地 学雑 誌

Joumal of Geography

104 (3) 392-407 1995

Formation of Backarc Basin and Genesis of Ophilolite

Hajimu KINOSHITA*, Yukari NAKASA*, Rie MORIJIRI** and Toshiya FUJIWARA***

Abstract

Geoscientific informations from the Japan Sea, a backarc basin between Japan and Siberian

continental mass, and from a part of the ophiolite belt along the southern rim of Japan,

Mineoka ophiolite belt, are presented to discuss a possible relation between the opening of the

Japan Sea and the formation of the ophiolite belt. It is speculated that the Central Japan

ophiolite belt was formed by slicing of the lithosphere and uplift of serpentinized mantle peridotite

in Miocene. Slicing of the lithosphere was induced by opening of the Shikoku Basin (northern

part of the Philippine Sea Plate) shortly after the opening of the Japan Sea.

I. Introduction

The northwestern Pacific is characterized by

a presence of number of backarc basins along

the Eurasian continental margin. One of the

most intensively studied basins is the Japan

Sea between Japan and the Eurasia. Elucida-

tion of the origin and evolution of the Japan

Sea is an interesting scientific subject. The

opening of the Japan Sea and the occurrences

of a small but conspicuous ophiolite belt in the

Central Japan is believed to be connected with

each other not only on their geologic time scale

but tectonic environments of this region (e. g.,

Arai and Okada, 1991).

Tectonic evolution of the Japan Sea (Fig. 1)

has been attracting attention of scholars of earth

sciences. The formation scheme of the Japan

Sea was discussed first by Terada (1934) who

noticed from the bathymetry feature of the

Japan Sea that the basin was formed behind

the drifting Japanese landmass off the Eurasian

continent by rifting of the continental crust, the

Siberian landmass, and leaving banks and highs

as remnant continental fragments behind Japa-

nese landmass (Terada, 1934). Since then, the

Japan Sea and its surrounding area, Japan,

Korea and Primorie of Russia, have been

objectives of extensive geoscientific studies.

W ell organized introductions on these subjects

were made recently by Ocean Drilling Program

research groups (Tamaki et al., 1990, 1992).

The magnetic anomaly patterns of this area

were studied earlier by Murauchi (1972), and

Isezaki (1986). Intensive and thorough map-

ping surveys in the eastern part of the Japan

Sea by Geological Survey of Japan were made

for years (e.g., Tamaki, 1986). These data

were incorporated with Russian researches to

build a new magnetic anomaly map by Izezaki

(1986). Kono (1986) has re-examined the mag-

netic lineation patterns for deducing formation

age of the Japan Sea. Later, Seama and Ise-

zaki (1990), and recently Nakasa and Kinoshita

*Earthquake Research Institute, University of Tokyo. Yayoi, Bunkyo-ku, Tokyo 113, Japan.**Geological

Survey of Japan. Tsukuba, Ibaraki 305, Japan.***Ocean Research Institute, University of Tokyo. Minamidai,Nakano-ku, Tokyo 164, Japan.

392

(1994) have reported weak but clear lineation

patterns and apparent fracture zones accompa-

nied by the magnetic lineation discontinuity in

the northeastern corner of the Japan Basin.

Nakasa and Kinoshita (1994) have relied only

on the magnetic data with a good quality of

ships positioning.

Gravity data collection from the Japan Sea

and its adjacent area was made from surface

gravity data by Tomoda (1973). Yoshii (1979)

made a compilation of gravity data along a

line across Japanese island including large part

of the Japan Sea and discussed upper mantle

heterogeneity. Teleseismic and surface wave

study of this area revealed Moho depth (Abe

and Kanamori, 1970 ; Evans et al., 1978) and

an anisotropic configuration in the seismic ve-

locity (Okada et al., 1978 ; Chung, 1992).

The Geological Survey of Japan has accumu-

lated also gravity data on the eastern half of

the Japan Sea (Honza, 1979). Tamaki (1986)

discussed a possible age of formation of the

Japan Sea by the bottom topography of the

area. Kono and Furuse (1989) have collectd

gravity data around the Japanese Islands includ-

ing part of the Japan Sea. Nakasa and Seno

(1994) and Hirata (1995) argue the gravitational

stability of the lithosphere of the Japan Sea.

