correspond

Hokkaido.

rocks

Thrust,

Japan

HMT-Hidaka

ages of Main

areas are the Quatemary

Site 439 and distribution

Basin, and IL-

Isotopic

of their intimate

to those in Fig. 5. (B) Distribution

line with QVF and dotted of DSDP

because

Kuril Basin.

of granitoids

in

a SiO2

(wt.%)

SiO2

(wt.%)

b Fig. 5. Plots of the Hidaka El-E2

and Wl-W5

Wl -W5. %a,0

Low. medium,

+ K20

fields

plutonic

correspond

rocks on Na,O.

to the Eastern

high, and extremely

correspond

!o areas

K,O,

Chain high

and NazO

and Western

+ K,O Chain,

K 2O fields are divided

of tholeiite,

high-alumina

basalt.

vs. SiO, diagrams respectively.

modified

by the lines of Gill (1981). and

alkali

from Maeda

See Fig. 4 for explanation basalt

series

et al. (1986). of El-E2

Low, medium. defined

by Kuno

and

and high

(1966),

respectively.

The close association of talc-alkalic and tholeiitic series rocks indicates that the Hidaka magmatism is of subduction-related arc type. It is noted that the trend of the Hidaka arc is nearly perpendicular to that of the modern Kuril arc-trench system (Fig. 4). Since no notable variation is ob-

served in K20 and Na,O + K20 contents from north to south, both in the Eastern and Western chains (Figs. 4 and 5), the origin of the Hidaka magmatic arc may not be explained by subduction along an E-W trending consuming boundary. The Hidaka arc is considered to be related to an old

OPENING

OF -WE

KURIL

243

BASIN

FeO*

FeO*

Fig. 6. Plots of the Hidaka tholeiitic

plutonic

rocks on an AFM (Na,O

(TH) and talc-alkalic

+ K,O-total

series rocks (CA) defined

N-S trending subduction zone rather than the E-W trending Kuril arc subduction system. As shown in Fig. 4B, acidic to intermediate plutonic rocks with ages of 28.5 to 32 Ma are reported from the southeastern end of Sakhalin (Firsov, 1964). Although the petrographic and petrochemical characters are not clearly shown, these plutons are likely to represent the northern continuation of the Hidaka arc. It is noted that the northern end of the Hidaka arc nearly corresponds to the northern limit of the Kuril Basin

iron as FeO-MgO)

by Irvine and Baragar

diagram.

The boundary

between

(1971) is also shown.

rocks at Site 439 did not belong to the Hidaka magmatism (Fig. 4B). The Hidaka magmatism is intimately associated in space and time with the Hidaka regional metamorphism. The Hidaka metamorphic rocks are derived from pelitic and psammitic rocks and MORB-type greenstones of the Hidaka Supergroup. Metamorphic grade ranges from greenschist to granulite facies (Osanai et al., 1986b). Migmatitic and

rocks bearing garnet, cordierite, andalusite. sillimanite (i.e., S-type granitoids) are also

(Fig. 4B). Arc-type acidic to intermediate volcanic rocks were obtained from DSDP Site 439 (Fujioka, 1980) about 150 km off the east coast of North-

exposed (Kizaki, 1964; Komatsu, 1983). The S-type granitoids are not plotted in Figs. 5 and 6, and are excluded from consideration because they are derived from meta-sediments by anatexis (e.g.

east Honshu (Fig. 4B). 39Ar-“aAr ages of the volcanic rocks are 22-24 Ma (Yanagisawa et al., 1980). Site 439 is located at about 150 km south of Cape Erimo (Fig. 4B). Because of their similarities in geographic alignment and age to the Hidaka plutonic rocks, it was thought by Moore and

Owada, 1989). Although a concept of paired metamorphic belts consisting of the low-pressure Hidaka belt and high-pressure Kamuikotan belt was formerly proposed (Miyashiro, 1961; Matsuda and Uyeda, 1971), this concept is definitely abandoned because of the discrepancy in metamorphic age between the two belts (Okada, 1974). The paleogeothermal gradient of the Hidaka metamorphic terrain, obtained by pressure-temperature analysis, is 33” C/km on average and corresponds to that of active magmatic arcs (Osanai et al., 1986a).

