Seafloor spreading magnetic anomalies in the southeastern Arabian Sea

MARINE GEOLOGY ELSEVIER

Marine Geology 128 (1995) 105-l 14

Seafloor spreading magnetic anomalies in the southeastern Arabian Sea A.K. Chaubey, G.C. Bhattacharya,

D. Gopala Rao

National Institute of Oceanography, Dona Paula, Goa 403004, India

Received 1 November 1994; revision accepted 16 May 1995

Abstract Study of closely spaced new marine magnetic profiles in the southeastern Arabian Sea has enabled fairly confident identification of E-W trending linear magnetic anomalies 24 through 20. These magnetic lineations are consistently right laterally offset along numerous fracture zones and extend eastwards up to the base of the western slopes of the Laccadive Ridge. Obliteration of magnetic lineations further eastward is attributed to the emplacement of volcanic material of the Laccadive Ridge over the pre-existing oceanic crust. DSDP Site 221 is observed to be located on the reversely magnetized oceanic crust between anomalies 22 and 21 which provide a reliable age constraint (48 Myr) for the oceanic crust at this site. The presence of additional unidentifiable normal magnetic anomalies suggests a relatively complex spreading pattern (ridge jump?) in the vicinity of the Laccadive Ridge during the interval of anomalies 24 and 23.

1. Introduction

The Laxmi and Laccadive Ridges are two very prominent bathymetric features in the Arabian Sea (Fig. 1). Earlier studies (McKenzie and Sclater, 1971; Whitmarsh, 1974; Schlich, 1975; Norton and Sclater, 1979; Naini and Talwani, 1982; Miles and Roest, 1993) suggested that a major portion of the Arabian Sea, westward of these ridges, was created by seafloor spreading along the Carlsberg Ridge in two distinct phases. The spreading during the older of these two phases commenced sometime during the Early Paleocene (anomaly 28 or anomaly 27) when the spreading centre jumped between India and Seychelles. This phase of spreading possibly ceased or became very slow sometime after the formation of anomaly 21. The spreading of the later phase commenced in its present geometry just before anomaly 11 and is continuing up to today (Chaubey et al., 1993). A recent study 0025-3227/95/$9.50 0 1995 Elsevier Science B.V. All SSDI 0025-3227(95)00089-5

rights reserved

(Bhattacharya et al., 1994a), however, suggests the existence of another spreading phase (pre-anomaly 27) which was responsible for the creation of the crust lying between the Laxmi Ridge and the western continental slope of India. In these previous studies the magnetic anomaly sequences 27 (or 28) to 21 were mapped mostly in the western part of the Arabian Sea (west of 66”E). Further eastwards identifications of anomaly 24 and younger anomalies were proposed, but those identifications were not unequivocal in all places. Even the anomalies adjacent to DSDP Site 221 were not conclusively identified. Furthermore, in close vicinity of the Laccadive Ridge so far no magnetic lineations could be identified. In the present study, we discuss the magnetic anomalies in this sparsely mapped sediment covered southeastern part (approximately between 63”E and the western slopes of the Laccadive Ridge) of the Arabian Sea and present an updated identification of the magnetic lineations

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Geology 128 (1995) 105-114

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Fig. 1. Generalized map of the Arabian Sea, showing magnetic lineations (solid lines), fracture zones (dashed lines) and main structural features (compiled from Schlich, 1982; Naini and Talwani, 1982; Bhattacharya et al., 1992, 1994a,b; Chaubey et al., 1993; Miles and Roest, 1993). SMP = Seychelles Mascarene Plateau; ESB = Eastern Somali Basin; C-R = Carlsberg Ridge; C-L-R = Chagos-Laccadive Ridge; P-R=Prathap Ridge; R=Raman Seamount; P=Pannikkar Seamount; IV= Wadia Guyot. L2, L3, and L4 represent magnetic lineations in the Laxmi Basin. Lineation LI represents extinct spreading center. Pseudo fault (PF) and transferred crust (TC) as inferred by Miles and Roest (1993). Location of DSDP sites are shown with dots. Bathymetric contours are in hundreds of meters. Location of the study area (dashed block) is shown in inset map.

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2. Data The present study is mainly based on new magnetic profiles (prefixed with NR and ANT in Fig. 2) collected on board DSV Nand Rachit during 1990 and M.V. Polar Circle during 1981-1982. The position fixing during these surveys were carried out with a satellite navigation system (Magnavox 1107). A proton precession magnetometer was used for the acquisition of total magnetic intensity data. Residual magnetic anomalies were calculated by removing the International Geomagnetic Reference Field (IGRF) of appropriate epochs as regional field. In addition to these new magnetic data we have also used a few other published profiles (Bhattacharya et al., 1992) and data obtained from the National Geophysical Data Centre (NGDC). The reduced magnetic data are presented as profiles along the ship’s tracks (Fig. 2) and as projected profiles (Fig. 3).

