Magnetic crust in the Bay of Bengal

Marine Geology, 99 (19913 257 261

257

Elsevier Science Publishers B.V., Amsterdam

Letter Section

Magnetic crust in the Bay of Bengal D.C. M i s h r a National Geophysical Research Institute, Hyderabad-500007, India

(Received October 24, 1990; revision accepted May 22, 1991)

ABSTRACT Mishra, D.C., 1991. Magnetic crust in the Bay of Bengal. Mar. Geol., 99:257 26I. Four marine magnetic profiles across the Bay of Bengal are analyzed and the magnetic nature of the upper part of the crust is investigated by inversion in the frequency domain. The magnetization distribution at the average depth of the basemen! below the sediments is computed and shows high magnetization with alternating polarity. The spectral decay provides a maximum estimate of the depth to basement of 8 and 10 km along profiles approximately parallel to 10~ and 14'N respectively. This suggests a thick pile of sediments in this region, with thickness increasing towards the north. The inversion of the recorded magnetic field along the I0°N profile at an average depth of 8 km provides alternating magnetization of approximately 1-2 x 10 --~ cgs units, corresponding to basic rocks. This indicates a magnetic basement in the Bay of Bengal with alternating polarities typical of oceanic crust. Greater magnetization (3 x 10 3 cgs units) over the 85" East Ridge and the pattern of computed magnetization distribution on either side of this ridge indicate that it might have been a spreading ridge at sometime in the geological past which subsequently aborted.

Introduction

Magnetic anomalies in the Bay of Bengal

The Bay of Bengal between the east coast of India and the Andaman Islands is of great significance due to its probable relationship with the break-up of the Indian plate from Gondwanaland and the occurrence of the large transform fault (the Ninety East Ridge). Due to the thick pile of sediments, analyzing the magnetic anomalies has been difficult and it has been impossible to obtain core samples from the basement for laboratory studies. The sediment pile has also increased the overall crustal thickness (Brune and Singh, 1986) making it even more difficult to distinguish between oceanic and continental crust. The seismic velocities towards the southern part of the crust south of Sri Lanka are typical of oceanic crust (Stark and Forsyth, 1983). This area depicts the usual seafloor spreading magnetic anomalies (Sclater et al., 1976). It was therefore considered worthwhile to analyze magnetic profiles over the northern part of the crust around Madras to distinguish the general nature of the crust in this region.

Some marine magnetic and bathymetric profiles across the Bay of Bengal (Fig. 1 and 2) obtained from the Scripps Institute of Oceanography have been published and interpreted in the classical terms of basement tectonics by Rao and Rao (1986), and it is apparent from these profiles, especially the two southernmost profiles approximately along 10° and 14°N, that there are distinct wave bands in these data, which makes them suitable for the spectral analysis described below. These alternating magnetic anomalies are unlikely to be due to normal granitic basement under the sediments because the latter will produce shortamplitude flat anomalies like those encountered in magnetic surveys of continental sedimentary basins. Instead, the anomalies observed in the Bay of Bengal are likely to be due to basic intrusions below the sediments. Although the short-wavelength anomalies are present in all the profiles of Fig. 1, they are most clearly defined along profiles 10 and 14. A close

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258

DC. MISHRA 80 °

8,5 °

90 °

95" E

INDIA

CALCUTTA 2O~N Ii

/

VISL-., AK MAPATNAM~IB18I

___

~

"

Z_\

Gom)AvARI~~R. 17

15° ---- -

17

BAY OF BENGAL

120°N

~< , ~

15°

/

I0 ' ~ ~ 80"

85"

~

~0°

90°

95°E

Fig. I. Layoul of protiles in Bay of Bengal.

examination of these anomalies suggests some correlation on either side of the magnetic signature of the 85 ° East Ridge which records a much larger anomaly than any of the adjacent anomalies. Because of the variable thickness of sediments along the profiles, the anomaly amplitudes may vary but their nature will remain the same. This may be used as a diagnostic feature for their identification. As shown in Fig.2 there are some major episodes on either side of the 85" East Ridge which can be recognized in the two profiles along 10° and 14~L and these episodes are even reflected in the other two northern profiles, although not clearly. The Ninety East Ridge was formed approximately l l0m.y, ago (Mahony et al., 1983); the events recorded in our magnetic profiles must pre-date this period, although in the absence of any direct dating of cores from this region the exact date remains unknown. Magnetic profiles 10 and 14 were digitized at a 2 km interval, providing 640 data points on each profile. Their power spectra were computed by segmenting them into six components with 128

