Magnetic model of the subduction zone in the northeast Japan Arc

~ecrQ~~~fi~~;~~, 192 (1991) 103- 1 I 5 Elsevier Science

Publishers

103

B.V.. Amsterdam

Magnetic model of the subduction zone in the northeast Japan Arc Yasukuni

Okubo

a, Masahiko

Makino

a and Sigeru

Kasuga

’

* Geologieai Suruey ofJapan, l-i-3 Higashi Tsukuba, lbaraki 305, Japan h H.~~r~graphi~ department, Maritime Safety Agency, S-3-1 Tsukfji Cjwo-ku, Tokyo 104, Japan (Received

by publisher

July 24, 1989)

ABSTRACT Okubo, Y., Makino. M. and Kasuga, Wasilewski and P. Hood (Editors),

S., 1991. Magnetic model Magnetic Anomalies-Land

of the subduction zone in the northeast Japan and Sea. Tectonophysics, 192: 103-115.

Arc.

In: P.

Magnetic modeling of the forearc side of the Tohoku (northeastern Japan) Arc was carried out using spectral analysis of shipborne magnetic data. The original data show anomalies caused by volcanic rocks on the western, shallow side and linear marine magnetic anomalies on the eastern, deeper side throughout the forearc region. As calculated from temperature gradient data of heat flow measurements and deep drilling, the Curie depth is deeper than the Moho on the eastern, deeper side. This suggests that the Moho may correspond to the base of the magnetic layer, because analysis of upper mantle xenolith suites indicates that the upper mantle should be non-magnetic. In addition, tht: Conrad discontinuity may correspond to the apparent top of magnetic layer because the lower crustal layer would be more mafic than the upper layer. It was estimated by 2D spectral analysis that the magnetic base and surface lie at about 20 km and 10 km respectively. This suggests that the Moho and the Conrad may roughly correspond to the magnetic base and surface. The magnetic anomalies in the Pacific basin are traced towards and over the inner side of the trench, and show .L gradual decrease in amplitude, disappearing eventually about 100 km west of the trench. This reflects the descent of the oceanic slab. The ID spectral analysis indicates that the marine magnetic layer is descending at an angle of about 4O in the Japan Trench. this angle increasing to 20° degrees below the coast, where the marine magnetic layer reaches a depth of more than 30 km. This agrees with the seismic reflection data from the trench, and because the magnetic layer can be traced down to a depth of 30 km it may be suggested that the Curie isotherm deepens to more than 30 km just east of the coast.

Introduction

ferromagnetic minerals, with the use of large data sets the Curie depth can often be estimated by

Several magnetic spectral analysis methods have been developed for estimating depth to the mag-

analysis

netic boundaries. oped the method

depth of the Japanese Islands estimated spectral analysis. The shallow Curie depths

Okubo

Spector and Grant (1970) develfor estimating an average depth

to the surface relief on an assemblage of magnetized bodies and Bhattacharyya and Lue (1977) introduced the use of the spectral moment of magnetic data for estimating the geometry of a single body. The algo~thms on those of the 3D model

of these were based introduced by Bhat-

Publishers

et al. (1985a,

spectral

b) have discussed

range.

the Curie from (less

The Curie depth in the forearc basins gradually increases towards the east. The numerical model explaining the heat flow observed (Hasebe et al., 1970) indicates that the sudden drop in the isotherms occurs between the volcanic front and the trench.

geometry of magnetic sources in the oceanic crust. Because the Curie transition depth (Curie depth) corresponds to the deepest occurrence of b 1991 - Elsevier Science

long-wavelength

than 8 km below sea level) occur within the Quaternary volcanic provinces and the higher level geothermal areas, and the deep Curie depths (more than 15 km) occur over the pre-Neogene structural belts and the forearc basins on the Pacific side.

tacharyya in 1966. Marine magnetic anomalies are commonly obtained in lD-shaped lineations. Thus, a model combining horizontally long and thin magnetized bodies is useful in estimating the

0040-1951/91/$03.50

of the

Figure 1 shows anomalies caused by igneous rocks on the west side and linear marine magnetic B.V.

Y. OKUBO

104

350 350 325

250

-250-27% -3cc-325%

ET AL.

4

1

-225 -250 --275 -3oe

“” BELOW LINDEFINED AREA

100 Fig. 1. Shipbome

200km

magnetic map of study area. The color scale is at 2S nT Intervals. Dashed curve indicates the Japan Trench. Rectangle indicates the area for the estimate of depth to the marine magnetic layer.

pp.105-108

144O +45O

b+?