3. Results The present analysis of magnetic data differs from previous analyses mainly in two ways. Firstly, most of our new profiles are oriented approximately orthogonal to the hitherto inferred magnetic lineations of this area, which allows a better correlation of the magnetic anomalies between profiles. Secondly, our identifications of anomalies 24 and 23 are based on detection of the fine scale structure of these anomalies. We have also reviewed all available anomaly identifications proposed for this area (Whitmarsh, 1974; Schlich, 1975; Naini and Talwani, 1982; Cande et al., 1989; Bhattacharya et al., 1992; Chaubey et al., 1993). In regions where we have not added new identifications or suggested modifications, the identifications by earlier workers are retained. For the computation of synthetic magnetic anomalies we have assumed that the magnetic anomalies have their source in the upper 2 km of the crust below the sediment and constrained the depth

Geology 128 (1995) 105-114

107

to the sediment-basement interface within the limits imposed by the published seismic results. In consistence with the assumptions made by the earlier workers, we have computed synthetic anomalies for a ridge striking N75”W at 10”s. We have used the recent geomagnetic polarity reversal time scale and the polarity chron nomenclature proposed by Cande and Kent (1992). The magnetic anomalies in the study area up to about 70”E are pronounced, whereas further eastwards over the western slopes of the Laccadive Ridge, they are subdued and depict no obvious correlation between adjacent profiles. In the study area the presence of linear magnetic anomalies (24 through 20) was initially proposed by Whitmarsh (1974). Later in a different interpretation proposed by Naini and Talwani (1982), eastward of 65”E only anomalies 23 and 24 were shown to be present. Following are the identifications proposed in the present study: 3.1. Anomaly 24 Bhattacharya et al. ( 1992) have shown that anomaly 24, characterized by a double troughed shape, is a conspicuous anomaly sequence in the Arabian Sea. Their identification of anomaly sequence 24 on profiles SK50-01 and SK50-03 are shown in Fig. 2. This double troughed shape is due to the presence of two closely spaced normal polarity events (subchrons) within the broad interval considered as chron 24 in the magnetic polarity time scales. We observe that this double troughed shape alone is not an adequate criterion to identify anomaly 24 on all profiles in our study area. Schlich (1975) recognized the existence of a brief normal polarity interval within these two relatively long normal polarity intervals of anomaly 24. This interval was included as the third subchron (24N2) in the recent geomagnetic polarity time scale of Cande and Kent (1992). Presence of these 3 subchrons results in a distinct shape of anomaly 24 which in the Arabian Sea is depicted as a small trough (tiny wiggle) within the two larger troughs of anomaly 24 (Fig. 4). We could observe this tiny wiggle within the confidently identified anomaly 24 in this area and used this event as a key indicator, along with the double

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Fig. 3. Synthetic and selected observed magnetic profiles in the southeastern Arabian Sea. Observed profiles are projected at an azimuth of 0”N. Synthetic magnetic profile is computed for a ridge striking N75”W at a latitude of 10”s and now observed at 9”3O’N and 68”3O’E. Normally magnetized blocks are indicated by solid black. Magnetized layer (susceptibility 0.01 cgs units) is flat 2.0 km thick with its top at 5.5 km below sea surface. Other details are as in Fig. 2.

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A.K. Chaubey et al. JMarine Geology 128 (1995) 105-l 14

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Fig. 4. Observed and simulated magnetic anomalies between 23Nl and 24N3 along selected profiles in the Arabian Sea. The profiles demonstrate; (1) the tiny wiggle corresponding to subchron 24N2, (2) the reduced width of crust, corresponding to magnetic anomaly 23N2, in the areas where an additional normal magnetic event c(was observed. Parameters for synthetic profile computation are as in Fig. 3. In model B, we have arbitrarily introduced an additional normal block and suitably adjusted the widths of blocks within the interval from 23N2 to 24Nl in order to model the additional normal magnetic event x

troughed shape described earlier, to establish the identification of anomaly 24 in other profiles (Figs. 2 and 3). We believe to have identified anomaly 24 with a fair degree of confidence in almost all the profiles across the study area and found them to strike approximately E-W and parallel to the other lineations in this area. However, on profiles SK50-03 and V3407-01 (Fig. 3), we feel, anomalies 24N2 and 24N3 are somewhat disturbed. 3.2. Anomaly