Fujioka (1980) and Cadet and Charvet (1983) that the volcanic rocks at Site 439 represent the southern continuation of the Hidaka arc. It is noteworthy that the southern continuation of the Hidaka arc extends farther south from the southern limit of the Kuril Basin, even if the volcanic

244

J MAEDA

1.2

0.6

0.4

0.2

0.0

I

I

0

0.7

0.8

0.9

I

I

I

1 .o

1

1

I

1.2

1.3

1.4

AICNK Fig. 7. Plots of the Pipairo-Toyokoro Na,O

+ K,O

respectively.

in molecules) Boundary

diagram

lines among

data of Hine et al. (1978), Murata

rocks (large open circle) on a NK/A modified them (I-type:

from Maeda

et al. (1986).

small solid circle, S-type:

(1982), Kawasaki

vs. A/CNK I, S, and

(Na,O

+ K,0/A120,

vs. AI,O,/CaO

A are fields of I-, S-, and A-type

small open circle, A-type:

(1980), Collins et al. (1982), Murakami

cross) are drawn

et al. (1983). and Whalen

+

granitoids, based on the

et al. (1987).

FeO* Pipairo- Toyokoro anorogenic-type

magmatism

The Pipairo-Toyokoro magmatism, the second stage of magmatism in Central Hokkaido, occurred at about 15 Ma (Fig. 4). Major-element chemistry indicates that the Pipairo-Toyokoro rock suite is alkalic and belongs to the anorogenic type or A-type of Loiselle and Wones (1979) (Fig. 7). In general, A-type rocks intrude late in a magmatic cycle, or intrude crystalline basements that have undergone previous ultrametamorphism and/or magmatism. The Hidaka plutonic and metamorphic rocks maybe correspond to such a crystalline basement. A-type rocks occur commonly in an extensional tectonic regime (Collins et al., 1982). It is likely that during Middle Miocene

/

\

Y

NazO+KzO

m0

Fig. 8. Plots of the Kamishiyubetsu diagram

plutonic

rocks on an AFM

OPENING

time,

OF THE

KURIL

Central

extensional

245

BASIN

was dominated

Hokkaido,

by

an

to Early outer

stress regime.

Eocene

rocks

exposed

such

as in

East

arc,

Lesser Kuril Kamishiyubetsu arc magmatism

modern studies

The third-stage

magmatism

began

at about

Ma and is called the Kamishiyubetsu (Fig. 4). It extends Hokkaido, to the

alignment

of the

present

Kuril arc and is restricted front.

are characterized K,O,

magmatism

direction.

chain

by intermediate

SiO,,

and low Na ,O + K *O contents,

talc-alkaline

nature

and distribution

indicate

evolution

rocks medium

terrain.

and Saroma and

They

thought

from a northern that

consisting

Okhotomorsk and Kontani

Block

the terrain,

of continental-

mentioned,

the

(1983)

et al. (1986) showed

in the central

Sea. As already

the

of the

groups in East

Kontani

were transported Land”,

and

sedimentological

Okhotsk Kiminami

part of the this terrain

in

this

is

paper.

(1983) also concluded

that

the Nemuro and Saroma groups accumulated in a fore-arc basin of the “Paleo-Kuril arc” which was

that the Kamishiyubetsu of the modern

on

type crust, was exposed called

and show a

Based

the Kuril

a basement

(1983), and Kontani

“Paleo-0khotsk

of arc type (Fig. 8). The age

magmatism was a forerunner arc magmatism. Tectonic kaido

volcanic

to the inner side

The Kamishiyubetsu

arc.

Kiminami

that the elastics

of the

constitute

of the Nemuro

Kiminami

It is parallel

volcanic

Kuril

Hokkaido,

in East as well as in Central

with a NE-SW

of the volcanic

10

Islands,

along

Hokkaido

Kuril

located at the southern margin of the “PaleoOkhotsk Land”. It is estimated that the ancient fore-arc basin extends from East Hokkaido to the

of the Ku14 Basin and Hok-

Vityaz submarine rise over several hundred kilometers along the Kuril outer arc (Kiminami

Was the Kuril Basin of entrapped or of spreading origin?

and Kontani, 1983). Their “Paleo-Kuril arc” is named “Proto-Kuril arc” in this paper. Based on

The Kuril Basin is separated Kuril arc from the Pacific Ocean.

the above evidence, it is suggested that the Kuril Basin is not oceanic floor entrapped by the Proto-Kuril arc but was formed by seafloor

by the modern Late Cretaceous

53-48Ma

48-43Ma

;I,flly’

A-

NA

EU

PA

PA

$

3 2_

43-37Ma

37-OMa 1

EU

Fig. 9. Relative

plate motions

of North

EU

America

and the Pacific

to Eurasia.