Heat flow data in the Japan Sea area were

studied extensively (e. g., Yoshii, 1972). These

data were used to consider the thermal regime

of the upper mantle beneath this area. Tamaki

(1986) and Kuramoto (1989) have estimated the

age of the Japan Sea using heat flow data and

opal A/TC boundary from the seismic reflection

profiles. They imply that the Japan Sea can be divided into three regions on average heat

flow values and noted that the western part of

the Japan Sea is younger in general. This

fact has led to some speculation that the Japan

Sea was formed passively under the influence

of shearing force between the Eurasian Plate

and the Pacific Plate due to obliquity of their

relative motion (Jolivet et al., 1991). Results

of ODP drilling (Leg 127: Tamaki et al.,

1990, and Leg 128: Tamaki et al., 1992) imply

that the Japan Sea was formed as early as 20-

25 Ma BP.

Seismic velocity structure of the crust of the

Japan Sea is one of keying parameters for un-

derstanding the evolution history of the region.

The seismic structures of the Japan Sea were

studied number of research groups (Murauchi,

1966, 1972 ; Ludwig et al., 1975 ; Hirata et

al., 1987, 1992 ; Tokuyama et al., 1987 ; Ka-

tao, 1988 ; Chung et al., 1990 ; Karp et al.,

1992 ; Shinohara and Suyehiro, 1992 ; Hirata

1995 Bikkenina et al., Submitted). It is noted

that the Japan Sea consists of combination of

Fig. 1 An index map showing approximate

positions of the Japan Sea, magnetic study area, and Mineoka ophiolite

Thick curve on the southern rim of the

Central Japan is an outline of outcrops

of the ophiolite belt. Trench and trough

systems are shown by thick curves with

triangles.

393

typical oceanic crust mainly in the Japan Basin

(northern part of the Japan Sea) and number

of blocks of continental crust under premature

extension. The tuffaceous (green tuff) layer of

4.2 km/s in seismic velocity is considered to be

distributed widely in the entire region of the

Yamato Basin (southern part of the Japan Sea)

which implies an occurrence of powerful vol-

canic activities in the past.

Magnetic characteristics of the basement lay-

ers of the oceanic lithosphere is studied in de-

tail using the experimental ODP Hole 504 B

(e. g., Kinoshita et al., 1989). It was found

also that there is a number of detachment faults

at its deeper part (e. g., Alt et al., 1993). It

is inferred from the study that the basement

magnetic lyer may be scraped off by a tectonic

shearing forces such as accretion due to sub-

duction (Kinoshita and Matsuda, 1989).

A magnetic characteristics of the Mineoka

ophiolite belt (Fig. 1) in the southern part of

Japan (the Boso Peninsula) suggests a shallow

burial of a thin and elongated magnetized body

(Kinoshita, 1995). The ophiolite belt consists

of serpentinized ultrabasic rocks and their brec-

ciated cumulates. Its main body is restricted

in a narrow band of 15 km. Geology (e. g.,

Kanehira, 1976 ; Geological Survey of Japan,

1980 ; Arai and Okada, 1991) shows that

the Mineoka belt intruded possibly along the

strike slip fault zone between the Shikoku Ba-

sin, northern tip of the Philippine Sea plate,

and the Japan landmass millions years ago in

conjunction with a large tectonic development,

such as opening of the Japan Sea followed by

the opening of the Shikoku Basin. Tonouchi

and Kobayashi (1983) proposed a model that the

Mineoka belt is buried within the upper part of

the crust and causes a broad band of geomag-

netic low in the aeromagnetic map (Utashiro

and Kondo, 1972). Ogawa and Taniguchi

(1987) note that the Mineoka Microplate devel-

oped through collision of the Izu Island-arc

with the Honshu landmass. Arai and Okada

(1991) suggest that the Mineoka belt was form-

ed by accretion of serpentine diapirs in the

forearc region. Soh et al. (1991) propose that

the Mineoka belt was produced by tectonic

fragmentation of the crust of the Izu-Bonin

arc collision. These studies are supplemented

by geophysical data on faults (Research Group

for Active Faults of Japan, 1991), gravity (Ko-

no and Furuse, 1989), and seismic structure

(Asano et al., 1979).