23

The geomagnetic polarity time scale of Heirtzler et al. (1968) did not contain any subchron within chron 23, but subsequent time scales contained two subchrons (23Nl and 23N2). The presence

of these subchrons predicts a distinct shape of anomaly 23 (Fig. 5). Using this distinct shape, Bhattacharya et al. ( 1992) have mapped anomaly 23 on profiles V3407-01, SK50-01 and SK50-03. Using the same diagonistic shape, we have identified anomaly 23 on all other profiles (Figs. 2 and 3). We believe that except between 66” and 68”E, the anomaly sequence 23 is well developed and can easily be identified. In the study area we have noticed the presence of an unidentifiable additional anomaly (labeled a) between confidently identifiable anomalies 23N2 and 24Nl on almost all the profiles east of 65”E (Fig. 2). It can be seen that event CI is well developed and correlatable from one profile to another and also parallel to other lineations. This anomaly can be modelled as an additional normal event within the reversed crust between anomalies 24Nl and 23N2 (Fig. 4). We will return to this point later. 3.3. Anomalies

near the DSDP

Site 221

In the Arabian Sea, GC-23 is the only profile which passes through DSDP Site 221 and also intersects many of the magnetic lineations of this area. Therefore, this profile could have been used to validate the anomaly identifications. However, earlier workers were not unanimous regarding the identification of the two prominent anomalies which lie immediately north and south of DSDP Site 221. The anomaly immediately north of this site was identified as anomaly 20 by Whitmarsh (1974) based on the Middle Eocene age of the oldest sediment (46 Myr) overlying the basement at this site and the geomagnetic polarity reversal time scale proposed by Heirtzler et al. (1968). Schlich (1975) expressed his reservation about this identification and suggested this anomaly to be A22. His identification was, however, based on comparable characteristic shapes of anomalies 24 and 23 observed in the Crozet and Madagascar basins and the anomalies on the CC-23 profile. Naini and Talwani ( 1982) apparently considered these anomalies as isolated ones and did not assign any identification to them. We have presented two more profiles; ANT4-01 and ANT1 -01 on either side of profile GC-23. Based on these additional profiles, we observe that the two prominent ano-

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Fig. 5. Segment of observed post-A24 lineations near DSDP Site 221 and corresponding synthetic magnetic anomalies to demonstrate the identification of anomalies 23 through 20. Model parameters and other details are same as in Figs. 2 and 3.

malies located immediately north and south of Site 221 on profile GC-23, are not isolated ones but are confidently correlatable from one track to another. These anomalies strike parallel to other lineations in this area and occur immediately next to and south of anomaly 23 identified by us. Therefore, we conclude that these two anomalies from north to south are anomalies 22 and 21, respectively (Figs. 2 and 5). 3.4. Pre-A24 Iineations near Laccadive Ridge Towards the close vicinity of the Laccadive Ridge, east of 69”E and north of anomaly 24N3, we have mapped two more well correlatable lineations and marked them as Cl and C2. A similar E-W lineation (C3) is also present further eastwards (Figs. 2 and 3). Since these lineations are parallel to the trend of other lineations in this region, their origin also can be attributed to seafloor spreading processes. However, their identification requires additional data.

3.5. Fracture zones

The identified magnetic anomalies in the study area are offset by a number of fracture zones. For reference to the text and figures these fracture zones are marked as FZl-FZ8 (Fig. 2). The locations of the fracture zones FZl-FZ4 were inferred in the previous studies (Naini and Talwani, 1982; Bhattacharya et al., 1992) and our recompiled data agree with their locations. However, further eastwards we infer the presence of three new fracture zones (FZ5-FZ7). Fracture zone FZ8 was proposed by Whitmarsh (1974) and named as Rudra Fracture Zone. However, he postulated the northern limit of this fracture zone up to 8”N, which according to the present data can be considered to extend further north across our study area.

