- Email: [email protected]
A study of the geomagnetic anomalies from deep crustal sources in the southeastern coastal region of China
358
Physics of the Earth and Planetary Interiors, 44 (1986) 358—365 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
A study of the geomagnetic anomalies from deep crustal sources in the southeastern coastal region of China Ren Guo-Tai and Yan Ya-Fen * Institute of Geophysics, Academia Sinica, Beijing (People’s Republic of China) (Received December 22, 1985; revised March 15, 1986)
Ren, G.-T. and Yan, Y.-F., 1986. A study of the geomagnetic anomalies from deep crustal sources in the southeastern coastal region of China. Phys. Earth Planet. Inter., 44: 358—365. This paper first makes use of a lot of data extracted from the magnetic anomalous information from deep crustal sources, then discusses the relations between magnetic anomalies and distribution of seismic belts and between the regional gravity field and deep geological structures.
1. Introduction
and deep geology, we have investigated the structure of the crust and other problems in this region..
We completed a geomagnetic survey of three elements (H, D, I) on the Earth’s surface, in the southeastern coastal region of China in 1958 and 1964. The whole survey region is about 130 000 j~2 and the number of observations is 266. The survey data are important for studying the structure of the earth’s crust in this region. Therefore, we have reduced them to~themain field values of 1960.0. Then we converted H, D and I into X, Y and Z in an orthogonal coordinate system for convenient analysis. We have set up two kinds of spatial distribution models based on the above data and the whole data of China in the 1960’s. Finally, we have computed the regional magnetic anomaly values that reflect the deeper magnetic medium stratum and we have drawn the corresponding maps. This paper is based on the above maps and is combined with the results of seismology, gravity **
This paper is from an international symposium on ‘Interpretation of Magnetic Anomalies from the Lower Crust presented at the 5th General Assembly of the International Association of Geomagnetism and Aeronomie held at Prague in August 1985.
0031-9201/86/$03.50
© 1986 Elsevier Science Publishers B.V.
2. The spatial distribution model of the geomagnetic field and the magnetic anomaly maps of a deep crustal source 2.1. The model of the regional magnetic field from 23°—26°N and 116 °—120°E for 1960.0 To study the magnetic anomaly that is from the magnetic medium stratum in the deep crust, we have firstly to set up the model of spatial distribution and variation of geomagnetic field in the corresponding region. This model must filter the noise from local magnetic bodies in the Earth’s surface and the upper crust. That is to say the model only reflects the magnetic field from the magnetic medium stratum in the deep crust and electric current system in the Earth’s core. According to the continuous character of the spatial variation of the geomagnetic field, we can use the Taylor polynomial to express its regional spatial distribution and spatial variation (Bhattacharyya and Morley, 1965). When the studied region is far larger than the local magnetic
359
anomalies within it, then the use of a proper order polynomial can filter out the noise of the local anomalies. In fact, the distnbutioiial range of 40 local magnetic anomalies in this region were all less than 1000 km2 in area, so the above condition was satisfied. After a thorough investigation and examination we took polynomial (n 3) to fit the spatial distribution of the geomagnetic field in this region. Its coefficients were computed using the data from 266 points
D
+ — —
=
H
(0.365674 — 0.234351 Ap
=
—
(—
=
~
=
0.024603 AX
+ 1.530871
AX2 + 63.6446673 Ap3
—143.6597438 Ap2AX + 222.2896070 Ap AX2
1.0756284 AX2— 23.30744378 Ap~
+ 42.11958160
0.463731 AX2 + 21.2639293 Aq3 59.0539864 ~2AX + 71.7444881 Aq AX2
— 11.9314182 AX3) X 3438’ ~ 6416+ 1 66351 A + 0 03865 AX . P —0.034030 Ap2 — 1.965102 Ap AX
—0.8738880 Acp2 + 0.3219339 Ap AX +
0.0373—0.27881 Aq — 0.09187 AX 0.096554 Ap2 — 0.070960 Aq AX
—57.0450493 AX3) X 3438’
Ap2AX + 2.53319354 Ap AX2
where:
~
=
—
~o’ AX
=
X
—
0 ~ 26°N, X0 118°E; the unit of Ap and AX is radian; the =
3) x iO~nT
=
—41 .81394476 AX
28:J:~ri~cTI~.j7TTrTfli~0 Iç$’~’ / 1 .1 1’~ PU,CH~Mr~
~,
(~/
:;://,/.i,/,;.,.....:
I’
27 ~
I, /
/
/ /
///,‘ I ~ 0.’ 25
-
..
