Interpretation of gravity data over the 85 E ridge and Afanasy Nikitin seamount using spectral method

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1 Indian Journal of Marine Sciences Vol. 34(3), September 2005, pp Interpretation of gravity data over the 85 E ridge and Afanasy Nikitin seamount using spectral method A.R. Bansal & V.P. Dimri* National Geophysical Research Institute, Uppal Road, Hyderabad , India [ vpdimri@ngri.res.in] The 85 E ridge is an enigmatic buried aseismic ridge having the peculiar negative gravity anomaly in the north and positive anomaly in the south. Different theories are proposed for the negative anomaly over the 85 E ridge. Highresolution two-dimensional satellite gravity data over 85 E ridge is interpreted using the spectral analysis method covering the area from 5 S to 18 N. The area consists of the Afanasy Nikitin seamount, partially buried hills and the 85 E ridge. Twelve overlapping blocks of 3 3 are selected to calculate the depth of anomalous sources. The two layers of anomalous sources are observed for all blocks. The different depth values of causative sources are found for different portion of the 85 E ridge. The deeper depth values for each block are representing the crustal thickness. The maximum thickness of 25.6 km was found for block 10 covering the area between latitude 12 N to 15 N. The average crustal thickness decreases from north to south along 85 E ridge. The reason of negative free air gravity anomaly below the 85 E may be due to the depression in the Moho. [Key words: Gravity data, 85 E Ridge, Afanasy Nikitin Seamount, spectral method, crustal thickness] Introduction The evolution of the Bay of Bengal starts from the early Cretaceous during the breakup of greater India from the eastern Gondwanaland and subsequent events result in the formation of 90 E ridge, the submerged 85 E ridge, fractures zones etc The 85 E ridge is found from the seismic reflection profiles as the basement rise following approximately 85 E longitude between 19 N and 6 N latitude then takes the NE - SW turn and extends up to 2 S, the northern extension of the Afanasy Nikitin seamount (ANS) and width varies from km 1,4,5. The 85 E ridge is buried under thick Bengal Fan to the north of 7.5 N, and has no surface expression 6. The ANS has positive free air gravity anomaly and is bounded by the Indira fracture zone and 84.5 E fracture zone to the west and east respectively 6. Various fractures zones are identified to the south of equator oriented in the N S direction. These include the 80 E, Indira, 84.5 E and 86 E fracture zones. The magnetic anomalies over the ANS suggest that this seafloor was not created by seafloor spreading 6. The 85 E ridge is an enigmatic aseismic ridge, having a peculiar negative gravity anomaly and positive magnetic *Ph.: Fax: anomaly. The different models are proposed for the origin of the 85 E ridge as the hotspot origin 3,7 and some suggest volacanism occurred due to a lithospheric weak zone within a short span of time 4. Curray & Munasinghe 7 proposed that Raj Mahal Traps, the 85 E ridge and the ANS were formed by a hot spot that subsequently formed the Crozet Islands on the Antarctic plate. Gopal Rao et al. 3 overviewed the four suggested models of 85 E ridge formation; (1) due to Crozet hotspot 7 ; (2) northern extension of the 86 E fracture zone 8 ; (3) volcanism along weak zone for a short span of time and (4) abandon spreading center 9. Curray & Munasinghe 7 proposed that negative free air anomaly at the northern part of the 85 E is due to the thick sediment cover and the negative free air anomaly disappears as the sediment thickness approach to 3-4 km. The free air anomaly becomes positive on the Afanasy Nikitin seamount, which is covered by thin sediments of the distal Bengal Fan. Krishna 6, however, proposed that the sediments over 85 E ridge are metasediments which are more denser than the volcanic rocks, so the flexure of the lithosphere can only explain the negative free air anomaly over the 85 E ridge. Krishna 6 studied four profiles covering 13 N, partial buried hills (0 ), subsurface structure (1 S) and the ANS (3 S), and