1390 42"+

+41° 144O

410+

+40° 400-e

+390

39Of

370 +

+42O

1430

2. Curie depth

map of Japan

indicating

the localities

denote inferred

of temperature Curie depths

measurements.

(km) below sea level.

Contour

interval

is 1 km. Contour

values

5@ + 37O 142O

MAGNETIC

MODEL

anomalies

OF THE

netic anomalies and Segawa

the trench anomalies

from

of Mesozoic

mafic

anomalies

cross the trench eventually

along

Estimation

of uncertain

origin

Party,

1980)

have

boundary

Mountains.

wavelength

the eastern

is

de-

between to the minor

in the midthe coastal the DSDP

identified

silicic

that

titanomagnetite

series

cluding

gisawa et al. (1980), these anomalies the silicic

plutonism,

by Yana-

may be correwhich

suggests

depth

analysis

to the in long-

fields. The first consideration

dominant

magnetite

magnetic xFe,TiO,(l

(Fe,O,),

loses

mineral,

the

- x)Fe,O,,

in-

its

ferromag-

at the Curie temperature (T,)and that the T,isotherm at depth is often a lower bound of the

netism

magnetized crust because the deeper layers with temperatures higher than T, possess only weak induced magnetization. The Curie temperature depends

creases

as was suggested

anomaly

considerations

of the

by spectral

the

the earliest

Miocene,

of general

in the estimation

Several

occur

are a number

involved

and gradually

plate.

There magnetic

and the coast. This is attributed of the oceanic

that

Nagata

with

of depth to the base of the magnetic

crust

plutonism in the lowermost Miocene of this area and if this plutonism did indeed take place during

lated

109

ARC

outcrop

disappearing

slope area of the quiet zone between and trench sides anomalies. However, (Scientific

Arc

JAPAN

to ultra-

age which

side of the Kitakami

magnetic

crease westward, subduction

high mag-

(1975) who suggest

are derived

intrusives

Trench

NGRTHEAST

by Ogawa and Suyama (1975)

and Oshima

along the coastal Japan

IN THE

along the coast of the Tohoku

the anomalies

The linear

ZONE

on the east side. Rem~kably

have been delineated

mafic

SUBDUCTION

netic

on the chemical minerals.

According

(1961), both

composition

as the

to the experiments amount

of titanium

the magnetization

Curie temperature ture of magnetite

of the mag-

intensity

of in-

and

the

decrease. The Curie temperais 580” C, and decreases with

earliest Miocene frontal arc volcanism beyond the main volcanic front (Honza, 1980). This further

increasing TiO, content to less than 100 o C. This suggests that Curie temperature at depth cannot

suggests that the magnetic anomalies of the island arc side reflect continental magnetic crust, and the crust can be assumed to consist of prismatic bodies, which is an approximation which is also be-

be easily defined. The temperature at the depth of the magnetic bottom was estimated by comparing the Curie depth map of the Japanese Islands compiled by the New Energy Development Organiza-

lieved to apply over the land. The magnetic boundaries can be estimated using the same algorithms that were used for the Curie

tion and the temperature gradients obtained from drilling which has had no effect on local convec-

depth estimation. Magnetic anomaly data from the forearc side of the Tohoku Arc were collected

It is inferred that the boundary between the crust and the mantle, or the Moho physicochemi-

by the Maritime the Association

Safety

Agency

and digitized

for the Development

by

of Earth-

quake Prediction. This paper discusses the magnetic boundary between the coast and the trench and presents an interpretation of the characteristics of the crust including aspects such as tectonic development, crustal thickness and thermal conditions. Another issue addressed here is the esti-

tion (Fig. 2).

cal boundary, bound

is, beneath

of the magnetic

1979) (i.e., where the

the continents, layer

the lower

(Wasilewski

T,isotherm

depth

et al.,

lies in the

mantle, as it does for some regions of oceanic crust and thin continental crust, the magnetic base occurs at the Moho). The landward side of the study area is an area of thin continental crust with no linear oceanic magnetic anomalies. The Research Group for Explosion Seismology (1977) noted that the Moho below the east coast of the

mated geometry of the magnetic layer in the oceanic crust as indicated by the 1D spectral analysis. Finally, the estimated magnetic boundaries on the land side and the geometry deduced from the marine magnetic anomalies are compiled, and the magnetic structure between the coast and trench