I.
/
/
/
~
I!
/
j
// /
fi /
~
—50
/
\
\
/
/
•‘.
.
~
~.
.
~
—
-—
~“i~~°
~
_/,,~
/
‘/7/
//
/
/
/
‘
~
50
‘,
“.
~
/‘ ~‘iXlAMEN ////,/1:/ //,i~.;
/
,f~
~t.• h,
I
..~
117°
/ ‘
,/74J7T~ ,‘
/
/
“/~
,/, 18”
. .~
:
~‘....—‘
\
/1/
,/
1
/
‘~
~
im ~
~
AN1’~J.~~7
-
I
,~-~/rii~
~
‘..
••“
/i
~~-//
,“ ,..‘/
-
/
//
/ /
I
16°
/
~
-
-~
. iOU
7 7. 24
,
/
,1
/
“i/Il I I—--I--- ‘~uQ~~ •/ / I II ~M \.. /
.
\
/
“
/ ~
~‘
,•‘
/
/
/ /1. / / . ~/.‘ /~ii / / ,1 ‘ / / I
26
—20
(
X
1~
(Z)
,
1I9~
120
Fig. 1. The anomalous vertical field component (Z) of the deep crustal source.
360
negative sign of Declination indicates that it was to the west of north, 2.2. Spatial distribution model of the whole country ~ normal geomagnetic field (1960.0) We want to get some information that reflects the anomalous sources in the deep crust, but the above model expresses the magnetic field distribution from a deep crustal source and from the Earth’s core. So we must set up another model that mainly expresses the magnetic field from the Earth’s core only. We selected the normal magnetic field of our whole country and its neighbouring area as the magnetic field from the following source. H
=
(0.33371
=
— —
0.0824299 AX2 + 0.31997908 Ap3
nT
—
0.00646989 Ap2AX + 0.37424377 Ap AX2
the intense magnetic anomaly (the largest anomalous value> 500 nT). According to the above the main basis and the
(—0.02792
AX3) X lO5nT,
0.08750 Ap
—
Ap2
—
—
0.17908 AX
0.923981 Ap AX
—0.106976 AX2 + 0.1967667 Ap3 —0.0728342 Ap2AX + 0.7451309 Ap AX2 + 0.2595494 =
The main basis of a divisional anomalous zone (or area) is the position of the zero contour (Z); at the same time reference can be made to the variational features of other elements contours. According to the largest anomalous value of Z in each zone (or area), we have divided them as follows: the weak magnetic anomaly (the largest anomalous value ~ 100 nT), the stronger magnetic ~anomaly nT < the largest anomalous value 200 nT),(100 the strong magnetic anomaly (200
—
+ 0.013908
I
3. Division of the magnetic anomalous zone (or area) of the deep crustal source
0.367475 Ap 0.001039 AX 0.3407651 Ap2 + 0.1248313 Ap AX —
+ 0.00899436
D
value from the above two kinds of model. To draw the anomaly maps conveniently, we have computed the value on the network (20’ X 20’), we have drawn the anomaly maps by the network values as follows.
(0.87529
+
AX3) x 3438’,
1.49127 Ap
—1.213123 Ap2
—
—
0.03220 AX
0.215389 Ap AX
—0.056948 AX2 + 0.5635299 Ap3 + 0.6745236 —
Ap2AX
—
0.4298905 A/ AX2
0.0786568 AX3) X 3438’.
In the above formulas except for p~=34°N and X 0 104°E,the other symbols and units are the same as the former model. =
2.3. The anomalous value ofa deep crustal source in the southeastern coastal region of China and the corresponding regional anomaly maps value the deep crustal source at The everyanomalous point was the ofdifference in computed
<
the largest anomalous value ~ 500 nT) and
standard, we put the studied region divided to the southeast of Fujian province with the western half of the Taiwan Strait weak positive anomalous zone; the western half of the Fujian province stronger negative anomalous area; the northern part of Fujian province and the southern part of the Zhejiang province intense positive anomalous area. 3.]. The southeast of the Fujian province with the western half of Taiwan Strait weak positive anomalous zone, hereafter called the weak positive anomalous zone 3.1.1. The range and the trend of the weak positive anomalous zone The western boundary of the zone was located in the Daiyun mountain and its extended direction. The eastern boundary was located in northeast direction through the middle part of the Taiwan Strait. The position ~6f the northern boundary in 260 20’N and the southern half of the zone extended to the south China Sea.2. The Thewhole zone area of the zone is about 100 000 km trends NNE.