Magnetic field surveys were carried out most-

ly using airborne proton' magnetometers (Geo-

logical Surgey of Japan, 1980 ; JICA, 1983 ;

NEDO, 1983 as referred to in NAKAI et al.,

1987). These data were processed by adjusting

the International Geomagnetic Reference Field

(IGRF) residuals and altitude (Nakai et al.,

1987 ; Okubo et al., 1987). The thickness

and depth of the Mineoka ophiolite belt is sug-

gested to be small (e. g., Arai and Okada,

1991). It is inferred that the deeper part of

the basement beneath the Mineoka hill is less

magnetic or preferably nonmagnetic to produce

a wide low band of the magnetic field (Kino-

shita, 1995).

II. Geophysical and Geological Data

The Japan Sea

(1) Gravity and heat flow

The entire region of the Japan Sea is mostly

compensated in terms of Airy type isostasy.

Free air gravity anomaly in the Japan Basin

traces exactly the bathymetric topography ex-

cept around conspicuous seamounts. However,

the residual gravity anomaly implies that the

upper mantle beneath this region consists of

less dense materials (Yoshii, 1972). Heat flow

value distribution shows insignificant variability

394

within the entire basin area of the Japan Sea.

Only exceptional high heat flow values are

observed at around the transition boundary be-

tween Japanese landmass and the Japan Basin

where the shallow earthquakes and the crustal

deformation along fault lines are frequently

observed. The regional gravitational and heat

flow stability implies a thermal and dynamic

equilibrium of the shallower part of the upper

mantle (Nakasa and Sum, 1994 ; Hirata, 1995).

(2) Upper crust and sedimentary layers

Several measurements were carried out and

maps of sediment thickness were produced

(Ludwig et al., 1975 ; Tokuyama et al., 1987 ;

Tamaki, 1988). Recent seismic reflection sur-

veys by the Russian Research Vessel provided

a new dataset. Two way travel time of reflec-

tion profiles is read out to obtain grid data of

the acoustic basement depths. The basement

contours were drawn by solving finite differ-

ence equations (Akima, 1970, 1974 ; Briggs,

1974) after applying empirical depth correction

to the sound velocity (Ludwig et al., 1975).

The surface of the basement feature of the

Japan Basin is fairly rugged covered by sedi-

ments as thick as 3 km and there is a number

of isolated seamounts (Nakasa and Kinoshita,

1994).

(3) Seismic structure of the crust

Seismic structure of the transient region be-

tween Siberia and the Japan Basin was studied

by Bikkenina et al. (Submitted) using two-

dimensional ray tracing method rucently. The

author describes in the remarks ; " It is sugges-

ted that the basin was formed in connection

with stretching and destruction of the continen-

tal crust ". It is clearly shown in the report

that the transition shows a thinning of the

continental crust gradually into the Japan Basin

structure.

Average thickness of the Japan Sea litho-

sphere was studied by surface waves observed

at land stations (Abe and Kanamori, 1970 ;

Evans et al., 1978) with its anisotropic char-

acter (Okada et al., 1978). Marine seismo-

logical surveys were run by many works which

were based on similar principles. (e. g., Hirata

et al., 1992 ; Hirata, 1994). The seismic data

(signals and time control) used for these anal-

yses are different each other in their quality

and, therefore, the ambiguity of each dataset

are not in the same figure.

From the results of the seismic refration ex-

periments so far, the crustal structure of the

Japan Basin reveals a typical oceanic basin

(Murauchi, 1972 ; Ludwig et al., 1975 ; Hi-

rata et al., 1992 ; Hirata, 1995). The crust

consists of three oceanic layers : sedimentary

layer, layer 2 (3.3-5.9 km/s), and layer 3. The

Moho surface is almost flat in the eastern part

and slightly deeper in the western part (Yoshii,

1972 ; Hirata et al., 1992). The surface of

sediment layer overlying basaltic basement is

almost flat in contrast to rugged topography of

basement.