Tohoku Arc decreases in depth toward the east to less than 30 km. In addition, an analysis of heat flow data shows a Curie depth of more than 30 km beneath the western side of the study area (Hasebe et al., 1970). Fujii and Kurita (1978)

along the Tohoku

reworked

Arc is discussed.

the diagrams

of Hasebe

et al. (1970) by

Y. OKUBO

110

consistent

with

estimated

from drilling

an average

backarc

side

Conrad

discontinuity

depth

and

(Okubo

et al.,

Beneath

the Pacific

on

the

estimate

the

magnetic

Fig. 3. Cross sections of heat flow (a) and structure (b) beneath

Tohoku

forearc

island arcs (Fujii and Ku&a,

mathematical

.I

1978). VP velocities from Re-

comprises

considering a descending mantle convection system (Fig. 3). It is suggested in Fig. 3 that the

the Curie

to the magnetic side, however,

the

boundary

is more than 10 km.

above

considerations, boundaries

area using

we can

beneath

the

analysis.

The

a collection

of random

to the

should

emphasize,

volcanic

prism is only a convenient ing the necessary theory,

front and the trench.

In addition,

m level indicate

drilling a low

temperature gradient of less than 2O”C/km (Okubo et al., 1989), but when the temperature gradient is constant at 20 o C/km, the depth to the 500° C isotherm remains at about 25 km. These magnetic base of the conthe coast and the trench

samples

is based from a

uniform distribution of rectangular prisms, each prism having constant magnetization. The model was introduced by Spector and Grant (1970), and has proven very successful in estimating average depths

from the 3000-4000

spectral

model on which our analysis

depth of the 500 o C isotherm is shallow below the land but drops to more than 50 km between the

results suggest that the tinental crust between may be at the Moho. The partitioning of sialic layer and a lower

the

--

search Group for Explosion Seismology (1977).

results

on the

Hence,

than

may be a magnetic

when the Curie depth Based

1989).

is deeper

discontinuity

gradient

data of 45”C/km

does not correspond

boundary. Conrad

temperature

ET AL.

geological

relief

of magnetized

however,

that

bodies.

We

the rectangular

geometry for developand is not a required

model.

Our principal result based on the Spector and Grant analysis is that the expected value of the spectrum for the assembly of prism-shaped magnetized bodies is the same as that of a single body with the average parameters of the assembly. We then develop the equations expressing the theoreti-

the crust into an upper mafic layer may be repre-

sented by the seismic expression (e.g., refraction velocity at a “Conrad” discontinuity or transition zone in the middle to lower crust) or by the petrological composition of the layers. For estimating the magnetic base by spectral analysis, the contribution of the lower mafic crustal layer is important because the upper horizon of this layer corresponds to one of the magnetic boundaries when the Curie depth is in the lower crustal layer or the mantle. The Conrad discontinuity beneath the backarc side of the Tohoku Arc lies at a depth of about 20 km, this depth decreasing gradually towards the east and eventually reaching about 10 km beneath the coast on the Pacific side of the arc (Research Group for Explosion Seismology, 1977). The average Curie depth inferred from the magnetic spectral analysis, on the other hand, lies about 10 km beneath the Japanese Islands. This is

5/64

IO/64

0

wavenumbar(kn-‘)

Fig. 4. Spectrum for the Curie depth estimate. The data used he within a 64 km x shown

in

in 1F(s)/s

Fig.

5.

64 km

square area whose center is point A

Crosses

denote

spectrum

defined

as

1,where s is the radial frequency and F(s) is the

spectrum of the magnetic anomaly. Solid line indicates mean values. Here, 10.6 km is obtained as the centroid depth (2,) using the slope gradient of spectrum between the 64 km/cycle and the 64/3

km/cycle.

The depth to top of magnetic crust in

this area is calculated to be 5.6 km; the bottom of the magnetic crust is therefore at about 15 km.

MAGNETIC

MODEL

cal spectrum tions,

OF THE

SUBDUCTION

ZONE

for the single body.

we can determine

as the depth

average

to the base,

IN THE

Using

NORTHEAST

the equa-

parameters,

by comparison

such of the

JAPAN

111

ARC

anomalies,

but

corresponds toured

the average

to the Moho

depth, depth

area. The estimated

about

beneath

depths

20 km, the con-

to the magnetic

spectrum for the observed anomaly with the theoretical anomaly. Based on 3D magnetic model

relief, on the other hand, range widely, from 5 to 10 km. This means that, excepting the local mag-

studies,

netic sources

it certainly

seems

useful in estimating

that

the algorithm

the Curie depth (Okubo

is

et al.,

correspond

rocks, the magnetic to the Conrad

dis-

continuity.