(1986).

I

Data from Maruyama

(1984) and Maruyama

and Seno

246

48-43Ma

1

\\

1

\

1

NORTH

\

EAST

AMERICAN

PLATE

HOKKAIDO

PACIFIC

CENTRAL

PLATE

HOKKAIDO

/ \ I

43-37Ma

EURASIAN

IIDAKA

NORTH

AMERICAN

PLATE

PLATE

MAGMATIC

EASTERN

1

\ \ \

PACIFIC

ARCP-0

PLATE

CHAIN \

@c */

O/ \‘\i

i‘\

J,r

? ?

q\HIDAKA

TRENCH

.

17-17(or

18)Ma

[

\

I

EURASIAN

‘IDAKA

AMERICAN

PLATE

PLATE

MAGMATIC

WESTERN

NORTH

\

ARC\

i

PACIFIC

o’*

PLATE

l

CHAIN ,\\,/

---k

?

’

?

’ 1

Fig. 10. Tectonic

reconstructions

around

Hokkaido

during

the Tertiary.

Relative

shown by arrows.

motions

of the Pacific to the Eurasian

plates are also

OPENING

OF THE

KURIL

BASIN

241

16-15Ma NORTH

EURASIAN

TOKORO

AMERICAN

PLATE

PLATE

TECTONIC

LI

PACIFIC

PLATE

‘-+Q

/ I I

1%12Ma

EURASIAN

\ \ \

NORTH

AMERICAN

PLATE

PLATE

PIPAIRO-TOYOKORO ANOROGENIC TYPE MAGMATISM PACIFIC

PLATE

l pr

NORTH

AMERICAN

PLAT 0

EURASIAN

PLATE

:AMISHIYUl3ETSU(KURILl MAGMATIC ARC PACIFIC HIDAKA

THRUST

Fig. 10 (continued).

PLATE

248

J. MAEDA

spreading along a nearly between the Okhotomorsk

E-W trending rift(s) Block and Proto-Kuril

arc at least later than Early Eocene

time.

continental

crust in the overriding

the accreted

Kula

also a favored

explanation

Hidaka magmatic

for

the

origin

of the

arc

Westward Eastern sponds

The origin of the Hidaka a N-S

trending

consuming

was active during We apply Hokkaido

to the

calculated

plate boundary

the period magmatic

the relative

sian, Pacific,

arc may be related of 43 to 17-16 history

motions

and North

by Maruyama

among

American (1984)

to

which Ma.

in Central the Eura-

plates (Fig. 9) and

Maruyama

and Seno (1986), based on the model by Engebretson et al. (1985). As already mentioned, the boundary between the North American plate and Pacific Basin plate during Late Cretaceous to Early Eocene time was probably E-W trending and was located along the Kuril outer arc, therefore the position of the Hidaka arc is not expected to match the North American-Pacific boundary. Although the boundary between the North American and Eurasian plates was nearly N-S trending, it was a divergent zone from 43 to 37 Ma and was an inactive convergent zone with a very slow convergence rate (about 1 cm/yr) after 37 Ma (Fig. 9). These relative motions do not explain the magmatic activity of the Hidaka arc during 43 to 17-16 Ma. Thus, it is expected that the Hidaka arc was formed along the convergent zone between the Eurasian and Pacific plates. The Pacific plate changed its direction of motion from NNW to WNW at 43 Ma (Clague and Jarrard, 1973; Maruyama and Seno, 1986), therefore the nearly N-S trending boundary between the Eurasian and Pacific plates might have changed its character from highly oblique and inactive subduction to active subduction. This event agrees well with the beginning of the Hidaka magmatism in the Eastern Chain at 43 Ma. The convergence rate of the Pacific plate increased from 6 to 10 cm/yr at 37 Ma. Tholeiite-dominated basic magmatism, which began in 36 Ma or slightly later in the Western Chain, may be explained by this increase in convergent rate (see Miyashiro, 1972; Gill, 1981, pp. 218-221). The absence of

Chain

Gill,

to Western

Chain

to either the westward

If the above interpretation of the Pacific

from the

probably

advance

corre-

or shallow-

Pacific slab. for the origin of the

arc is valid, an influence

swing motion

is

1981, pp. 218-221).