The other part of the Japan Sea, Yamato

Basin to south of the Japan Basin, has a semi-

continental crustal structure and its crust is as

twice thick (14 km) as that of the Japan Basin

in average (Murauchi, 1972 ; Hirata et al.,

1987, 1992 ; Katao, 1988 ; Chung et al., 1990 ;

Shinohara and Suyehiro, 1992 ; Chung, 1992 ;

Hirata, 1995). Accumulation of these data al-

lows us to draw a two dimensional cross section

of the lithosphere from continent (Siberia),

over backarc basin (Japan Sea) and island arc

(Japan) to trench (Japan or Izu Ogasawara)

systems (Fig. 2).

(4) Ages

The formation ages of the entire Japan Sea

area estimated by different approaches range

from 10 to 30 Ma (Tamaki, 1988 ; Tamaki

395

et al., 1990 Kaneoka et al., 1992). Most

of these age estimates encompass entire Japan

Sea region and refer also to various data sources

from land masses around this region. The

ODP Leg 127 reached basement volcanic layers

which give the radiometric ages as 15-19 Ma

(Tamaki et al., 1990 Kaneoka et al., 1992).

The layer reached by ODP is still under con-

troversy whether this is a part of the basement

massive flow units. The basaltic rock sample

from the Bogorov Seamount (B with a thick ar-

row in Fig. 3) indicates K-Ar age 18 Ma

(Sahno and Vasiliev, 1974).

The age of formation of the entire Japan

Sea is not clearly difined yet. It is noted how-

ever, the formation of the Japan Basin occur-

red first before 20 Ma and then the formation

of the Yamato Basin occurred later referring

number of recent data not earlier than 1980's.

For instance, the latest crustal stretching of

the Yamato Basin occurred at around 15 Ma

(e. g., Tamaki et al., 1990), significantly later

than the formation of the Japan Basin. The

Japan Sea opening is advocated by the indirect

evidence, i.e., a possible bending of the Japa-

nese Island arc at about 19-20 Ma (e.g., Otofuji

and Matsuda, 1983).

III. Age Constraints for the Japan Basin

Clear magnetic lineation patterns are observ-

ed only in the northern part of the Japan

Sea, i.e., the Japan Basin (Fig. 3). The geo-

magnetic data are applied to the age estimate

of the opening of the Japan Basin. The data

are available from the National Geophysical

Data Center (NGDC) of the National Oceanic

and Atmospheric Administration (NOAA), U.

S.A., and the Japan Oceanographic Data Cen-

ter (JODC), Geological Survey of Japan

(GSJ), Japan Maritime Safety Agency (JMSA),

Ocean Research Institute, University of Tokyo

(ORI), Japan Meteorological Agency (JMA) and

institutions of U.S.A., Russia and ourselves.

Using 77 cruise data, a magnetic anomaly map

of the Japan Basin was made with a track line

crossover error check. All the data are re-

adjusted to IGRF 90 (International Geomag-

netic Reference Field 1990: IAGA Division

V Working Group 8, 1991).

The analysis is restricted within a rectangle

of 40-43•‹N, 131-138•‹E. In this area, a number

of magnetic lineations are found. A set of

grid data every 10 km interval from magnetic

anomalies are made after Akima (1970, 1974).

The alignments of magnetic anomaly lineations

are exaggerated by stacking the anomaly val-

ues along a fixed strike to give rise to the

best coherency (Nakasa and Kinoshita, 1994).

The model simulation is performed to iden-

tify anomaly chron (Harland et al., 1989).

There is no unique combination among param-

Fig. 2. Cross section of a continent-backarc basin-island arc region based on the study of seismic velocity structure from number of areas around Japan Original data from Yoshii (1972), Hirata et al. (1992), and Shinohara and Suye- hiro (1992). Mono depth in the Japan Basin is approximately 12-13km.

396

eters due to their trade off. A synthetic

two-dimensional block model (e.g., Bhatta-

charyya, 1964) is compared with stacked pro-

files adjusting spreading rate and direction,

layer thickness, magnetization intensity, incli-

nation and declination. The alignment of ma-

gnetic anomalies and their chron are shown in

Fig. 3 from four regions described before

estern part of region 1, age 22-23 Ma, or 23-24

Ma western part of region 1, age 16-18 Ma.

A dredge haul rock sample from the Bogorov

Seamount in the west of region 1 gave a com-

patible absolute age 18 Ma (Sahno and Vasiliev, 1974). The central region 2 is either 13-15

Ma or 22-24 Ma. The former value is too

much younger compared to other informations

and this value is discarded in the interpretation.