1985a). The algorithm to calculate extensive was fixed liminary

requires

the

Curie depth

an extensive

spectrum.

Hence,

2D data set the

inferred

should be an average depth within

square

area.

This block

size used here

at 64 km over all areas study

using

a block

an

after

dimensions

(magnetic

sources)

Estimation

of the geometry

of subducted

oceanic

crust

a pre-

size of 128 km was

carried out. Based on 3D magnetic model studies, a minimal ratio of 12 : 1 or 13 : 1 for block size to prism

such as intrusive

relief may roughly

is necessary

for reasonable estimates. The implication from this for the magnetic data of the studied area is that a minimum block size of about 60 km is

The magnetic anomalies over the trench, which trend at about 60 o N, gradually decrease in amplitude and eventually the trench.

disappear

The decrease

about

in amplitude

100 km from reflects

sub-

duction of the oceanic crust. The original Vinee Matthews model (Vine and Matthews, 1963) pro-

necessary for resolving the smallest anomalies (approximately 5 km). Also, in the case of a 64 km block size the maximum depth to the base is

posed a homogeneous 20 km thick source layer. Subsequent anomaly models generally reduced this to a uniformly magnetized layer of about 0.5 km in thickness (e.g., Talwani et al., 19’71). More

calculated to be about 20 km from the 3D magnetic model. Hence, the estimated depth of more

recent work, however, suggests that the source layer is neither homogeneous nor thin (e.g., John-

than

son,

20 km exceeds

the maximum

depth

which

1979;

Strangway

Banerjee, (1987),

1984). in

has been resolved precisely. Figure 4 shows a representative sample of the spectrum for the Curie

evolution

depth estimate. The low-frequency range

netic layer of more than decrease in the amplitude

frequency

spectrum is stable in the rather than in the high-

range. Thus the depth

the magnetic crust using the spectrum

to the middle

of

(centroid, Z,,) is estimated of the low-frequency range

using the least-squares method. When the spectrum in the low-frequency range shows a linear feature, the centroid is accepted as reliable. Then the depth to the top of the magnetic crust is estimated using a method which is very similar to that

used for the centroid

bottom (Z,) = 22, - z,.

is calculated

and

the depth

to the

from these values:

Z,

Figure 5 shows the depth to the magnetic base estimated in the western part of the studied area. This includes small oceanic magnetic anomalies. The magnetic bottom is at around 20 km, as mentioned above, some results from deeper than 20 km are unreliable. The results from the eastern part may be disturbed by the oceanic magnetic

caused

considering

of a subducting

by: (1) increase

netic layer, (2) magnetic layer with increasing Makino and analysis method depth to the

Arkani-Hamed

plate,

the estimate

and thermal a mag-

10 km in thickness. The of the anomalies can be in the depth

to the mag-

disappearance or collapse of the by friction, (3) demagnetization temperature. Okubo (1988) developed a spectral using 1D data for estimating the middle of the magnetic layer

(centroid), responsible for producing the oceanic magnetic anomalies. The magnetic model consists of several

thin and infinitely

long prisms

extend-

ing at right angles to the data line and each prism is normally or reversely magnetized. The algorithm for this model may be useful for estimating the distance from the observed line to the middle of the descending magnetic layer. Model studies suggest that if the prism is thin enough compared to the depth to the centroid, the method of Makino and Okubo (1988) will be successful.

112

‘J. OKUBO

ET AL

Tohoku

+

200

0

I Fig. 5. Estimated superimposed

depth

to bottom

on the magnetic

of magnetic

map.

layer (heavy

Black squares

contour

are centers indicated

lines at 5 km intervals

of 64 km’ areas

with question

marks.

km

and values

for the spectral

analysis.

attached

LO black

Questionable

squares) values

are

MAGNETIC

MODEL

OF THE

SUBDUCTION

ZONE

IN THE

NORTHEAST

JAPAN

113

ARC

The study area for estimating the depth to the centroid of the magnetic layer is shown in Fig. 1. Magnetic data along N-S

profiles which cross the

trench at an angle of less than 10” were obtained, so the data lines are roughly parallel to the trench. However, they do not extend at right angles to the magnetic lineations, by 20-30”

and accordingly they deviate

from the perpendicular;

in the model

it was proved that a deviation of less than 30” can be ignored. Figure 6 shows the magnetic anomaly of the 1D data of the N-S

profiles and the accompanying

spectrum. Figure 7 is the result of depth estimates using the 1D marine magnetic data and shows that the magnetic layer begins to descend at the trench at a low angle of about 4”. This angle seems to agree with the angle of the subducted slab at the trench as indicated by multi-channel