4. Discussion

and conclusions

Based on a large number of well oriented and relatively closely spaced profiles, we have presented

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Geology 128 (1995) 105-114

an updated identification of the magnetic anomalies in the southeastern Arabian Sea (Figs. 2 and 6). Our identified magnetic anomalies show that

the location of DSDP Site 221 falls on the reversely magnetized oceanic crust between anomalies 21 and 22. The corresponding magnetic age of this

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A.K. Chaubey et aLlMarine Geology 128 (1995) 105-114

reversely magnetized crust is about 48 Ma according to the recent magnetic polarity reversal time scale (Cande and Kent, 1992). This age is closely comparable to the 46-Ma micropaleontological age (Whitmarsh, 1974) of the oldest sediment overlying the basaltic crust at Site 221. It may be further noted (Table 1) that for this site the depth to the basement predicted from the cooling plate model agrees well with the actual depth (after sediment load correction) determined from drill hole results. This age and crustal depth compatibility, we believe, further strengthens the validity of the updated magnetic anomaly identifications proposed by us in this study. The identification presented in this study as well as in the earlier works, indicates that anomalies 24 through 20 are consistently E-W in the entire Arabian Sea, whereas, according to the recent study by Miles and Roest (1993) anomalies 28 through 25 in the northern Arabian Sea depict a slightly different trend (Fig. 6). We therefore infer that at least by the time of anomaly 24 the spreading centres all over the Arabian Basin became stabilized in a uniform E-W direction. The updated fracture zone lineations show a systematic right lateral offset pattern up to FZ8 (Rudra Fracture Zone). East of this fracture zone, we could not identify any anomalies. However, DSDP Site 220, which lies east of the Rudra fracture zone (Fig. 2), is underlain by an oceanic crust of age not less than 51 Myr (Whitmarsh et al., 1974). Considering the magnetic age (- 48 Myr) of the oceanic crust at DSDP Site 221 Table 1 Comparison of observed DSDP Site 221 Observed basement load correction Basement model

and

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(1)d,,=d,+t,{(p,-p,)/p,--p,)}afterCrough(1983),where, water depth (d,) = 4650 m, sediment thickness (tJ = 270 m and average sediment density (p,) = 1.8 g/cm3 are from drill hole data after Whitmarsh et al. (1974). Water density (p,,,)= 1.03 g/cm3 and mantle density (p,) = 3.3 g/cm3 are assumed. (2) dc,=2700+300(t)‘iz after Hays (1988), where, t=48 Myr age of the crust inferred in the present study.

113

postulated in this study, we observe an offset in the age of the oceanic crust across this fracture zone too and this offset also is right lateral. We therefore feel that the right lateral offset pattern is prevalent throughout the Arabian Basin. As mentioned earlier we have mapped an anomaly M between anomalies 23N2 and 24Nl in the areas east of FZ3. Westwards from this fracture zone no such extra anomaly is observed between anomalies 23N2 and 24Nl. In the magnetic reversal time scales proposed so far there is no such normal event between these anomalies. Further we observe that the width of anomaly 23N2 is comparatively narrower (Fig. 4) in the areas where we have noticed the presence of anomaly CI. The narrow width of anomaly 23N2 is particularly apparent approximately between 68”E and 70”E (Fig. 2). We believe these two phenomena are genetically linked and anomaly a represents a short lived ridge jump/parallel spreading event (?) which took place during anomaly 23N2 time onto the reversed crust between anomalies 24Nl and 23N2. Moreover, in the region between FZ6 and FZ7 we observed comparatively greater width of crust and an additional lineation [24Nl(?) in Fig. 21 between anomaly CIand 24Nl. This anomalous extra crust possibly resulted from a localized ridge jump. These situations perhaps suggest that compared to the western part of the Arabian Basin, the southeastern part (near Laccadive Ridge) experienced a relatively complex spreading pattern between anomaly 24Nl and 23Nl times. Considering the age (- 5 1 Myr) of the oceanic crust at DSDP Site 220 and the right lateral offset pattern of anomalies established in this study, we feel, some of the post-A28 anomalies should have been present in the area east of 70”E between 9” and 13”N (Fig. 6). It can be seen that this area falls on the western slopes of the Laccadive Ridge and as mentioned earlier our profiles indicate that the magnetic anomalies in this area are of reduced amplitude and lack obvious correlations. It may be mentioned here that the Chagos-Laccadive Ridge is considered to be a linear volcanic feature formed during the northward motion of the Indian Plate over the Reunion hot spot (Whitmarsh, 1974; Morgan, 1981; Shipboard Scientific Party, 1988). We therefore surmise that in this area the

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Laccadive Ridge was emplaced over an oceanic crust and thereby obliterated/obscured the magnetic anomaly signatures associated with that crust.