The western region 3 has basement age 13-15

Ma.

IV. Ophiolitt. Belt in the Boso Peninsula

(1) Crustal features

Tectonic and petrographical studies of the

region (Mineoka of Fig. 1) indicate that the

ophiolite is a group of isolated and fragmented

small bodies of serpentinized gabbroic sheet of

Cenozoic era (e. g., Arai and Okada, 1991).

This part of the crust forms a line of hills

which is bounded on both sides by fault lines

running almost east to west (Research Group

for Active Faults of Japan, 1991). Bouguer

gravity anomaly within the pair of fault lines reveals intermediately low values as low as -30m gals , and these faults are seismically inac-

tive (Kono and Furuse, 1989). Seismic structure

indicates that the longitudinal wave speed of

the basement (5-6 km/s) layer is lower than that

of surrounding basement by about 8-10 %

(Asano et al., 1979).

(2) Magnetic anomaly

The ground level (ca. 1,200 points every

100-400 meters along routes) and aeromagnetic

surveys of various altitudes (430, 1,350, and

3,150 meters : Nakai et al., 1987) were carried

out over the eastern edge of the ophiolite suits.

Diurnal variation of the magnetic field was

corrected to the total magnetic field every min-

ute by use of the observed data at the Geo-

detic Observatory, Geographical Surgey Insti-

tute of Japan near the work area. Magnetic

anomalies were calculated by subtracting IGRF

90 (IAGA Division V Working Group 8,

1991). Figure 4 is a magnetic anomaly map

based on the ground level surveys which is fil-

tered to cut off wave length shorter than 200

m. A characteristic feature for the magnetic

anomaly field of various altitudes is a NW-SE

trending deep low at higher altitudes and sharp

dipole type anomalies in the ground level pat-

terns.

(3) Model of Magnetic Crustal Structure

A low of the magnetic field can be produced

either by a magnetized body which has a nearly

horizontal south seeking magnetization vector

(e.g., Nakai et al., 1987) or by a large non-

magnetic body in the magnetic environment.

The latter case is more plausible in the present

case from the seismic study, gravity field, and

relatively low geothermal gradient (ca. 30 K/

km). The nonmagnetic basement body can

possibly exist due either to burial of sedimen-tary materials or demagnetization of basement

formations for number of tectonic reasons

brecciation, hydrothermal alteration.).

The 3-dimensional inverse modeling is car-

ried out to fix the initial model. Then, aero-

magnetic anomalies were separated into re-

gional (long wavelength) and residual (short

wavelength) components by using upward con-

tinuation after Gupta and Ramani (1980).

After removing a long wavelength magnetic

field variation, a block model is used for for-

397

Magnetic Anomaly

398

ward modeling for much detailed magnetic

field variation. Some of the conspicuous bumps

in the ground level profiles can be attributed

to a small magnetic body. A numerical model-

ing is achieved by assuming a structural situa-

tion (Fig. 5) where the position and extent of

central blocks (MB : stands for magnetic block,

and NMB : non-magnetic depression) is read-

justed to obtain better fit to the observed mag-netic configuration. A schematic cross section

of the magnetic anomaly is reproduced in Fig.

6 along an artificial track line (Monitor track in

the lower inset) which shows a composite

magnetic field and topography profiles observed

or calculated at various altitudes. The most

probable burial depth of the top face of the

Fig. 4 Magnetic anomaly map of Mineoka ophiolite belt region obtained on the ground level

Contour interval is 40nT. Lightly dotted areas indicate magnetic heighs and green to bluish

areas indicate magnetic lows.

Fig. 3 An index map of magnetic anomaly of the Japan Basin and a colored magnetic anomaly patterns in three regions (bottom figure) Solid lines are magnetic lineation patterns, Positive and Negative (R) magnetic anomalies in three rectangle regions (dotted line) corresponding to the study area of Fig. 1. The entire area is divided into three parts on the basis of three different directions of magnetic lineations. A character B with a thick arrow shows position of the Bogorov Seamount.

399

magnetized prism is between 0.5 to 1 km and

the thickness is about 0.5 km according to this

model. This value gives an approximate size

of the Mineoka ophiolite body. More detailed

modeling would be possible if we have more

magnetic dataset from wider region of this

area in the future.