361
28 ~
27
/
26
-~-~-~-\~
/
j/
-
,/
\
,/~
,i~.J~FUZHcLu
\
\
\~
i~-~ JH-7~±t1~t1 /~
_
25 I ( ~ 24~~
/~/~Y~
1~
/~rY//~/~-T
%4~
2.3~j~H
cx,
Fig. 2. The anomalous north field component (X) of the deep crustal source.
3.1.2. The main features of the magnetic field A common feature is the symmetry in the magnetic anomalous spatial distribution of the above three anomalous zones (or areas). The symmetrical axis of the weak positive anomalous zone is its central axis and it trends NNE. The position of the northern half of the symmetrical axis is located in the Changle—Xiamen deep-large break zone. The southern half has broken away from the continent and extends to the south China Sea. 3.2. The western half of the Fujian province stronger negative anomalous area, hereafter called the stronger negative anomalous area 3.2.1. The range and the trend of the stronger negative anomalous area The eastern side of this area is the weak posi-
tive anomalous zone. Its western zero contour is located along the Wuyi mountain. The northern part of the area extends from the northeast part of Jiangxi province to the southwest part of Zhejiang province. The other boundary has not been determined because of lack of data. Its distributed range is more than 100 000 km2.
3.2.2. The main feature of the magnetic field The anomalous intensity was —200 nT at the centre and is located in Chongan county of the Fujian province. The form of the contours corresponds to an ellipse. The trend of the ellipse’s long axis is in a north—south direction within the northern part of 27°N. In the southern part of 27°Nthe trend of the long axis changes to NNE.
362
28/~4J~~ r •‘S~
~L—~ ~
I
PUCHEj’J
-~
0~
~
Jk~ 27
~
.~
~
2~ 1160
I
...~
___
~
26
250
Ys.
~
___
~
__
—
/
c~,
4
,,/ 117~
~uj~
~-/-7 ~
/,//
~
/,/
~////~~
(Y)
1190
120~
Fig. 3. The anomalous east field component (Y) of the deep crustal source.
3.3. The intense positive anomalous area north of Fujian province and south ofZhejiang province, hereafter called the intense positive anomalous area 3.3.1. The range of the intense positive anomabus area The southeast coastal region of China was mostly covered by a weak anomaly, but in the north of Fujian province together with the south of Zhejiang province there was an intense positive anomalous area. The largest anomalous value was more than 500 nT. In the southern and western side of the area is the weak positive anomalous zone. Its eastern part extends to the east China
Sea, but its boundary has not been determined because of lack of data. 3.3.2. The main features of the magnetic field Firstly, both the anomalous intensity and its spatial variation rate are the largest in this area compared with other anomalous zones (or areas). Secondly, the position and trend of the contour symmetrical axis coincides with the extended part of the symmetrical axis of the weak positive anomalous zone. Thirdly, magnetic anomalous intensity of two elements (X and Y) are small and positive, this indicates that the magnetic material in the deep crust was magnetized vertically.
363
4. Further study on the anomalous zone (and area) of the deep crustal’ source 4.1. The relation between the anomalous zone (or area) and seismic activity (a) All of the strong earthquakes occur within the weak positive anomalous zones. On the westem side of the stronger negative anomalous area, there is a weak positive anomalous zone too, and many strong earthquakes have occurred in the past. The epicentre data (see Fig. 4) have proven that the boundaries of above magnetic anomalous zones are the control lines of the southeast coastal seismic zone. (b) The weaker earthquakes occurred within the weak positive zone too. Figure 5 shows, on the zero contours eastern side, that the frequency of weaker earthquakes exceeds those from the westem side. (c) In both the stronger negative anomalous
251
area and the intense positive anomalous area, no strong earthquakes or very small weaker earthquakes have occurred, so they are seismically stable areas. (d) The boundary of the weak positive anomalous zone coincides with the control line of the strong earthquakes (see Fig. 4). It is shown that the area of the weak positive anomalous zone coincides with the famous southeast coastal seismic zone of China. This point is very important, because it provides a basis for the division of seismic zones from the geophysical field data. 4.2. The relation between the magnetic anomalous zone (or area) and the regional gravity field According to the results of the gravity study, we can divide the southeast coastal region of China into two units of the first magnitude. They are the gravity gradient zone of the southeast of province Fujian and Pucheng-Yongan negative gravity field.