of the magmatism

ing in dip of the down-going Hidaka

before,

of such basic magmatism

1972;

transition

plate, which is

as mentioned

condition

(see Miyashiro, Plate-tectonic

plate

of the oscillatory

plate (Jackson

et al.,

1975) to the Hidaka

arc might

though

swing means the “absolute”

motion

the oscillatory of the Pacific

along the Eurasia-Pacific fected. Counterclockwise

plate,

be expected.

the relative

AI-

motion

boundary would be afepisodes at about 35 Ma

and 20 Ma coincide with the remarkable intrusive activity at 36-35 Ma and 20-17 Ma in the Hidaka arc, respectively. On the contrary, the clockwise episodes synchronize with the relatively quiescent periods of magmatism. Based on the above

considerations,

it is con-

cluded that the Hidaka arc was formed relating to a N-S trending subduction zone, the Hidaka trench, along which the Pacific plate subducted westward beneath the Eurasian plate (Fig. 10). As already mentioned, the westward subduction zone of the Kula plate beneath the Eurasian plate during was located

the Cretaceous (i.e., the Yezo trench) on the west of the Tertiary Hidaka

arc. Therefore, since the Late Cretaceous to Middle Eocene, the subduction zone was stepped toward the east to the Hidaka trench and the part of the Kula plate between the Yezo trench and Hidaka trench was accreted to the Eurasian plate. This event may be related to the subduction of the Kula-Pacific ridge at about 60-55 Ma. opening

of the Kuril Basin

The Hidaka arc magmatism terminated at 17-16 Ma, although the motion of the Pacific plate relative to the Eurasian plate did not significantly change since 37 Ma. Moreover, on the east of Central Hokkaido, there is no N-S trending present-day topography indicating the subduction of the Pacific plate. Instead, the Kuril Basin and East Hokkaido are located there.

OPENING

OF THE

KURIL

249

BASIN

These problems demonstrate that the Km-i1 Basin was not present and the Pacific plate was present on the east of Central Hokkaido during the period when the Hidaka magmatism was taking place. Accordingly, it is likely that the ProtoKuril arc-trench system retreated southw~d and the Kuril Basin was formed between the retreated Proto-Kuril arc and the Okhotomorsk Block at least later than 17-16 Ma (Fig. 10). During the opening of the Kuril Basin, the Hidaka trench may have been converted into a transform fault and the westward subduction of the Pacific plate beneath the Hidaka arc stopped (Fig. 10). Thus, the opening of the Kuril Basin resulted in the cessation of the Hidaka arc magmatism. It is shown that the opening of the Kuril Basin proceeded rather rapidly, because the terminal age of the Hidaka magmatism is coeval (about 17-16 Ma) along the whole of the Hidaka magmatic arc, as far as examined in Hokkaido (Fig. 4). At about 15 Ma, the Pipairo-Toyokoro A-type magmatism was active in Central Hokkaido (Fig. 4). In general, activity of A-type rocks indicates a tensional stress regime. Such a stress field during Middle Miocene time is consistent with the formation of graben-like sedimentary basins in Central Hokkaido, such as the base-Monbetsu Oki and Tokachi basins (S. Miyasaka, pers. commun., 1986). This tensional stress field can be attributed to subsiding of the Pacific slab covered by the retreated Proto-Kuril arc due to the transform movement along the Hidaka trench. Therefore, it is likely that the opening of the Kuril Basin took place during 16-15 Ma. Beginning of new subduction of the Pacific plate beneath Hokkaido along the modern Kuril trench in the Late Miocene The Hidaka arc became a remnant arc after the opening of the Kuril Basin. it is not a usual remnant arc such as discussed by Karig (1972), because the subduction zone related to its arc ma~atism did not retreat but was converted into a transform fault. In general, remnant arcs suffer subsidence, such as those in the Philippine Sea (Karig, 1972). The