Acknowledgements We are grateful to Mr. R.R. Nair, Head of the Geological Oceanography Division for his encouragement to carry out this study and to Dr. E. Desa, Director, National Institute of Oceanography for permission to publish this work. We would also like to thank Tony Thottam, K. Ramani and D.K. Naik for technical support on board and to T. Ramprasad for his help and constructive comments. We are grateful for the critical review and constructive comments by Dr. J.M. Bull and an anonymous reviewer, which helped to improve the paper. The authors are thankful to the Department of Ocean Development, New Delhi for providing partial study. this NIO/ financial support to CONTRB/S-95.

References Bhattacharya, G.C., Chaubey, A.K., Murty, G.P.S., Rao, D.G., Scherbakov, V.S., Lygin, V.A., Philipenko, A.I. and Bogomyagkov, A.P., 1992. Marine magnetic anomalies in the northeastern Arabian Sea. In: B.N. Desai (Editor), Oceanography of the Indian Ocean. Oxford and IBH, New Delhi, pp. 5033509. Bhattacharya, G.C., Chaubey, A.K., Murty, G.P.S., Srinivas, K., Sarma, K.V.L.N.S., Subrahmanyam, V. and Krishna, K.S., 1994a. Evidence for seafloor spreading in the Laxmi Basin, northeastern Arabian Sea. Earth Planet. Sci. Lett., 125: 211l220. Bhattacharya, G.C., Murty, G.P.S., Srinivas, K., Chaubey, A.K., Sudhakar, T. and Nair, R.R., 1994b. Swath bathymet-

Geology 128 (1995) 105-114 ric investigation of the seamounts located in the Laxmi Basin, eastern Arabian Sea. Mar. Geodesy, 17: 1699182. Cande, SC. and Kent, D.V., 1992. A new geomagnetic polarity time scale for the Late Cretaceous and Cenozoic. J. Geophys. Res., 97: 13,917-13,951. Cande, S.C., La Brecque, J.L., Larson, R.L., Pitman, W.C., Golovchenko, X. and Haxby, W.F., 1989. Magnetic lineations of the world’s ocean basins, 1 sheet. AAPG, Tulsa, Okla. Chaubey, A.K., Bhattacharya, G.C., Murty, G.P.S. and Desa, M., 1993. Spreading history of the Arabian Sea: some new constraints. Mar. Geol., 112: 3433352. Crough, S.T., 1983. The correlation for sediment loading on the seafloor. J. Geophys. Res., 88: 6449-6454. Hays, D.E., 1988. Age-depth relationships and depth anomalies in the southeast Indian Ocean and south Atlantic Ocean. J. Geophys. Res., 93: 2931-2954. Heirtzler, J.R., Dickson, G.O., Herron, E.M., Pitman, W.C. and Le Pichon, X., 1968. Marine magnetic anomalies, geomagnetic field reversals, and motions of the ocean floor and continents. J. Geophys. Res., 73: 2119-2136. McKenzie, D.P. and Sclater, J.G., 1971. The evolution of the Indian Ocean since the Late Cretaceous. Geophys. J. R. Astron. Sot., 25: 4377528. Miles, P.R. and Roest, W.R., 1993. Earliest sea-floor spreading magnetic anomalies in the north Arabian Sea and the ocean continent transition. Geophys. J. Int., 115: 102551031. Morgan, W.J.. 1981. Hotspot tracks and the opening of the Atlantic and Indian Oceans. In: C. Emiliani (Editor), The Sea. Wiley-Interscience, New York, 7. pp. 443-487. Naini, B.R. and Talwani, M., 1982. Structural framework and the evolutionary history of the continental margin of western India. In: J.S. Watkins and CL. Drake (Editor), Studies in Continental Margin Geology. AAPG Mem., 34: 1677191. Norton, 1.0. and Sclater, J.G., 1979. A Model for the evolution of the lndian Ocean and the breakup of Gondwanaland. J. Geophys. Res., 84: 680336830. Schlich, R., 1975. Structure et age de l’ocean Indien occidental. Mem. Hors-Ser. Sot. Geol. Fr., 6, 103 pp. Schlich, R., 1982. The Indian Ocean: aseismic ridges, spreading centers and Ocean basins. In: A.E.M. Nairn and F.G. Stehli (Editors), The Ocean Basins and Margins. Plenum, New York, 6: 51.-147. Shipboard Scientific Party, 1988. Site 715. Proc. ODP, Init. Rep., 115: 917-946. Whitmarsh, R.B., 1974. Some aspects of plate tectonics in the Arabian Sea. Init. Rep. DSDP, 23: 5277535. Whitmarsh, R.B., Weser, O.E. and Others, 1974. Sites 220 and 221. Init. Rep. DSDP, 23: 117-210.