_
F /~ ____
.1
_______
116°
117~
J 118°
Fig. 4. The map of the strong earthquake epicentres.
/ T0M>B O7>M~6
A’ / i
119°
06>M~5~ p5’M~4~~ 120°
364
A
-.1 1
L-~.~
~-~•-‘~
27~.~L
6i~
;/
_________
2c~N~HE~
~
ZH0~
~
/
__ 116°
__;i:
117~
1180
‘/~.
I
2~M~3
~
119°
120°
Fig. 5. The map of the weak earthquake epicentres.
Both the distributed range and the trend of the gravity gradient zone coincide with the weak positive anomalous zone and the intense positive anomalous area. The gravity anomalous value and its contour, after being extended up to 25 km, decrease with height except for the Bouguer anomalous value. The range of the whole gradient zone, the gradient zone’s trend and the gradient value and so on coincide with the Bouguer gravity anomaly map on the Earth’s surface. It is shown that the gradient zone, in this region, is related to the deep structure of the Earth’s crust. At the same time it is indicated that there is a magnetic anomalous source deep within the Earth’s crust (see Fig. 6). The range of the negative gravity field coincides with the strong negative anomalous area too (see Fig. 6). The thickness of the Earth’s crust is the greatest within the negative gravity field. According to the theory of isostasy, the mean
density of the Earth’s crust is different between the strong negative anomalous area and the weak positive anomalous zone. That is to say they are composed of different materials. 4.3. The relation between the magnetic anomalous zone (or area) and the deep geological structure 4.3.1. Geological sense of the boundaries of the geomagnetic anomalous zone (or area) In the whole region studied there are two boundaries that possess an important geological sense. They are located on both sides of the weak positive anomalous zone. The western one extends along the Daiyun mountain. Some geological data have proven that it coincides with a deep fault. The eastern one passes the middle part of the Taiwan Straits from the northeast to the southwest. Owing to it being a control line of strong earthquakes and a boundary of positive and nega-
365
________ ________
30
0
)~
and the eastern end of the Nanling latitudinal structural system. Now, the problem is whether or not the two structural systems belong to the same
structural ural system unit is in further this they region. divided into three secondstructing, class Xianyou-Zhangping structural belts, and areThe Lianjiang-Ninghua the Nanling Xiamen-Nanj-
‘J’~~
28°
two magnitude 1 structural systems in this area. They are the new Cathaysian structural system
second-class structural belts. In the local magnetic anomaly maps of the three elements, all of them
L_.)
have three local magnetic anomalous belts existing second-class structural belts do exist. However, in the anomaly map of the deep crustal source (Z), the contours trend NNE or NE, that is to say they are coincident with the trend of the new Cathay-
~ _______
~~~cEMG
maps (X, Y) possess the above character too. in Therefore, the abovethey area. indicate So it isthat demonstrated the effectualthat depth three of latitudinal sian structural structural system. system The isother more two shallow element than the new Cathaysian system. That is to say the former ought to be a second-class structural unit as compared with the latter. Acknowledgement We express our sincere thanks to Dr. Xia GuoHui for providing help in this work.
Fig. 6. The map of the regional gravity field after continuing upwards to 25 km.
tive magnetic anomalous zones, we think it j~ located over a deep and large fault. 4.3.2. The effectual depth of the latitudinal structural system in the southeast coastal region From the viewpoint of geomechanics, there are
References Bhattacharyya, B.K. and Morley, L.W., 1965. The delineation of deep crustal magnetic bodies from total field aeromagnetic anomalies. J. Geomag. Geoelectr., 17: 237—251.
