Sri Lanka

(1)

RESEARCH ARTICLE

DOI: http://dx.doi.org/10.4038/jnsfsr.v46i4.8624

,QWHUSUHWDWLRQRIJUDYLW\DQRPDOLHVRYHUWKHHLJKW\¿YH(DVW5LGJH

LQWKH%D\RI%HQJDO

'$7DQWULJRGDDQG0030)HUQDQGR^*

Department of Physics, Faculty of Applied Sciences, University of Sri Jayewardenepura, Gangodawila, Nugegoda.

5HYLVHG0DUFK$FFHSWHG-XQH

* Corresponding author ([email protected]; https://orcid.org/0000-0003-2758-4934)

This article is published under the Creative Commons CC-BY-ND License (http://creativecommons.org/licenses/by-nd/4.0/).

7KLVOLFHQVHSHUPLWVXVHGLVWULEXWLRQDQGUHSURGXFWLRQFRPPHUFLDODQGQRQFRPPHUFLDOSURYLGHGWKDWWKHRULJLQDOZRUNLV

properly cited and is not changed in anyway.

$EVWUDFW6HYHUDOSUR¿OHVRIWKHQHJDWLYHJUDYLW\DQRPDO\RYHU

the submerged 85^o E ridge in the Bay of Bengal of the Indian Ocean have been interpreted in two dimension and results are combined together to give a detailed picture of its morphology and thickness of surrounding sediments. The observed negative gravity anomaly has been explained as the combined effect due to the positive mass anomaly caused by the replacement RIVHGLPHQWVE\KLJKGHQVHLJQHRXVURFNVRIWKHULGJHDQGWKH

negative mass anomaly caused by replacing the high dense XSSHU PDQWOH PDWHULDO E\ WKH RFHDQLF FUXVW ZKLFK KDV EHHQ

bended and sunk into the upper mantle due to the pressure exerted by the ridge. Downward migration of the oceanic crust has been also calculated assuming that the oceanic crust is EHKDYLQJDVDWKLQLQ¿QLWHHODVWLFSODWHUHVWLQJRQLQYLVFLGÀXLG

half space and is found to closely agree with the results of the gravity study. Both studies indicate that the thickness of the ridge vary from 11 km to 16 km while the oceanic crust has undergone a depression of 9 km to 14 km.

.H\ZRUGV %D\ RI %HQJDO HLJKW\¿YH (DVW 5LGJH JUDYLW\

DQRPDO\,QGLDQ2FHDQ6UL/DQND

INTRODUCTION

Interpretation of gravity anomalies provides a relatively inexpensive way of revealing the shallow crustal structure of the Earth. A gravity anomaly is a manifestation of mass anomalies inside the Earth. Low dense sediments in sedimentary basins surrounded by high dense country URFNVJLYHULVHWRQHJDWLYHDQRPDOLHVZKLOHKLJKGHQVH

igneous intrusions emplaced in an environment of relatively low dense country rocks give rise to positive

anomalies. Most of the gravity anomalies that we have observed adhere to this general principle. An apparent exception to this general principle can be observed along the 85^oE longitude in the Bengal Fan in the Indian Ocean.

Existence of a buried ridge known as 85^o East Ridge of high dense volcanic material within the sediments in this region has been established by Curray and Moore DQG0RRUHet al. (1974).

One of the most prominent features of the satellite gravity anomaly map of the Indian Ocean east of Sri Lanka (Figure 1) is a series of elongated negative anomalies running over the region suspected to occupy the 85^o( 5LGJH 7KLV DQRPDO\ ZKLFK DSSHDUV DURXQG

20ôN in the northern Bay of Bengal continues as a near linear feature up to 5ôN and then curves towards west and continues to do so up to 2ôN. The length of the anomaly is about 1850 km and it covers an approximate area of 2 × 10⁵ km². The intensity approximately varies from - 60 mGal to -15 mGal. The bathymetry of the region is JHQHUDOO\VPRRWKZLWKDJHQWOHQRUWKWRVRXWKJUDGLHQW