A/\

A

0 vvv

/J

-700 t

(a)

-400;

o

n

I

I

40

/

I

80

I

I

120

/

I

160

11 200

Ckml

Fig. 7. Estimated depth to middle of magnetic layer in the oceanic crust. TA denotes approximate location of the Japan Trench. See text for discussion.

60 km from the trench and then increases to 20 o _ Hasegawa et al. (1978) revealed that the deep

200 -

+

0

seismic reflection records (Nasu et al., 1980). This angle remains constant up to a distance of about

,O”5 -

301

j

300

100

~14.5

8

km

seismic zone in this region is distinctly separated into two planes, which are almost parallel to each other. If the estimated magnetic layer continues to descend at an angle of 20 O, the magnetic layer will lie at a depth of about 60 km beneath the eastern coast. This corresponds to the depth of the upper deep seismic zone. The deepest estimated magnetic sources are at about 30 km, which means that the Curie depth

km 1

occurs at at least > 30 km around 143” E, and that the magnetic layer deeply penetrates the mantle. Compiled magnetic structure and conclusions

cycle/lOOkm Fig. 6. Magnetic profile (a) and its spectrum for the estimate of the marine magnetic layer depth (b). The magnetic profile trends N-S and runs down the center of the area indicated in Fig. 1 by the box.

T= magnetic intensity and P = power

spectrum of T, which is defined as P(w) = Kd2 exp( -2wh), where K is a constant, w is the angular frequency, d is the half width of the magnetic layer, and h is the depth to the centroid of magnetic layer. Here, 14.5 km is obtained as the centroid depth (h).

Figure 8 shows a summary of the magnetic model across the Tohoku Arc. Beneath the Tohoku Arc the Curie depth is less than that of the Conrad and Moho discontinuities. The base level of the magnetic layer is at the Curie isotherm. The Curie depth between the coast and the trench is more than 30 km because the oceanic magnetic anomalies may be traced to a depth of 30 km at a distance of 100 km from the trench. This means that Curie depth increases suddenly from 10 to

Y. OKUBO

114

f&F coast

tkNSW

ET AL.

coast

VF

TA

0

5OL km Fig. 8. Summarized magnetic model across the Tohoku Arc. VF and TA denote volcanic front and the trench axis, respectively.

more than 30 km in the vicinity of the coast. Hasebe et al. (1970) have discussed the thermal model beneath the Tohoku Arc and according to their results the observed heat flow data can be explained by assuming the descent of the cold oceanic plate. This suggests that the sudden increase in Curie depth may be due to the effect of this descending cold oceanic plate, and the Conrad and than

Moho

discontinuities

the Curie isotherm.

Hence,

become

shallower

the depth

to the

magnetic base may correspond to the Moho, and the magnetic relief may correspond to the Conrad discontinuity. suggests that

The magnetic spectrum analysis the depths to the base and upper

horizon of the magnetic crust roughly correspond to those for the Conrad and the Moho derived from the analysis of the data obtained by marine seismic studies. The magnetic layer which generates the oceanic magnetic anomalies was interpreted as descending at a shallow angle of about 4”) which agrees well with the seismic reflection data at the trench. The oceanic magnetic layer can be traced to a depth of at least 30 km, and it can be extrapolated to the upper surface of the deep seismic zone beneath coast.

the

subject. We also wish to thank the staff of the Association for the Development of Earthquake Prediction for the use of the digitized shipborne magnetic map.

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B.K., 1966. Continuous spectrum of the total-

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Acknowledgements

Geophys. Space Phys., 17: 215-226. Makino, M. and Okubo, Y., 1988. Spectrum analysis of marine magnetic anomalies. Butsuri-Tansa (Geophys. Explor.), 41:

We wish to thank E. Honza of the Geological Survey of Japan for critically reading the manuscript, and M. Ozima of the University of Tokyo, Y. Honkura of the Tokyo Institute of Technology, S. Oshima of the Maritime Safety Agency and Y. Ogawa of the Geological Survey of Japan for much encouraging discussion on this

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