extensional regime observed in Central Hokkaido during the Middle Miocene indicates the presence of a tendency to subsidence. After the completion of the opening of the Kuril Basin, the Pacific plate began to subduct obliquely along the modem Kuril trench and the Kuril arc magmatism became active at about 10 Ma (Fig. 10). The active subduction of the Pacific plate along the Kuril trench has no genetic relationship to the opening of the Kuril Basin. The new subduction since the Late Miocene changed the stress field from extension to compression (Kimura, 1981; Yamagishi and Watanabe, 1986). Watanabe (1986) and Yamagishi and Watanabe (1986) showed that the a,,,, since the Late Miocene is WNW-ESE and is parallel to the subduction direction of the Pacific plate. It is likely that the compressional regime prevented the Hidaka arc from subsiding. On the other hand, subsidence of the Okhotomorsk Block since the Late Miocene (Savostin et al., 1983) may be related to the southward retreat of the Proto-Kuril arctrench system. Oblique subduction of the Pacific plate caused the westward migration of the Kuril arc and resultant collision of East Hokkaido against Central Hokkaido since the Late Miocene (Kimura, 1981, 1986). By this event, the southern part of the Hidaka magmatic arc was upthrust toward the west (Figs. 4 and lo), and a vertical section of the Hidaka magmatic arc, consisting of upper-mantle peridotites, grarmlites, gneisses, schists, and homfelsic rocks, as well as plutonic rocks of various levels, is observed in southern Central Hokkaido (Komatsu et al., 1983). As a result of the westward migration of the Kuril arc, a new NE-SW trending dextral strikeslip fault zone, the Kamishiyubetsu Tectonic Zone (Fig. 4), was formed in East and Central Hokkaido along the volcanic front of the Kuril arc (Kimura, 1981). The alignment of the Hidaka arc was dextrally displaced about 50 km along the Ka~s~~betsu Tectonic Zone (Kimura, 1981). Discussion

on the origin of the Kuril Basin

There are two kinds of kinematic ideas concerning the formation of back-arc basins (e.g., Dewey, 1980; Uyeda, 1982), that is, the “retreat-

250

ing trench”

model and the “anchored

(Seno and Maruyama, of the Kuril fied

into

Basin proposed until the “anchored

now are classisiab-retreating

either

Okhotomorsk

Block” model (Savostin

Kimura

Tamaki,

and

Proto-Kuril

trench”

slab” model

1984). Models on the origin

1986b) model

et al., 1983;

or the “retreating

(Niitsuma

ference Nemuro

in the location of VGP between the Peninsula and China-Siberia would be

explained

if the Kuril

the Nemuro southern

Basin were not present

Peninsula

margin

and

area were located along the

of the Okhotomorsk

ing the Late Cretaceous

Block dur-

to Early Eocene.

and Akiba,

1986; Jolivet,

1987). My model given in this paper

demonstrates

the “retreating

Proto-Kuril

trench”

Boundary ~{}kka~do

between

East

Hokkaido

and

Central

model. Kimura based

and Tamaki

on the discussion

(19868)

(1978), the age of the subducting the time of the opening young to cause Basin opened Okhotomorsk reorganization

mentioned

of Molnar

that,

and Atwater Pacific

plate at

of the Kuril Basin was too

trench retreat, and that the Kuril by the northward retreat of the Block, which was related to the of the terrains within the Eurasian

continent due to the India-Eurasia collision since the Eocene. However, Carlson and Melia (1984) indicated that no clear correlation is observed between the trench hinge motion and the age of subducted oceanic plate. Moreover, Uchimura et al. (1988) concluded that northeastern Asia was consolidated in the Cretaceous and has been stable since then. Uchimura et al. (1988) mentioned that the effect of the India-Eurasia collision is too small to be detected eastern Asia.

by pal~magnetism

Evidence for the retreat of the Proto-Kuril

in north-

trench

The topography of the Kuril-Kamchatka arc does not shown a single smooth arch; there is a cusp near the pivot of the fan-like shaped Kuril Basin (Fig. 1). The presence of the cusp and the configuration ward retreat

of the basin indicate that the southof the Proto-Kuril arc-trench system

accompanied a counterclockwise rotation. Recently, Tanaka and Uchimura (1989) investigated the pal~magnetism of the sedimental rocks of the Nemuro Group exposed in the Nemuro Peninsula. The ages of the examined samples range from Late Cretaceous to Early Eocene. They mentioned that the Nemuro Peninsula area has rotated counterclockwise by the amount of 29.4 _t 10.1” with respect to China-Siberia since the Early Eocene, and concluded that the dif-

According migrated

the opening boundary

to

my

southward of the between

model,

East

to the present Kuril East

Basin. Hokkaido

Hokkaido

location

during

Therefore, and

the

Central

Hokkaido, the Tokoro Tectonic Line (Fig. lo), should be a dextral transform, which is supposed to extend along the western margin of the Kuril Basin. Niitsuma and Akiba (1986) confirmed the boundary in the Tokachi area (Fig. 3). To the east of the Tokachi River, Paleogene sediments conformably overlie the Cretaceous Nemuro Group and are conformably overlain by Neogene marine sediments. In the Neogene sediments, an unconformity gap from

1.5 to 12 Ma is observed.