Physics of the Earth and Planetary Interiors, 44 (1986) 358—365 Elsevier Science Publishers B.V., Amsterdam — Printed in The Netherlands
A study of the geomagnetic anomalies from deep crustal sources in the southeastern coastal region of China Ren Guo-Tai and Yan Ya-Fen * Institute of Geophysics, Academia Sinica, Beijing (People’s Republic of China) (Received December 22, 1985; revised March 15, 1986)
Ren, G.-T. and Yan, Y.-F., 1986. A study of the geomagnetic anomalies from deep crustal sources in the southeastern coastal region of China. Phys. Earth Planet. Inter., 44: 358—365. This paper first makes use of a lot of data extracted from the magnetic anomalous information from deep crustal sources, then discusses the relations between magnetic anomalies and distribution of seismic belts and between the regional gravity field and deep geological structures.
1. Introduction
and deep geology, we have investigated the structure of the crust and other problems in this region..
We completed a geomagnetic survey of three elements (H, D, I) on the Earth’s surface, in the southeastern coastal region of China in 1958 and 1964. The whole survey region is about 130 000 j~2 and the number of observations is 266. The survey data are important for studying the structure of the earth’s crust in this region. Therefore, we have reduced them to~themain field values of 1960.0. Then we converted H, D and I into X, Y and Z in an orthogonal coordinate system for convenient analysis. We have set up two kinds of spatial distribution models based on the above data and the whole data of China in the 1960’s. Finally, we have computed the regional magnetic anomaly values that reflect the deeper magnetic medium stratum and we have drawn the corresponding maps. This paper is based on the above maps and is combined with the results of seismology, gravity **
This paper is from an international symposium on ‘Interpretation of Magnetic Anomalies from the Lower Crust presented at the 5th General Assembly of the International Association of Geomagnetism and Aeronomie held at Prague in August 1985.
0031-9201/86/$03.50
© 1986 Elsevier Science Publishers B.V.
2. The spatial distribution model of the geomagnetic field and the magnetic anomaly maps of a deep crustal source 2.1. The model of the regional magnetic field from 23°—26°N and 116 °—120°E for 1960.0 To study the magnetic anomaly that is from the magnetic medium stratum in the deep crust, we have firstly to set up the model of spatial distribution and variation of geomagnetic field in the corresponding region. This model must filter the noise from local magnetic bodies in the Earth’s surface and the upper crust. That is to say the model only reflects the magnetic field from the magnetic medium stratum in the deep crust and electric current system in the Earth’s core. According to the continuous character of the spatial variation of the geomagnetic field, we can use the Taylor polynomial to express its regional spatial distribution and spatial variation (Bhattacharyya and Morley, 1965). When the studied region is far larger than the local magnetic
359
anomalies within it, then the use of a proper order polynomial can filter out the noise of the local anomalies. In fact, the distnbutioiial range of 40 local magnetic anomalies in this region were all less than 1000 km2 in area, so the above condition was satisfied. After a thorough investigation and examination we took polynomial (n 3) to fit the spatial distribution of the geomagnetic field in this region. Its coefficients were computed using the data from 266 points
D
+ — —
=
H
(0.365674 — 0.234351 Ap
=
—
(—
=
~
=
0.024603 AX
+ 1.530871
AX2 + 63.6446673 Ap3
—143.6597438 Ap2AX + 222.2896070 Ap AX2
1.0756284 AX2— 23.30744378 Ap~
+ 42.11958160
0.463731 AX2 + 21.2639293 Aq3 59.0539864 ~2AX + 71.7444881 Aq AX2
— 11.9314182 AX3) X 3438’ ~ 6416+ 1 66351 A + 0 03865 AX . P —0.034030 Ap2 — 1.965102 Ap AX
—0.8738880 Acp2 + 0.3219339 Ap AX +
0.0373—0.27881 Aq — 0.09187 AX 0.096554 Ap2 — 0.070960 Aq AX
—57.0450493 AX3) X 3438’
Ap2AX + 2.53319354 Ap AX2
where:
~
=
—
~o’ AX
=
X
—
0 ~ 26°N, X0 118°E; the unit of Ap and AX is radian; the =
3) x iO~nT
=
—41 .81394476 AX
28:J:~ri~cTI~.j7TTrTfli~0 Iç$’~’ / 1 .1 1’~ PU,CH~Mr~
~,
(~/
:;://,/.i,/,;.,.....:
I’
27 ~
I, /
/
/ /
///,‘ I ~ 0.’ 25
-
..