2 280 INDIAN J. MAR. SCI., VOL. 34, No.3, SEPTEMBER 2005 modeled the gravity data using the constraints from seismic studies. The profile data is good for fitting the sub-surface model along it, however, if we wish to study the detail depth information of anomalous bodies in the continetal crust 2-D gravity data should be used. The aim of this study is to determine the detailed depth information of anomalous sources below the 85 E ridge from 5 S to 18 N and explain its negative free air gravity anomaly. Materils and Methods The high-resolution satellite gravity data 10 is used for carrying out the depth values of the anomalous sources. The high-resolution satellite gravity data is derived from reprocessing the raw altimeter data of the geodetic mission data of ERS-1 and GEOSAT. The derived gravity data 10 is compared with data of Sandwell & Smith 11 and ship borne gravity data 4 sets. The various statistical analyses such as power spectrum, correlation coefficients, root mean square errors (RMS) have proved the data is of high resolution 10. The dataset has a full wavelength resolution of 10 km and 3 mgal accuracy 12. The depth values are calculated by the spectral analysis method 13,14. which calculates the Fourier transform from the auto correlation of data and is popularly known as Blackman & Tukey approach 18. In this study, the twodimensional power spectrum was calculated by using the periodogram method. Then 2-D power spectrum is annular averaged to get 1-D power spectrum. The log of power spectrum versus wavenumber was plotted to calculate the depth values. Gravity data over 85 E ridge and Afanasy Nikitin Seamount Twelve overlapping blocks covering different portion of ridge were selected as shown in Fig. 1. Block 1 covers the southern portion of the ANS between 5 S and 2 S and block 2 covers the northern part of the ANS between 3.5 S and 0.5 S. Block 3 (2 S 1 N) and block 4 (1 S-2 N) covers the partial buried hills. The remaining blocks from block 5 block12 cover the southern and northern part of the 85 E ridge. Results and Discussion The high-resolution gravity data 10 derived from altimeter data over the Bay of Bengal is shown in Spectral analysis method The spectral analysis method is good in providing the average depth values to the top of a statistical ensemble of blocks of anomalous sources These anomalous sources can be interpreted in terms of subsurface structures. The depth values are simply related to the power spectrum as: P( w) Ae (1 e ) 2 d1 w w ( d2 d1) 2 = (1) where P( w ) is the power spectrum, w- wavenumber, A is a constant for random distribution of sources and d 1 and d 2 are depth to the first and second anomalous sources. The depth values are found from the slope of straight line fitting to the plot of log power spectrum versus wavenumber. In the plot of log power spectrum versus wavenumber more than one straight line can be fitted 3, and subsequent depth values provide the depth of the shallow sub-surface bodies. There are two spectral analysis techniques based on the Fourier transform: (1) the direct method, which calculates the Fourier transform directly from the data by using fast Fourier transform; this is known as Cooley & Tukey approach 17, (2) the indirect method, Fig. 1 Tectonic and location map of the Bay of Bengal and adjacent Indian ocean showing 12 overlapping blocks of this study.

3 BANSAL & DIMRI: GRAVITY DATA OVER 85 E RIDGE 281 Fig. 2. The 85 E ridge is shown by low gravity values < 60 mgal in the north following almost 85 E longitude. It takes an arcute shape from 10 to 2 N with the gravity values between 20 mgal to < 60 mgal. A strong gravity high of magnitude -20 mgal to 0 mgal is observed around 4.5 N to 3.5 N latitude and 1.5 N to 1 S correspond to partial buried hills. Further south another gravity highs of > 20 mgal corresponding to Afanasy Nikitin seamount is observed around 2 S latitude and 83 E longitude. The 90 E ridge follows almost 90 E longitude with the positive gravity values > 60 mgal. The depth Fig. 2 The high-resolution gravity map of the Bay of Bengal.

4 282 INDIAN J. MAR. SCI., VOL. 34, No.3, SEPTEMBER 2005 Fig. 3 The plot of log power spectrum versus wavenumber to calculate the depth values for blocks Table 1 The depth values found for different blocks Block Moho depth (km) Depth of 1 st layer (km) values for each block (shown in Fig. 1) are calculated by using the Eq. (1). The plot of log power spectrum versus wavenuber is shown in Fig. 3. The depth values are also shown in the Fig. 3 and Table 1. The depth values of 16 and 17.2 km found from the blocks1 and 2 over the ANS may be representing the crustal depth below it. Krishna 6 found it around 18 km from the seismic studies and gravity data modelling. The crustal depth value for the block 3 and 4 are 16.8 km and 22.8 km respectively. The depth values of 16.8 km may be related with the values of 15 km found by the Krishna 6 around 1 S. The depth value for block 4 is exceptionally high and this may represent the depth values of surrounding area and not the partially buried hills. The depth values for the remaining blocks provide the crustal structure below the 85 E ridge, and vary from 15.5 km to 25.6 km. The shallowest depth value of 15.5 km for block 6 corresponds to the gravity anomaly of the order of around 35 and 40 mgal, which is higher than the adjacent northern and southern block. The crustal structure below the 85 E indicates that the reason of negative free air gravity anomaly over the 85 E ridge is due to crustal thickening below it. The depth value of 5-6 km for the upper layer indicates the top depth of the 85 E ridge. Subrahmanayam et al. 19 reported the projection of 85 E ridge around 6 km below the mean sea level. The depression in Moho below the 85 E ridge is in close agreement with the depth

5 BANSAL & DIMRI: GRAVITY DATA OVER 85 E RIDGE 283 Fig. 4 (a) Gravity data, (b) contours of the estimated depth values in the area and (c) crustal depth along XY (85 E longitude) profile.