On the

other hand, to the west of the Tokachi River, Neogene marine mudstone aged 15 to 12 Ma covers the Mesozoic rocks. Since 12 Ma, however, common sedimentary sequences were deposited in both areas. So it is expected that the two parts were juxtaposed with each other and a common sedimentary basin was formed since the Middle Miocene. These observations are consistent with my model on the opening of the Kuril Basin. Although the northern extension is not clear. the Tokoro Tectonic Line is believed to extend from the Tokachi River area to the N-S trending boundary between the Hidaka Supergroup and Yuubetsu Group (Fig. 3) where the volcanic rocks of the late Middle (Yahata and Nishido,

Miocene to Late Miocene 1989) are widely exposed.

Euidence observed in the Kuril Basin The model of rapid opening at 16-15 Ma proposed in this paper is consistent with the occurrence of the sedimentary cover in the Kuril Basin. As already mentioned, the thick sedimental cover is undeformed and horizontal and was not af-

OPENING

OF THE

KURiL

251

BASIN

fected by the rugged basement topography. Such an occurrence is believed to indicate that the crustal extension was not active during the accumulation of the sedimentary cover (Gnibidenko and Svarichevsky, 1984). Short-term formation of the basement followed by static accumulation of the sedimentary cover is consistent with the observed data. Comments on other Proto-Kuril trench retreation models An origin of the Kuril Basin caused by the southward retreat and/or counterclockwise rotation of the Proto-Kuril arc-trench system was already proposed by Niitsuma and Akiba (1986) and Jolivet (1987). Niitsuma and Akiba (1986) considered that the turbiditic conglomerate of Middle Miocene age, distributed along western Central Hokkaido in a N-S direction, was deposited around the mountain area which was formed by the compressional stress field in a “collisional” junction of the Proto-Kuril arc and Central Hokkaido. Because the age of the conglomerate is older in the north (15 Ma) and younger in the south (13 Ma), they concluded that the position of the junction between the Proto-Kuril arc and Central Hokkaido moved southward as a result of the opening of the Kuril Basin from 15 to 13 Ma. However, the origin of this conglomerate is not explained as a result of compression between the Proto-Kuril arc and Central Hokkaido but between West Hokkaido and Central Hokkaido (see Hoyanagi et al., 1986). As clearly discussed by Kimura (1981, 1986), a compressional regime dominated since the Late Miocene between the (Proto-)Kuril arc and Central Hokkaido. My model given in this paper demonstrates that the opening of the Kuril Basin during 16-1.5 Ma was accompanied by counterclockwise rotation of the Proto-Kuril arc, and does not constrain the opening force of the Kuril Basin. On the other hand, Jolivet (1987) considered the mechanism of the opening of the Kuril Basin on a global scale. In his model, a convergent zone between the Eurasian and North American plates since 55 Ma, in compensation for the spreading of the Arctic Oce-

an, has been located between the Eurasian continent and the Japanese Islands; that is, the Japanese Islands belonged to the North American plate. He mentioned that the convergence was accommodated by southward motion of the the southern tip of the North American plate, i.e., the Japanese Islands, along a dextral strike-slip shear zone, and that the Sea of Japan was formed as a pull-apart basin between the Eurasian continent and the Japanese Islands during the Early Miocene. Further, he explained that the easternmost part of the southern tip of the North American plate (i.e., the eastern half of Hokkaido and the Kuril Islands) migrated southward along the ‘“Hidaka Shear Zone”, and the Kuril Basin was formed behind it during the Middle Miocene. Three points must be discussed in his model. First, his idea that the Japanese Islands were belonging to the North American plate since the Eocene is not supported by the regional geology of the Japanese Islands (e.g., Maruyama and Seno, 1986). Second, his “Hidaka Shear Zone” is located in mid Hidaka magmatic arc and it is difficult to support his idea that the “Hidaka Shear Zone” is a transform resulting in the opening of the Kuril Basin. The western boundary of the Kuril Basin is about 200 km east of the “Hidaka Shear Zone”, and the transform boundary is thought to be located along the Tokoro Tectonic Line as mentioned above. Third, the origin of the Hidaka arc magmatism is not explained in his model. Opening of the Kuril Basin and Japan Basin The age and process of fo~ation of the Japan Basin have been intensively discussed in recent years. Paleomagnetic examinations showed that Southwest Honshu rotated by 47’ clockwise at about 15 Ma (Otofuji et al., 1985, 1986) and Northeast Honshu rotated by 23” counterclockwise during the period later than 20-22 Ma and earlier than 15 Ma (Hamano and Tosha, 1985; Tosha and Hamano, 1988). Based on these data, it was proposed that the Sea of Japan opened by the oceanward retreat of the Japanese Islands at about 15 Ma (Otofuji et al., 1986; etc.). This hypothesis was supported by marine paleobiogeographic study (Chinzei, 1986) and geological examinations (Ni-