I.
/
/
/
~
I!
/
j
// /
fi /
~
—50
/
\
\
/
/
•‘.
.
~
~.
.
~
—
-—
~“i~~°
~
_/,,~
/
‘/7/
//
/
/
/
‘
~
50
‘,
“.
~
/‘ ~‘iXlAMEN ////,/1:/ //,i~.;
/
,f~
~t.• h,
I
..~
117°
/ ‘
,/74J7T~ ,‘
/
/
“/~
,/, 18”
. .~
:
~‘....—‘
\
/1/
,/
1
/
‘~
~
im ~
~
AN1’~J.~~7
-
I
,~-~/rii~
~
‘..
••“
/i
~~-//
,“ ,..‘/
-
/
//
/ /
I
16°
/
~
-
-~
. iOU
7 7. 24
,
/
,1
/
“i/Il I I—--I--- ‘~uQ~~ •/ / I II ~M \.. /
.
\
/
“
/ ~
~‘
,•‘
/
/
/ /1. / / . ~/.‘ /~ii / / ,1 ‘ / / I
26
—20
(
X
1~
(Z)
,
1I9~
120
Fig. 1. The anomalous vertical field component (Z) of the deep crustal source.
360
negative sign of Declination indicates that it was to the west of north, 2.2. Spatial distribution model of the whole country ~ normal geomagnetic field (1960.0) We want to get some information that reflects the anomalous sources in the deep crust, but the above model expresses the magnetic field distribution from a deep crustal source and from the Earth’s core. So we must set up another model that mainly expresses the magnetic field from the Earth’s core only. We selected the normal magnetic field of our whole country and its neighbouring area as the magnetic field from the following source. H
=
(0.33371
=
— —
0.0824299 AX2 + 0.31997908 Ap3
nT
—
0.00646989 Ap2AX + 0.37424377 Ap AX2
the intense magnetic anomaly (the largest anomalous value> 500 nT). According to the above the main basis and the
(—0.02792
AX3) X lO5nT,
0.08750 Ap
—
Ap2
—
—
0.17908 AX
0.923981 Ap AX
—0.106976 AX2 + 0.1967667 Ap3 —0.0728342 Ap2AX + 0.7451309 Ap AX2 + 0.2595494 =
The main basis of a divisional anomalous zone (or area) is the position of the zero contour (Z); at the same time reference can be made to the variational features of other elements contours. According to the largest anomalous value of Z in each zone (or area), we have divided them as follows: the weak magnetic anomaly (the largest anomalous value ~ 100 nT), the stronger magnetic ~anomaly nT < the largest anomalous value 200 nT),(100 the strong magnetic anomaly (200
—
+ 0.013908
I
3. Division of the magnetic anomalous zone (or area) of the deep crustal source
0.367475 Ap 0.001039 AX 0.3407651 Ap2 + 0.1248313 Ap AX —
+ 0.00899436
D
value from the above two kinds of model. To draw the anomaly maps conveniently, we have computed the value on the network (20’ X 20’), we have drawn the anomaly maps by the network values as follows.
(0.87529
+
AX3) x 3438’,
1.49127 Ap
—1.213123 Ap2
—
—
0.03220 AX
0.215389 Ap AX
—0.056948 AX2 + 0.5635299 Ap3 + 0.6745236 —
Ap2AX
—
0.4298905 A/ AX2
0.0786568 AX3) X 3438’.
In the above formulas except for p~=34°N and X 0 104°E,the other symbols and units are the same as the former model. =
2.3. The anomalous value ofa deep crustal source in the southeastern coastal region of China and the corresponding regional anomaly maps value the deep crustal source at The everyanomalous point was the ofdifference in computed
<
the largest anomalous value ~ 500 nT) and
standard, we put the studied region divided to the southeast of Fujian province with the western half of the Taiwan Strait weak positive anomalous zone; the western half of the Fujian province stronger negative anomalous area; the northern part of Fujian province and the southern part of the Zhejiang province intense positive anomalous area. 3.]. The southeast of the Fujian province with the western half of Taiwan Strait weak positive anomalous zone, hereafter called the weak positive anomalous zone 3.1.1. The range and the trend of the weak positive anomalous zone The western boundary of the zone was located in the Daiyun mountain and its extended direction. The eastern boundary was located in northeast direction through the middle part of the Taiwan Strait. The position ~6f the northern boundary in 260 20’N and the southern half of the zone extended to the south China Sea.2. The Thewhole zone area of the zone is about 100 000 km trends NNE.