UHÀHFWLQJ WKH QRUWKZDUG LQFUHDVLQJ VHGLPHQW WKLFNQHVV

and southerly transport of the turbidities comprising the fan. A positive free air anomaly is expected to observe RYHUWKHEXULHGULGJHUHÀHFWLQJKLJKGHQVLW\RIWKHULGJH

UHODWLYHWRVXUURXQGLQJVHGLPHQWV+RZHYHUDVLQGLFDWHG

above the satellite gravity anomaly depicted in Figure 1 shows a negative gravity anomaly over the region of the ridge. The only possible way to have a negative anomaly over the region is depression of the oceanic crust below the ridge due to the weight of the ridge replacing high

(2)

488 D.A. Tantrigoda & M.M.P.M. Fernando

December 2018 Journal of the National Science Foundation of Sri Lanka 46(4)

dense upper mantle material by relatively low dense oceanic crust (Liu et al1HJDWLYHDQRPDO\FDXVHG

by this density difference may outweigh the positive anomaly resulting from the positive density contrast between the ridge and the sediments giving rise to the REVHUYHGQHJDWLYHDQRPDO\,QWKLVVWXG\WKLVK\SRWKHVLV

has been tested by interpreting gravity anomalies over WKHULGJHDQGDOVRE\VWXG\LQJWKHÀH[XUDOGHÀHFWLRQRI

the oceanic crust due to the pressure exerted by the ridge.

$VH[SODLQHGDERYHJUDYLW\DQRPDOLHVRYHUWKH^oE Ridge haveWZRVRXUFHUHJLRQV7KH¿UVWLVWKHSDUWRI

WKH ULGJH ZKLFK LV EXULHG LQVLGH ORZ GHQVH VHGLPHQWV

DQGWKHVHFRQGLVWKHSDUWRIWKHRFHDQLFFUXVWZKLFKKDV

migrated downward into the high dense upper mantle due WRWKHSUHVVXUHH[HUWHGE\WKHULGJH7KH¿UVWSURGXFHVD

positive anomaly while the second produces a negative anomaly. In this interpretation of gravity DQRPDOLHVERWK

these sources have to be considered. Two dimensional LQWHUSUHWDWLRQ RI JUDYLW\ DQRPDOLHV LV VXI¿FLHQW LQ WKLV

case as the anomaly is a linear feature indicating that it is GXHWRDERG\ZLWKRQHKRUL]RQWDOGLPHQVLRQWKDWLVPXFK

larger than the other.

METHODOLOGY

$ WZR GLPHQVLRQDO LQWHUSUHWDWLRQ RI VHYHQ SUR¿OHV

(Figure 1) of satellite gravity anomalies over the 85^o E Ridge has been carried out using the method of Talwani et al. (1959). In the interpretation it was assumed that the thickness of oceanic crust is 6.0 km and densities RIVHDZDWHURFHDQLFVHGLPHQWVRFHDQLFFUXVWDQGXSSHU

mantle are 1030 kgm^-3 NJP^-3 NJP^-3 and 3300 kgm^-3UHVSHFWLYHO\:RROODUG%RWW

)XUWKHULWZDVDOVRDVVXPHGWKDWWKHUHLVDUHJLRQDOOHYHO

of - 60 mGal over the 85^oE Ridge in the Bay of Bengal.

8

Sri Lanka

P-02

P-03 P-04

P-05

P-07 P-06

85

^o

E Ridge

P-01

)LJXUH Gravity anomalies in mGal over the 85^o E Ridge. Line segments give the position of WKHJUDYLW\SUR¿OHVLQWHUSUHWHG

(3)

Journal of the National Science Foundation of Sri Lanka 46(4) December 2018

Downward migration of the oceanic crust due to the pressure distribution acting on the 85^oE Ridge has also been derived following a method similar to Liu et al.