252

J. MAEDA

itsuma et al., 1985) in the Sea of Japan region, and petrological Tertiary 1986; worthy

and geochemical

igneous Kurasawa

rocks and

examinations

on

Honshu

Konda,

that the opening

(Takahashi,

1986).

process

of the

and

Pacific slab, covered by the retreated Proto-Kuril arc during the opening of the Kuril Basin. The idea of rather

It is note-

is supported

age of the

rapid opening

by the occurrence

deformed

sedimentary

Km-i1 Basin given in this paper are very similar

to

basement

of the Kuril

those of the Japan

et

cusp

Basin proposed

al. (1986 etc.). Both back-arc formed

in the Middle

oceanward though

retreat

Miocene

of an

the relative motion

plate and subducting

basins

by Otofuji

were rapidly

by rotation

arc-trench between

and

system,

al-

the overriding

Pacific plate was different

both settings and, moreover, the overriding was not the same, (i.e., the North American for the Km-i1 Basin and the Eurasian

in

plate plate

plate for the

Japan Basin). Therefore, the opening of these back-arc basins may not be genetically related to the subduction of the Pacific plate. Migration of a hot region unrestricted by an overriding plateboundary may have played an important role in the opening

of back-arc

Pacific rim (Miyashiro,

1986).

The elastics which constitute the Late Cretaceous to Early Eocene Proto-Kuril arc rocks distributed along East Hokkaido to the Lesser Kuril Islands were transported from the Okhotomorsk Block which is now separated from the Kuril arc by the Kuril Basin. Accordingly, the Kuril Basin was not entrapped by the Proto-Kuril arc but was by back-arc

Kuril

the pivot

arc-trench

Basin.

The Kuril

the rugged

The presence

of the fan-like

system

counterclockwise formed

cover overlying

that the southward

retreat

Kuril

of a Basin

of the Proto-

was accompanied

by a

rotation.

Basin and Japan

in the Middle

Basin were rapidly

Miocene

of

by rotation

and

system,

the

oceanward

retreat

Proto-Kuril

island arc and the Japanese

respectively.

Ma

Migration

arc-trench of

a hot

island arc,

region

unre-

stricted by an overriding plate-boundary may have played an important role in the opening of backarc basins

in the northwestern

Pacific rim.

Acknowledgements

basins in the northwestern

Conclusions

formed

near

indicates

at 16-15

of the thick non-

spreading

at least later

than

Early Eocene time. The Hidaka magmatic arc was active during the period of 43 to 17-16 Ma and is located on the west of the Kuril Basin. The relative motions among the Eurasian, North American, and Pacific plates indicate that the Hidaka arc was formed by the westward subduction of the Pacific plate beneath the Eurasian plate. Therefore, the Kuril Basin did not exist before 16 Ma on the east of the Hidaka arc and was formed by the southward retreat of the Proto-Kuril arc later than 16 Ma. The presence of both A-type ma~atism and graben-like depression indicates an extensional tectonic setting in the Middle Miocene. The extensional regime is attributed to subsidence of the

I express my thanks to Prof. M. Matsui, Prof. Y. Katsui, Prof. S. Uozumi, Prof. K. Nakamura, Dr. S. Kawachi and Dr. T. Watanabe of Hokkaido University for criticism and encouragement, Dr. S. Miyasaka and Dr. K. Hoyanagi of Hokkaido University for discussion, and Mr. T. Hirama, Miss Y. Igarashi, and Miss A. Tanisawa of Hokkaido University for help in preparation of the manuscript. I wish to thank an anonymous reviewer for useful comments. The work was supported in part by a grant-in-aid for scientific research from the Ministry of Education, Science and Culture, Japan (Nos. 60740455 and 63540605). References Cadet,

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