361
28 ~
27
/
26
-~-~-~-\~
/
j/
-
,/
\
,/~
,i~.J~FUZHcLu
\
\
\~
i~-~ JH-7~±t1~t1 /~
_
25 I ( ~ 24~~
/~/~Y~
1~
/~rY//~/~-T
%4~
2.3~j~H
cx,
Fig. 2. The anomalous north field component (X) of the deep crustal source.
3.1.2. The main features of the magnetic field A common feature is the symmetry in the magnetic anomalous spatial distribution of the above three anomalous zones (or areas). The symmetrical axis of the weak positive anomalous zone is its central axis and it trends NNE. The position of the northern half of the symmetrical axis is located in the Changle—Xiamen deep-large break zone. The southern half has broken away from the continent and extends to the south China Sea. 3.2. The western half of the Fujian province stronger negative anomalous area, hereafter called the stronger negative anomalous area 3.2.1. The range and the trend of the stronger negative anomalous area The eastern side of this area is the weak posi-
tive anomalous zone. Its western zero contour is located along the Wuyi mountain. The northern part of the area extends from the northeast part of Jiangxi province to the southwest part of Zhejiang province. The other boundary has not been determined because of lack of data. Its distributed range is more than 100 000 km2.
3.2.2. The main feature of the magnetic field The anomalous intensity was —200 nT at the centre and is located in Chongan county of the Fujian province. The form of the contours corresponds to an ellipse. The trend of the ellipse’s long axis is in a north—south direction within the northern part of 27°N. In the southern part of 27°Nthe trend of the long axis changes to NNE.
362
28/~4J~~ r •‘S~
~L—~ ~
I
PUCHEj’J
-~
0~
~
Jk~ 27
~
.~
~
2~ 1160
I
...~
___
~
26
250
Ys.
~
___
~
__
—
/
c~,
4
,,/ 117~
~uj~
~-/-7 ~
/,//
~
/,/
~////~~
(Y)
1190
120~
Fig. 3. The anomalous east field component (Y) of the deep crustal source.
3.3. The intense positive anomalous area north of Fujian province and south ofZhejiang province, hereafter called the intense positive anomalous area 3.3.1. The range of the intense positive anomabus area The southeast coastal region of China was mostly covered by a weak anomaly, but in the north of Fujian province together with the south of Zhejiang province there was an intense positive anomalous area. The largest anomalous value was more than 500 nT. In the southern and western side of the area is the weak positive anomalous zone. Its eastern part extends to the east China
Sea, but its boundary has not been determined because of lack of data. 3.3.2. The main features of the magnetic field Firstly, both the anomalous intensity and its spatial variation rate are the largest in this area compared with other anomalous zones (or areas). Secondly, the position and trend of the contour symmetrical axis coincides with the extended part of the symmetrical axis of the weak positive anomalous zone. Thirdly, magnetic anomalous intensity of two elements (X and Y) are small and positive, this indicates that the magnetic material in the deep crust was magnetized vertically.
363
4. Further study on the anomalous zone (and area) of the deep crustal’ source 4.1. The relation between the anomalous zone (or area) and seismic activity (a) All of the strong earthquakes occur within the weak positive anomalous zones. On the westem side of the stronger negative anomalous area, there is a weak positive anomalous zone too, and many strong earthquakes have occurred in the past. The epicentre data (see Fig. 4) have proven that the boundaries of above magnetic anomalous zones are the control lines of the southeast coastal seismic zone. (b) The weaker earthquakes occurred within the weak positive zone too. Figure 5 shows, on the zero contours eastern side, that the frequency of weaker earthquakes exceeds those from the westem side. (c) In both the stronger negative anomalous
251
area and the intense positive anomalous area, no strong earthquakes or very small weaker earthquakes have occurred, so they are seismically stable areas. (d) The boundary of the weak positive anomalous zone coincides with the control line of the strong earthquakes (see Fig. 4). It is shown that the area of the weak positive anomalous zone coincides with the famous southeast coastal seismic zone of China. This point is very important, because it provides a basis for the division of seismic zones from the geophysical field data. 4.2. The relation between the magnetic anomalous zone (or area) and the regional gravity field According to the results of the gravity study, we can divide the southeast coastal region of China into two units of the first magnitude. They are the gravity gradient zone of the southeast of province Fujian and Pucheng-Yongan negative gravity field.