(1982). In this derivation it has been assumed that the RFHDQLFFUXVWLVEHKDYLQJDVDWKLQLQ¿QLWHHODVWLFSODWH

UHVWLQJRQLQYLVFLGÀXLGKDOIVSDFH'RZQZDUGPLJUDWLRQ

]x, y) of an elastic plate due to a pressure distribution of p(x, y) can be written as (Liu et al

^x ^y

y p z x

D ⁴z₄ ⁴₄

¸¸¹·

¨¨©§

w

w w w

ZKHUH ' LV WKH ÀH[XUDO ULJLGLW\ RI WKH RFHDQLF FUXVW

which is a measure of its resistance to bending. The IROORZLQJ H[SUHVVLRQ IRU ]x, y) can be obtained by VROYLQJWKHHTXDWLRQXVLQJ)RXULHUWUDQVIRUPV

»»

¼ º

««

¬ ª

¿¾

½

¯®

) (

) ( ) (

) ( ) ) (

(

4 1

c m

y x w s y x w c

g k D

k k S k

k F H

y x z

U U

U U U

U

9

Gra; Ano;

(mGal)

83.00 84.00 85.00 86.00 87.00

Longitude (Deg) 2.00

3.00 4.00 5.00 6.00 7.00 8.00 9.00 10.00

Latitude (Deg)

-75 -65 -55 -45 -35 -25 -15 -5

)LJXUH Free air gravity anomaly map over the 85^o E Ridge and the surrounding area. Contours in intervals of 5 mGal

ZKHUH^S

^k^x^k^y

is the Fourier transform of the thickness RI VHGLPHQWV ^H

^k^x^k^y is the Fourier transform of WKHWRSRJUDSK\RIWKHULGJHk is the wave number and UwUsUc and U_mDUHWKHGHQVLWLHVRIVHDZDWHURFHDQLF

VHGLPHQWVRFHDQLFFUXVWDQGXSSHUPDQWOHUHVSHFWLYHO\

Details of the derivation can be found in Tantrigoda (2004) and Fernando (2005).

RESULTS AND DISCUSSION

A gravity anomaly map and bathymetry map over the 85^o ( 5LGJH LV JLYHQ LQ )LJXUHV DQG UHVSHFWLYHO\

5HVXOWV RI WKH LQWHUSUHWDWLRQ RI JUDYLW\ SUR¿OHV RYHU

the ridge are depicted in Figures 4 and 5. Maps of depression of the lower layer of the oceanic crust and sediment thickness have been compiled combining the LQIRUPDWLRQJLYHQLQWKHDERYH¿JXUHV)LJXUHVDQG

UHVSHFWLYHO\,QWKHFRQWRXUPDSYDULDWLRQVRIVHGLPHQW

WKLFNQHVV IDU DZD\ IURP WKH ULGJH ZKLFK DUH RI WKH

order of 200 m have been ignored to enhance its clarity.

)OH[XUDOGHÀHFWLRQRIWKHORZHUVXUIDFHRIWKHRFHDQLF

crust has been depicted in Figure 8 using a 3-D surface map and a contour map.

10

Bathymetry (m)

83.00 84.00 85.00 86.00 87.00

3.00 4.00 5.00 6.00 7.00 8.00 9.00

Latitude (Deg)

-4400 -4300 -4200 -4100 -4000 -3900 -3800 -3700

)LJXUH Bathymetry map over the 85^oE Ridge and the surrounding area. Contours in intervals of 50 m

(4)

An isopach map over the 85^oE Ridge has been compiled using the results of two-dimensional LQWHUSUHWDWLRQ RI VHYHQ SUR¿OHV RI VDWHOOLWH JUDYLW\

DQRPDOLHV )LJXUH $V FDQ EH VHHQ IURP WKLV PDS

sediment thickness over the ridge at the extreme north is very low and is about 200 m. It goes up to about 3 km ZLWKLQDVKRUWGLVWDQFHRINPDORQJWKHSUR¿OH

Isopach map compiled in this study closely agrees with the isopach map compiled by Levchenko et al.

(1993) based on seismic studies. This ridge may have LQÀXHQFHG WKH VHGLPHQW GLVWULEXWLRQ DURXQG 6UL /DQND

7KHUH LV D FUHVFHQW VKDSHG ULFK VHGLPHQW GLVWULEXWLRQ

which has a thickness in the order of 2.0 – 3.0 km. The 85^o(5LGJHPD\KDYHEORFNHGWKHIUHHÀRZRIVHGLPHQWV

forcing them to get accumulated around Sri Lanka.