6 284 INDIAN J. MAR. SCI., VOL. 34, No.3, SEPTEMBER 2005 values reported by Subrahmanayam et al. 19. But this study differs from earlier studies in the sense that it provides the detailed crustal structure from 5 S to 18 N using the 2-D high-resolution gravity data. The gravity data along 85 E ridge is shown in Fig. 4a and the corresponding depth values are presented in terms of contour in Fig. 4b. The depth values along a profile XY is shown in Fig. 4c. The least square fit to depth values indicate a decrease in crustal thicknesses from north to south along the 85 E ridge, except block 4. Spectral analysis method is used to calculate the depth values of anomalous sources from the highresolution gravity data over 85 E ridge. Twelve overlapping blocks are selected along the 85 E ridge to know detailed depth information about sub-surface sources. The study shows that the crustal thickness is maximum 25.6 in north(12-15 N) and decreases towards south along the 85 E ridge. The negative gravity anomaly over 85 E ridge may be due the depression in Moho. Acknowledgement ARB is thankful to the Department of Science and Technology, New Delhi for granting the BOYSCAST Fellowship. Authors are grateful to Prof. J. D. Fairhead (GETCH, Leeds, U. K.) for providing his propriety softwares for processing the raw altimeter data. References 1 Curray, J.R. & Moore, D.G., Growth of the Bengal Deep - Sea Fan and Denudation in Himalayas, Geol. Soc. Am. Bull., 82(1971) Krishna, K.S., Rao, D.G., Ramana, M.V., Subrahmanyam, V., Sarma, K.V.L.N.S., Pilipenko, A.I., Shcherbakov, V.S. & Murthy, I.V.R., Tectonic model for the evolution of oceanic crust in the northeastern Indian Ocean from the Late Cretaceous to the early Tertiary, J. Geophys. Res., 100(1995) Gopal Rao, D., Krishna, K.S. & Sar, D., Crustal evolution and sedimentation history of the Bay of Bengal since Cretaceous, J. Geophys. Res., 102 (1997) Ramana, M.V., Subrahmanyam, V., Chaubey, A.K., Ramprasad, T., Sarma, K.V.L.N.S., Krishna, K.S., Desa, M. & Murty, G.P.S., Structure and origin of the 85 E ridge, J. Geophys. Res., 102(1997) Curray, J.R., Emmel, F. J., Moore, D. G. & Raitt, R. W., Structure, tectonics and geological history of the northeastern Indian Ocean, in: The ocean basin and margins, Vol. 6, The Indian Ocean, edited by A. E. Nairn and F. G. Stehli, (Plenum, New York) 1982, pp Krishna, K.S., Structure and evolution of the Afanasy Nikitin seamount, buried hills and 85 E Ridge in the northeastern Indian Ocean, Earth Plan. Sci. Lett., 209(2003) Curray, J.R. & Munasinnghe, T., Origin of the Rajmahal Traps and the 85 E Ridge: Preliminary reconstructions of the trace of the Crozpet hotspot, Geology, 19(1991) Kent, R. W., Storey, M., Saunders, A. D., Ghose, N. C. & Kempton, P. D., Comment and reply on Origin of the Rajmahal Traps and the 85 E Ridge Preliminary reconstructions of the trace of the Crozet hotspot, Geology, 20(1992) Mishra, D. C., Magnetic crust in the Bay of Bengal, Mar. Geol., 99(1991) Bansal, A.R., Fairhead, J.D., Green, C.M. & Fletcher, K.M.U., Revised gravity for offshore India and the isostatic compensation of submarine features, Tectonophysics, 404 (2005) Sandwell, D.T. & Smith, W. H. F., Marine gravity anomaly from GEOSAT and ERS-1 satellite altimetry, J. Geophys. Res., 102 (1997) Fairhead, J. D., Green, C. M. & Fletcher, K. M. U., Hyderocarbon screening of the deep continental margins using non-seismic methods, First Break, 22(2004) Spector, A. & Grant, F.S., Statistical model for interpreting aeromagnetic data, Geophysics, 35 (1970) Treitel, S., Clement, W.G. & Kaul, R.K., The spectral determination of depth of buried magnetic basement rocks, Geophys. J. Roy. Astr. Soc., 24(1971) Dimri, V.P., Deconvolution and inverse theory: Application to geophysical problems, (Elsevier, Amsterdam) 1992, pp Naidu, P.S. & Mathew, M.P., Analysis of geophysical potential fields: A digital signal processing approach, (Elsevier, Amesterdam)1998, pp Cooley, J.W. & Tukey, J.W., An algorithm for the machine calculation of complex Fourier series, Math. Comp., 19 (1965) Blackman, R.B. & Tukey, J.W., The measurement of power spectra from the point of view of communication engineering, (Dover Publications, New York) 1958, pp Subrahmanyam, V., Krishna, K.S., Murthy, I. V. R., Sarma, K. V. L. N.S., Desa, M., Ramana, M. V. & Raju, K.A. K., Gravity anomalies and crsutal structure of the Bay of Bengal, Earth Planet. Sci. Lett., 192(2001)

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