V. Discussion

There is a variety of speculative interpreta-

tions on formation of the Japan Sea. These

speculations are fundamentally based upon geo-

metrical considerations to put number of patches

of islands, banks and apparent continental

fragments in the Japan Sea back together to

the coast of Siberia (left side of Fig. 7). The

splitting of the continental block was followed

by ocean floor spreading in the Japan Basin

due to external shear force (right side of Fig.

7 modified from Arai and Okada, 1991) and

then by the crustal stretching of the Yamato

Basin, southern part of the Japan Sea.

The petro-physical approach to the mecha-

nism of generation of stretching force in the

backarc region such as the Japan Sea is still

open to discussion. A mantle return flow in-

duced by subducting Pacific Plate leads to ana-

texis (high temperature hydration), dragging

Fig. 5 A schematic two-dimensional cross section model of the tectonic and structural configuration of the Mineoka ophiolite belt and its adjacent area Seismic velocities of the crust are referred to from Asano et al. (1979). Bouguer Gravity (BG) curve from Kono and Furuse (1989) , ultrabasic emplacement (a dark block near surface in the center) from Arai and Okada (1991) , and fault planes (dipping lines from surface) from Research Group for Active Faults of Japan (1991). Dotted section (MB) is magnetized prism parallel to local geomagnetic field. Blank region (NMB) is nonmagnetic due to some tectonic or depo-sitional environments.

400

hydrated peridotite deeper, dehydration, up-

welling of partial melt, then formation of wet

magma sources, or a local plume from dry

partial melt from subducted slabs (e.g., Hasebe

et at., 1970 ; Ida, 1987 ; Tatsumi, 1989).

The wedge flow is most probably governed by

thermal energy rather than mantle drag (Ida,

1987). Some discussion are given to mobility

of lighter elements (or volatiles such as water,

carbondioxide etc.) in the light of hydrofractur-

ing process which reduces effective viscosity

(e. g., Nakashima, 1993). The hydrofracturing

process is one of candidates in the interpreta-tion of high mobility of fluids within mantle.

A retreat or drooping of the subducting

lithosphere due to differential motion in the

upper and lower part of the asthenosphere may

be possible. It is likely that the upper part

of the mantle beneath the Japan Sea is unique

in a way that the mantle material over the

subducting Pacific Plate consist of lighter ma-

terials (Yoshii, 1979). This implies either that

the mantle beneath the Japan Sea has compara-

tively higher temperature or consists of lighter

materials. The higher temperature scheme

seems to be advocated by distribution of fairly

high heat flow values of the Japan Sea com-

pared to the cooling plate model.

Fig. 6 Summary of all the data available so far of the magnetic anomaly along an artificial track line (Monitor Track in the bottom inset) which is equal to full extent of horizontal axis of the top left figure

The topography, composite magnetic field profiles at various altitudes (ordinates) along the moni-tor track line (abscissa) are shown. From top to bottom of the left figure, the magnetic field cross section of 3,150 m upward filtering (Nakai et al., 1987), magnetic field profile at 1,350 m altitude

(Nakai et al., 1987), the same at 430 m altitude (Utashiro and Kondo, 1972 ; Geological Survey of Japan, 1980), ground level magnetic field profile (Kinoshita, 1995), and the topography of this area. Altitude is given in m, horizontal distance in km, and relative value of the magnetic total force of 600 nT is given by a vertical thin ar-rows in the upper part of the top right figure. Right side is showing an altitude dependencies of magnetic total force anomaly due to a two dimensional single block of a magnetized body

(magnetization intensity Jn = 1.5 A/m) exten- ding perpendicular to the face of this page. Width of the buried sheet is 8 km. Thickness of the body is about 500 m. Some magnetic field variation expected from this body are shown in the top right columnar figure. A pair of thin funnel shaped vertical lines on the left figure is a copy of those from the right figure.

401

The volcanic activities indicate that the vari-

ation of the geometry of the mantle wedge

(return flow) shifts back and forth. It is diffi-

cult to maintain a persistent return flow in a

wide area for a long period of time. Hot spot

upwelling is not applicable to the formation

of the Japan Sea because it is unlikely that

the subduction and deep rooted hot spot must

coexist or juxtapose at the same time.