1RUPDOO\ PRVW of the oceanic ridges produce SRVLWLYHJUDYLW\DQRPDOLHV%RWW$JRRGH[DPSOH

for this is the 90^o( ULGJH 0XNKRSDGK\D\ .ULVKQD

1994) situated close to the 85^oE ridge. This is because WKH RXWFURSSLQJ SDUW RI D ULGJH EXULHG LQ ORZ GHQVH

sediments gives rise to a positive anomaly due to the positive density contrast between the sediments and

11 Sediments

Oceanic Crust

Upper Mantle

Ridge Sediments distribution

Depth [m]

Longitude (Deg) Longitude (Deg)

Observed gravity anomaly and calculated gravity anomaly

Gravity anomaly (mGal)

)LJXUH ,QWHUSUHWDWLRQRIWKHSUR¿OH3RI)LJXUH

(5)

12 Figure 5

Longitude

Longitude Sediments

Oceanic Crust

Upper Mantle

Ridge P-02

Sediments distribution

Longitude (Deg)

Observed Gra Ano and Calculated Gra Ano

Depth [m]Gravity anomaly (mGal)

P-02 ^P-03

Sediments Oceanic Crust

Upper Mantle

Longitude (Deg)

Depth [m] Gravity anomaly (mGal)

P-03

P-04

Upper Mantle

Longitude (Deg)

P-04 ^P-05

Upper Mantle

Longitude (Deg)

Depth [m] Gravity anomaly (mGal)

P-05

P-06

Sediments Oceanic Crust Upper Mantle

Longitude (Deg)

P-06 ^P-07

Upper Mantle

Longitude (Deg)

P-07

)LJXUH ,QWHUSUHWDWLRQRISUR¿OHV3WR3RI)LJXUH

P-02

P-04

P-06

P-03

P-05

P-07

(6)

13

83.00 84.00 85.00 86.00 87.00

3.00 4.00 5.00 6.00 7.00 8.00 9.00

Latitude (Deg)

-26000 -24000 -22000 -20000 -18000 -16000 -14000 Dow; Dep;

(m)

)LJXUH Map of downward depression of the lower layer of the oceanic crust over the 85^oE ridge and the surrounding area. Contours in intervals of 1000 m

83.00 84.00 85.00 86.00 87.00

3.00 6.00 7.00 8.00 9.00

0 400 800 1200 1600 2000 2400 2800 3200 3600 Sedi; Thi;

(m)

)LJXUH Map of sediment thickness over the 85^oE ridge and the surrounding area.

Contours in intervals of 200 m

(7)

15 Figure 8

85.00 85.50 86.00 86.50 87.00 Longitude (Deg)

5.00 5.50 6.00 6.50 7.00 7.50 8.00 8.50 9.00 9.50 10.00

Latitude (Deg)

-18000 -17000 -16000 -15000 -14000 -13000 -12000 -11000 -10000 -9000 -8000 -7000 -6000 -5000 -4000 -3000 -2000 -1000 Flexural Deflection (m)

a

85.0

87.0 5.0

10.0

Longitude

Latitude Flexural

Deflection (m)

86.0

7.5

b

85.5 86.0 86.5 87.0

15000 10000 5000

0

Flexural Deflection (m)

Longitude

c

)LJXUH 'RZQZDUG GHÀHFWLRQ RI RFHDQLF FUXVW EHORZ WKH ^o( ULGJH ZKHQ ÀH[XUDO ULJLGLW\ RI WKH RFHDQLF FUXVW LV

1.8×10²¹ Nm (a : contour map; b : 3D map and c : cross section along 8^olatitude)

PDWHULDORIWKHULGJHZKLFKhave a density more or less equal to that of the oceanic crust. It has been revealed in the gravity interpretation that oceanic crust has sharply migrated downward replacing high dense upper mantle material by low dense material of the ridge. As seen in Figure 8the oceanic crust bends down as much DV௅NPGXHWRWKHSUHVVXUHH[HUWHGE\WKHULGJH

producing an intense negative gravity anomaly. This

negative anomaly outweighs the positive anomaly due to the part of the ridge buried in the sediments making the resultant anomaly negative.