_
F /~ ____
.1
_______
116°
117~
J 118°
Fig. 4. The map of the strong earthquake epicentres.
/ T0M>B O7>M~6
A’ / i
119°
06>M~5~ p5’M~4~~ 120°
364
A
-.1 1
L-~.~
~-~•-‘~
27~.~L
6i~
;/
_________
2c~N~HE~
~
ZH0~
~
/
__ 116°
__;i:
117~
1180
‘/~.
I
2~M~3
~
119°
120°
Fig. 5. The map of the weak earthquake epicentres.
Both the distributed range and the trend of the gravity gradient zone coincide with the weak positive anomalous zone and the intense positive anomalous area. The gravity anomalous value and its contour, after being extended up to 25 km, decrease with height except for the Bouguer anomalous value. The range of the whole gradient zone, the gradient zone’s trend and the gradient value and so on coincide with the Bouguer gravity anomaly map on the Earth’s surface. It is shown that the gradient zone, in this region, is related to the deep structure of the Earth’s crust. At the same time it is indicated that there is a magnetic anomalous source deep within the Earth’s crust (see Fig. 6). The range of the negative gravity field coincides with the strong negative anomalous area too (see Fig. 6). The thickness of the Earth’s crust is the greatest within the negative gravity field. According to the theory of isostasy, the mean
density of the Earth’s crust is different between the strong negative anomalous area and the weak positive anomalous zone. That is to say they are composed of different materials. 4.3. The relation between the magnetic anomalous zone (or area) and the deep geological structure 4.3.1. Geological sense of the boundaries of the geomagnetic anomalous zone (or area) In the whole region studied there are two boundaries that possess an important geological sense. They are located on both sides of the weak positive anomalous zone. The western one extends along the Daiyun mountain. Some geological data have proven that it coincides with a deep fault. The eastern one passes the middle part of the Taiwan Straits from the northeast to the southwest. Owing to it being a control line of strong earthquakes and a boundary of positive and nega-
365
________ ________
30
0
)~
and the eastern end of the Nanling latitudinal structural system. Now, the problem is whether or not the two structural systems belong to the same
structural ural system unit is in further this they region. divided into three secondstructing, class Xianyou-Zhangping structural belts, and areThe Lianjiang-Ninghua the Nanling Xiamen-Nanj-
‘J’~~
28°
two magnitude 1 structural systems in this area. They are the new Cathaysian structural system
second-class structural belts. In the local magnetic anomaly maps of the three elements, all of them
L_.)
have three local magnetic anomalous belts existing second-class structural belts do exist. However, in the anomaly map of the deep crustal source (Z), the contours trend NNE or NE, that is to say they are coincident with the trend of the new Cathay-
~ _______
~~~cEMG
maps (X, Y) possess the above character too. in Therefore, the abovethey area. indicate So it isthat demonstrated the effectualthat depth three of latitudinal sian structural structural system. system The isother more two shallow element than the new Cathaysian system. That is to say the former ought to be a second-class structural unit as compared with the latter. Acknowledgement We express our sincere thanks to Dr. Xia GuoHui for providing help in this work.
Fig. 6. The map of the regional gravity field after continuing upwards to 25 km.
tive magnetic anomalous zones, we think it j~ located over a deep and large fault. 4.3.2. The effectual depth of the latitudinal structural system in the southeast coastal region From the viewpoint of geomechanics, there are
References Bhattacharyya, B.K. and Morley, L.W., 1965. The delineation of deep crustal magnetic bodies from total field aeromagnetic anomalies. J. Geomag. Geoelectr., 17: 237—251.