If we introduce a persistent magma upwelling

in a area comparable to the Japan Basin, we

can reach the mechanism of the backarc basin

formation. It is shown by age identification

of magnetic lineation patterns over the north-

western corner of the Pacific Plate that the

age of its western part decreases toward north

or northwest (e. g., Nakanishi et al., 1989).

The timing of the consumption of the extinct

spreading center (ridge of the Kula plate) is

little known. Magnetic lineation pattern stre-

tches over about 2,000 km from 110 to 180 Ma,

from north nearby Hokkaido, Japan, to the

south nearby Ogasawara. We estimate a half

spreading rate of this part of the cule plate as

30-40 mm/year. If this rate had been persist-

ent also from 0 to 110 Ma in this region, the

stretch of about 3,000 km younger part was con-

sumed by subduction in latest 30 Ma. Number

of backarc basins along western Pacific has

occurred in 60-20 Ma. Their occurrences seems

to be interconnected although it is hard to

show their relation quantitatively. It is assum-

ed that there had once been existing ran active

opening center moving north or northwest-

ward which collided with the Eurasian Plate

and caused a drastic change in tectonic scheme

of the Pacific Plate. The oceanic ridge seg-

ments collided one after another with the com-

Fig. 7 Configuration of distribution of continental fragments around Japan Japan at present (gray dotted part in the left figure) and fragmented blocks in Miocene (patchy blocks in the left figure) , with number of tectonic lines ; trenches, faults and backarc ridges

(right figure), activated by tectonic development in accordance with the Japan Sea and the Shikoku basin openings (modified from Arai and Okada, 1991).

402

tinental fringe. From ages of backarc basins

around this area, the southern and northern

ridge segments must have collided first to form

South China Sea and Bering Sea. However,

the formation of Bering Sea can be different

and it may have been formed by entrapment of

a part of the Kula plate as proposed by Marlow

and Cooper (1983).

Cross section of the upper mantle along a

line across islandarc-basin-continent is drawn

in Fig. 2 referring to Bikkenina et at. (Submit-

ted) , Shinohara and Suyehiro (1992), and Yoshii

(1972) to discuss the vertical scheme of the sub-

duction, formation of backarc basin and intru-

sion of ophiolite (diapir) in the forearc region.

A mechanism of evolution of this system can

be as follows : (1) Subduction of the oceanic

lithosphere with active spreading center. (2)

Liquid out of the subducting slab lowers solidus

of asthenospheric materials. (3) Differentiation

of magma wells up lighter materials from the

reservoir.

These big events occurred in accordance with

the time of Japan Sea development. The esti-

mate of the age can be determined by the mag-

netic anomaly of the Japan Basin. The for-

mation age of the Japan Basin is reexamined

by model experiments to give 13-24 Ma for

the opening of the Japan Basin. Radiometric

ages, estimates from heat flow and bathymet-

ric depths range 18-26 Ma in general. While

the Japan Sea was formed followed by opening

of the Shikoku Basin, there occurred a mega-

shear motion on the southern rim of the Jap-

anese landmass (e. g., Arai and Okada, 1991)

and diapir suits occurred along this shear zone

as schematically shown in Figs. 2 and 7

(right.)

Basement layers of the oceanic lithosphere

is covered by hydrothermally altered basaltic

and ultrabasic rocks. The magnetism of the

basement rocks decay within depth 1 km from

its surface (e. g., Kinoshita et al., 1989). The

mechanical properties changes significantly in

the same depth range as observed by a change

in sound velocity of basement formations (e.

g., Alt et al., 1993). The basement magnetic

layer can easily be scraped off by accretion

and subsequent underplating (Kinoshita and

Matsuda, 1989). The accreted part of the ultra-

basic materials may play a major role in dia-

pirism of an Ophiolite body.