The downward migration of the oceanic crust below the ridge was also calculated using elastic properties of the oceanic crust approximating the oceanic crust to a YHU\WKLQSODWHO\LQJRQDYLVFRHODVWLFÀXLG,WKDVEHHQ

(8)

/HYFKHQNR 29 0LODQRYVNL\ 9<H 3RSRY $$

A sediment thickness map and the tectonics of the distal Bengal Fan. Oceanology 33(2): 232 – 238.

/LX&66DQGZHOO'7 &XUUD\-57KHQHJDWLYH

JUDYLW\ ¿HOG RYHU ^o E Ridge. Journal of Geophysical Research 87(B9): 7673 – 7686.

DOI: KWWSVGRLRUJ-%L%S

0RRUH '* &XUUD\ -5 5DLWW 5: (PPHO )-

Stratigraphic-seismic section correlations and implications to Bengal Fan history. Initial Reports of the Deep Sea Drilling Project HGV && YRQ GHU %RUFK -* 6FODWHU

et al SS ௅ 86 *RYHUQPHQW 3ULQWLQJ 2I¿FH

:DVKLQJWRQ'&86$

Mukhopadhyay M. & Krishna M.B.R. (1994). Gravity anomalies and deep structure of the Ninetyeast Ridge QRUWKRIWKHHTXDWRUHDVWHUQ,QGLDQ2FHDQDKRWVSRWWUDFH

model. Marine Geophysical Researches 17(2): 201 – 216.

7DOZDQL 0 :RU]HO -/ /DQGLVPDQ 0 5DSLG

gravity computations for two-dimensional bodies with DSSOLFDWLRQV WR WKH 0HGRFLQR VXEPDULQH IUDFWXUH ]RQH

Journal of Geophysical Research 64: 49 – 59.

DOI: KWWSVGRLRUJ-=LS

Tantrigoda D.A. (2004). An Interpretation of Gravity Anomalies over the Indian Ocean Region around Sri Lanka. Final report of the research project funded by WKH1DWLRQDO6FLHQFH)RXQGDWLRQRI6UL/DQND3URMHFW1R

5*3SS

Woollard G.P. (1959). Crustal structure from gravity and seismic measurements. Journal of Geophysical Research 64௅

DOI: KWWSVGRLRUJ-=LS

also assumed that the region under investigation has reached vertical equilibrium by the combined effect of upthrust on the root of the seamount and support of stresses within the lithosphere.

CONCLUSION

The thickness of the 85^oE ridge varies from 11 km to 16 km. The oceanic crust has undergone a depression of 9 to 14 km as a result of the pressure exerted on the oceanic crust by the ridge. The negative gravity anomaly observed over the 85^oE ridge can be explained as a result of negative anomaly caused by replacement of high dense upper mantle material by downward migrated low GHQVHRFHDQLFFUXVWZKLFKH[FHHGVWKHSRVLWLYHDQRPDO\

caused by igneous rocks of the ridge intruded into the oceanic sediments.

REFERENCES

Bott M.H.P. (1982). Interior of the Earth 0F*UDZ +LOO

(GZDUG$UQROG/RQGRQ8.

&XUUD\ -5 0RRUH '* *URZWK RI WKH %HQJDO

deep-sea fan and denudation of the Himalayas. Geological Society of America Bulletin 82௅

Fernando M.M.P.M. (2005). Interpretation of gravity anomalies over the Indian Ocean region around Sri Lanka. PhD thesis

SS8QLYHUVLW\RI6UL-D\HZDUGHQHSXUD6UL/DQND

Sri Lanka

RESEARCH ARTICLE

,QWHUSUHWDWLRQRIJUDYLW\DQRPDOLHVRYHUWKHHLJKW\¿YH(DVW5LGJH

LQWKH%D\RI%HQJDO

Sri Lanka

85

E Ridge

,QWHUSUHWDWLRQRIJUDYLW\DQRPDOLHVRYHUWKHHLJKW\¿YH(DVW5LGJH