data in each segment corresponding to a wavelength (L) of 256 km and by averaging the energy corresponding to the same frequencies to obtain a stable spectrum (Bfith, 1974). The natural logarithm of the power spectrum is plotted versus frequencies in Fig. 3, the slope providing maximum depths of 8 and 10 km to the magnetic basement (Mishra, 1981; Mishra and Pedersen, 1982; Hahn et al., 1976) along these profiles. These depths conform to the maximum thickness of sediments obtained from seismic studies of the same profiles (Curray et al., 1982). The power spectrum corresponding to high frequencies of 8 x 27r/L and above do not fall on the linear segment, and may correspond to noise or to shallow sources, as in the case of profile 14 shown by the dashed line providing a depth of 3-4 km - - almost the same as the depth to the top of the sediment.

Distribution of magnetization The subsurface magnetization distribution can be obtained from the amplitudes and the phase of

259

M A G N E T I C C R [ S T IN T H E BA3" OF B E N G A L

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i

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88"

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92"

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BOTTOM TOPOGRAPHY STACKED ON PROFILES I0, 14,16,1T, 18 AND 20

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I

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l

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82" 84" 86" 88" 90" 92" 94" TOTAL FIELD MAGNETIC ANOMALIES STACKED ON PROFILES I0, 14,17, ANO ZO

Fig. 2. Bathymetry and magnetic profiles in Bay of Bengal (Scripps Institute of Oceanography).

the power spectrum computed above through an appropriate filter described below. The expression for the transform of the magnetic field in a plane due to an assembly of prismatic blocks or dykes has been given by Spector and Grant (1970) and Pedersen (1978). However, a simpler expression for the transform of a magnetic anomaly due to a vertical dyke along a perpendicular profile is

given by (Mishra et al., 1980): 27c

T(f) = (2~. J ( f ) . exp ~- (fh)) • H(/') where T(f) is the transform of the observed field, f = frequencies given by 0_+1, _+2...+n/2 (n - 1)/2

( 1)

or

I) C MISHRA

260 13

°° k~t "00~

SPECTRAL DELAY( S l o p e s ) ,O°N-

\~'~

(I)

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= 8Km = IOKm

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_=

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I 8 O0

FREQUENCY

I \_ I0 O0

-I _ 12 O0

I _ t4 O0

]" X2]T/L

Fig. 3. Power spectrum of magnetic data along profiles 10 and 14, providing an estimate of maximum depth ahmg these profiles. n = the number of observations or digitized data H(f) is the transform of the shape of the body, which for a rectangular body is a sine function given by (Sinfa)/ifa (a = the half width of the body) L is the wavelength equal to the length of the profile h is the average depth J(f) is the magnetization distribution given by:

K(.f). F(Sin

I - i C o s / . C o s D)

• (Sin 4 - i Cos Ir COS Dr) where F is the intensity of the Earth's magnetic field I and D are the inclination and declination of the Earth's present-day magnetic field and X I0 - 5 >'-

400

J

200

Units

•

,oo ,,, ¢-) (/)

CGS

AA ,.

-

I, and D, are the inclination and declination of the remanent magnetization K(f) is the susceptibility variation The transform of the magnetization distribution (J(f)) can be computed from the spectrum of the magnetic field T(f) and the other parameters in eqn. (1). Its inverse transform provides magnetization per unit depth at every point of observation or digitization. The digital field values along profiles 10 and 14 referred to above are used to compute the magnetization distribution along the profiles. A typical computed profile is shown in Fig. 4, which provides a variation from + 3.0 to - 2 . 5 × 10-3cgs units, with most of the values around _+1 x 10 3. The negative values correspond to the reverse magnetization. This range suggests basic intrusives with alternating polarity at depths of approximately 8 l0 kin, as discussed above. The magnetization corresponding to 85 ~> East Ridge (3 x 10 -3 cgs units) is the highest in the entire section and there appears to be some correspondence in the polarity on either side of this ridge, as referred to above. This suggests that at sometime in geological history the 85 ° East Ridge was probably an active ridge across which seafloor spreading anomalies formed. As there is no direct evidence to assign ages to the subsurface rocks and the corresponding magnetic anomalies, it is inferred that the start of eruption might have coincided with the break-up of Gondwanaland, which was the single most significant event in this region. The period of this break-up is assigned differently by different workers but generally appears to lie between 140 and 120 Ma (Besse and Courtillot 1988; Curray et al. 1982; Norton and Sclater 1979). Late Jurassic

An k/

V

etT.

sg"

L~'

[00

3-2oo -300

2

Fig. 4. Computed magnetization distribution depicted as repetitive patterns on either side of 85' East Ridge. The negative values indicate the reverse magnetization.