The amplitude of the magnetic anomalies

observed at ground level decreases rapidly at

higher altitude implying that the entire size

of Mineoka ophiolite bodies is small and con-

fined in a narrow band. A simplified two-di-

mensional crustal model of ultrabasic suits

around the Mineoka belt indicates that the di-

mension of intrusive mass is no thicker than

0.5 km and its horizontal extent is as narrow

as 8-10 km. The present two dimensional mod-

el is simplified enough to show only an approx-

imate dimension of a buried body. The root

of the diapir had to be much deeper when it

intruded. The rest of the diapir sank back

deep into the upper mantle leaving only a top-

most part of the intruded body. The three-

dimensional forward modeling can only be

obtained through much detailed informations

from the entire Mineoka area. The magnetic

intensity of this body is much stronger than

that of surrounding formations of the same level

(or crustal depth). The prism is located in the non-magnetic depression which explains

the broad band of the geomagnetic low extend-

ing to NW over the Boso Peninsula. The

Mineoka ophiolite bodies are probably floating

in non-magnetic environments. This implica-

tion is interesting for understanding a tectonic

development of this ultramafic suits.

403

VI. Summary

The trends of magnetic anomaly lineations

in the eastern, central, and western area of the

Japan Basin are slightly different in its orien-

tation of alignments. The age, spreading rate,

thickness of magnetic basement, and magnetic

intensity values seem to vary from the eastern

to western areas of the basin. The eastern

part of the east (region 1 : Fig. 3) indicates

the age around 22-24 Ma. The western part

of the east (region 1) shows younger age 16 to

18 Ma. The central region 2 has two possible

ages ; 13-15 or 22-24 Ma of which the younger

one can be discarded. The westernmost region

3 has obviously formed at pround 13-15 Ma,

the youngest age of the whole basin analyzed

magnetically so far. All these values cover a

similar range comparable to other age estima-

tes based on radiometric dating, bathymetric

features and heat flow values. Though the

magnetic modeling do not show very clear fea-

ture of the Japan Basin formation and evolu-

tion, it seems likely that the opening of the

Basin at the eastern and central parts occurred

almost at the same age. These opening centers

propagated westward with varying migration

speed during 24 and 13 Ma.

Diapir in the Japan forearc region may have

occurred in conjunction with the Japan Sea

opening. Ground magnetic surveys were car-

ried out in order to define the dimension of the

intrusive body in the southern part of the Boso

Peninsula. It is concluded that the size of dia-

pir left on land is comparatively small 0.5

km •~ 8 km in its cross section which may have

been a result of diaprism along shear zone pro-

duced by backarc basin formations.

Acknowledgment

The author would like to express thanks to. Dr. E. Kikawa, Geological Survey of Japan, for his critical arguments and suggestions. The author would hear-tily appreciate Prof. K. Suyehiro, Ocean Research Institute, Prof. N. Hirata, Earthquake Research In-stitute, for providing data and scientific comments. This program was partly supported by the Grant-In-Aid for International Scientific Research Program, Ministry of Education, Science and Culture.

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(Received 9 January, 1995 ; Accepted 20 March, 1995)

406

背弧 海盆 の形成 とオ フィオ ライ トの成 因

木 下 肇* 仲佐ゆか り* 森尻理恵** 富士原敏也***

日本海の北部 日本海盆の海上地磁気異常縞模様

配列から,海 盆の形成年代を改めて推計した。こ

の解析には,従 来の情報に加えて1989-1991年 度

の目露海域共同研究等によって得 られた各種の地

学的新情報を投入し,日 本海盆の地磁気による推

定形成年代を13-24Maと した。一方房総半島南

部に露出するオフィオライ ト岩体の地磁気探査か

ら,地 震波速度構造等を参考にして,地 磁気によ

る地殻構造モデルを作 った。この帯は従来より,

日本海,四 国海盆形成 にほぼ同期 して発生 したと

する説があり,テ クトニクス的に目本海や四国海

盆形成とこのオフィオライ ト帯形成の関連に興味

が もたれている。筆者 らは沈み込み付加帯下地殻

の磁気構造と減衰,並 びに海洋底基盤岩層の磁気

的特性の研究をおこなってきた。本論説 の内容は

その延長である。但 し日本海と日本南部のオフィ

オライ ト形成の時間ウィンドウについては嶺岡山

帯の形成時間の決定不足 もあ り,今 回の議論では

完全に絞 り込めたとは言えない。やや問題提起 も

含めて,成 果の総論を示す。

* 東京大学地震研究所

** 工業技術院地質調査所

*** 東京大学海洋研究所

407