261

M A G N E T I C C R U S T IN T H E BAY O F B E N G A L

Early Cretaceous magnetic anomalies have been r e p o r t e d f r o m the e a s t e r n I n d i a n O c e a n a d j a c e n t to N o r t h w e s t

Australia

(Fullerton

et al.,

1989;

R o y e r a n d S a n d w e l l , 1989). T h e g r e a t e s t signific a n c e o f the a l t e r n a t i n g p o l a r i t y m a g n e t i c a n o m a lies o f the Bay o f B e n g a l lies in the s u g g e s t i o n t h a t the crust in this r e g i o n a p p e a r s to be o c e a n i c in n a t u r e a n d , a l o n g 1 0 - 1 4 ° N , is b u r i e d d e e p u n d e r 8-10 km of sediment.

Acknowledgement I a m g r a t e f u l to the D i r e c t o r o f the N a t i o n a l G e o p h y s i c a l R e s e a r c h I n s t i t u t e at H y d e r a b a d for his e n c o u r a g e m e n t a n d p e r m i s s i o n to p u b l i s h this work.

References B~th, B.M., 1974. Spectral analysis in Geophysics. Elsevier, Amsterdam, 563 pp. Besse, J. and Courtillot, V., 1988. Palaeogeographic maps of the continents bordering the Indian Ocean since Early Jurassic. J. Geophys. Res., 93:11791 11808. Brune, J.N. and Singh, D.D., 1986. Continent-like crustal thickness beneath the Bay of Bengal sediments. Bull. Seismol. Soc. Am., 76:191 203. Curray, J.R., Emmel, F.J., Moore, D.G. and Raitt, R.W., 1982. Structure, Tectonics and Geological History of the Northeastern Indian Ocean. (The Ocean Basins and Margins.) Plenum, Vol. 6, pp.399 450. Fullerton, L.G., Sager, W.W. and Bandsch/imacher, D.W., 1989. Late Jurassic-Early Cretaceous evolution of the East-

ern Indian Ocean adjacent to North-East Australia. J. Geophys. Res., 94: 2937-2953. Hahn, A., Kind, E.G. and Mishra, D.C., 1976. Depth estimation of magnetic sources by means of Fourier amplitude spectra. Geophys. Prospect., 24: 287-308. Mahony, J.J., Macdougall, J.D., Lugmair, G.W. and Gopalan, K., 1983. Kerguelen hot spot source for Rajmahal traps and 90~' East Ridge. Nature, 303: 385-389. Mishra, D.C., 1981. Crustal structure in the north Arabian Sea from magnetic surveys. Mar. Geophys. Res., 4: 427-436. Mishra, D.C., 1984. Magnetic anomalies India and Antarctica. Earth Planet. Sci. Len., 71: 173-180. Mishra, D.C. and Pedersen, L.B., 1982. Statistical analysis of potential fields from subsurface relief. Geoexploration, 19: 247 265. Mishra, D.C., Murthy, K.S.R. and Rao, T.C.S., 1980. A general expression for the spectrum of magnetic anomaly due to a long tabular body and its characteristics. Indian J. Mar. Sci., 9: 250-252. Norton, I.O. and Sclater, J.G., 1979. Indian Ocean and breakup of Gondwanaland. J. Geophys. Res., 84: 6803-6830. Pedersen, L.B., 1978. A statistical analysis of potential field data using a circular cylinder and a dike. Geophysics, 43: 943-953. Rao, T.C.S. and Rao, V.B., 1986. Some structural features of Bay of Bengal. Tectonophysics, 124: 14I 153. Royer, T.Y. and Sandwell, D.T., 1989. Evolution of the eastern Indian Ocean since the Late Cretaceous: Constraints from GEOSAT altimetry. J. Geophys. Res., 94:13755 13782. Sclater, J.G., Luyendyk, B.P. and Meinke, k., 1976. Magnetic lineations in the southern part of the central Indian basin. Geol. Soc. Am. Bull., 87: 371-378. Spector, A. and Grant, F.S., 1970. Statistical models for interpreting aeromagnetic data. Geophysics, 35:293 302. Stark, M. and Forsyth, D.W., 1983. The Geoid, small-scale convection and differential travel time anomalies of shear waves in the central Indian ocean. J. Geophys. Res., 88: 2273 2288.