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Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data
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Earth and Planetary Science Letters 171 (1999) 237–251 www.elsevier.com/locate/epsl
Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data C. Subrahmanyam a,Ł , N.K. Thakur a , T. Gangadhara Rao a , Ramesh Khanna a , M.V. Ramana b , V. Subrahmanyam b a National b
Geophysical Research Institute, Hyderabad 500 007, India National Institute of Oceanography, Dona Paula, Gao 404 004, India Received 10 December 1998; accepted 16 June 1999
Abstract Recently released satellite-derived free air gravity anomalies and the existing ship-board geophysical data provide new insights into the tectonics of the Bay of Bengal with respect to the structure and regional extension of the buried 85ºE ridge and the tectonics of the Eastern Continental Margin of India (ECMI). The 85ºE ridge can be visualized extending inland via the Mahanadi basin volcanics to the Rajmahal Traps. A large volcanic province in eastern India encompassing the Rajmahal and Sylhet Traps and volcanics in the Bengal and Mahanadi basins, almost on the scale of the Deccan volcanic province along the west coast, can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 Ma. The 85ºE ridge represents the deep-ocean volcanic trace of this magmatic activity. Towards the south, the ridge continues in an arcuate manner to the Afanasy–Nikitin seamount at equatorial latitudes in the central Indian Ocean. Gravity models of the ridge are indicative of hotspot-related crustal underplating processes beneath the ridge. The ECMI can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data, which bear similarities with the conjugate East Antarctica margin. 1999 Elsevier Science B.V. All rights reserved. Keywords: Bay of Bengal; tectonics; satellite methods; geophysical surveys
1. Introduction The evolution of the Bay of Bengal (BoB) marks the early breakup history of eastern Gondwanaland in the northeastern Indian Ocean [1,2]. The breakup between the east coast of India and East Antarctica led to the development of the Eastern Continental Margin of India (ECMI) associated with subsidence and sediment deposition in the marginal basins [3– 5]. Subsequently, the uplift of the Himalayas and Ł Corresponding
author. Fax: C91 40 7171 564; E-mail: [email protected]
their erosion resulted in the development of the Bengal fan over the Bay of Bengal ocean floor where the thickest pile of sediments are deposited and this activity continues even today [6,7]. Three contentious issues restrict our understanding of the structure, tectonics and evolution of the Bay of Bengal ocean floor in a clear manner. The first concerns the time of breakup between India and East Antarctica and the evolution of BoB ocean floor. Most of the plate reconstructions attempted for this segment of the Indian ocean lithosphere have remained conjectural [1,2] mainly for the lack of identification of magnetic lineations of probable
0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 4 8 - X
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Mesozoic age both in the Bay of Bengal and its conjugate Enderby basin. A first attempt of identifying Mesozoic magnetic lineations in BoB was made recently based on newly acquired marine magnetic data [8]. On the basis of several NW–SE-trending ship track data, anomalies ranging from M11 to M0 were inferred with half spreading rates of 3.6 to 3.4 cm=yr during anomalies M11–M9 and M4–M0 and a faster spreading rate of 10.0 cm=yr during anomalies M9–M4. This puts the time of evolution of the Bay of Bengal ocean floor between 132 and 118 m.y. Several fracture zones disturbing the continuity of the magnetic lineations from track to track were also inferred [8]. In contrast to this view, there is an inference that the entire BoB ocean floor has evolved during the Cretaceous Magnetic Quiet Period, viz. during anomalies M0 to A34 [9], an inference based on two E–W-trending tracklines. Lawver et al. [10] observed that the pull apart of India from East Antarctica cannot be placed as older than anomaly M10 (130 Ma) due to space problems and argued that the separation cannot be determined to closer than anomalies M5 to M0 times (127–118 Ma). In general, the arguments favoring the evolution of the Bay of Bengal ocean floor prior to anomaly M0 appear to be stronger. The second aspect relates to the mode of emplacement of the 85ºE ridge, a longitudinal ridge running the length of the Bengal fan and completely buried under the fan sediments (Fig. 1). One view projects the ridge as a hotspot trace on the ocean floor, connecting it through the Afanasy-Nikitin seamount in the central Indian Ocean, to either the Crozet hotspot [11] or the Conrad Rise [12] in the southern Indian Ocean. Another inference [13] based on interpretation of marine magnetic data, considers this ridge as outpouring of volcanic material through a crack in the ocean floor. Ship-borne gravity data were interpreted as the ridge being composed of a homogeneous volcanic material of density 2.9 g=cm3 [13]. The main issue appears to be whether the ridge is a hotspot trace or not. The northward extension of the ridge is also not clearly resolved, whether it is connected to the Rajmahal Traps observed in northeastern India via the Bengal basin [11,13] or the ridge abuts the east coast of India, further south at about 20ºN in the proximity of the Mahanadi basin [14], both inferences based on marine magnetic data.
The third important point, which relates to the evolution of the continental margin (ECMI), is whether the margin is a total pull-apart margin or there is a segment, which possibly could have evolved as a sheared or transform margin. The development of coastal sedimentary basins along the ECMI [3] and the development of prograding deltas in the offshore regions give the impression of a pullapart nature of the margin in its entirety. However, the disposition of the coastline north and south of 15ºN, with a NE–SW trend in the north and a N–S trend in the south, casts some doubt on the total pullapart nature of the entire ECMI. Inferences based on examination of bathymetry contours from the conjugate margins of the Coromandel coast of eastern India (southern segment of ECMI) and the Kron Prins Olav Kyst region of Antarctica suggest that the margins could initially be of the transform-type allowing a transform motion of about 87 km [1]. In this study we have examined four different data sets from the Bay of Bengal, viz. satellite-derived free air anomalies [15] and shipborne gravity [13], line drawings of multichannel seismic reflection data [16], analog seismic sections obtained from the National Geophysical Data Center, USA, and bathymetry profiles across the ECMI, to address some of the problems stated above, namely structure and extension of the 85ºE ridge and the nature of the ECMI.
2. Data presentation The satellite-derived free air anomalies, along with single channel analog seismic records, were examined to outline the N–S extension of the 85ºE ridge and its probable connectivity with the AfanasyNikitin seamount in the central Indian Ocean basin. From the 20 ð 20 free air anomaly gridded database [15] a contour map is prepared (Fig. 2). This map was also examined to study the probable distinction between the northern and southern segments of the ECMI. Shipborne gravity data were used to model the density distributions beneath the 85ºE ridge. The gravity contour map prepared from these data, are shown in Fig. 3. Two sets of data were used here, one obtained from the National Geophysical Data Center, Boulder, USA, comprising all available data
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Fig. 1. Bathymetry, tectonics and sedimentation in the Bay of Bengal and adjacent central Indian Ocean. (a) Tectonic map after Curray [6]. Sediment thickness contours shown at a 2-km interval. Hachured circle in the north indicates the probable extent of hotspot-related volcanism in eastern India. See text for explanation. (b) Bathymetry contours in the northeastern Indian Ocean (from Curray et al [4]). Contour interval 100 m.
from cruises of international agencies, while the second set comprises the data obtained on board ORV Sagar Kanya during the 1992–93 cruises [8,13]. Line sections drawn from two processed multichannel reflection seismic sections [16] are shown in Fig. 4
and their locations are shown in Fig. 3. These sections have clear information from the seafloor to the basement. Sedimentation in the BoB can be categorized into two distinct phases, an older phase prior to collision along the Himalayan front, marked by
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Fig. 2. Satellite-derived free air anomaly distribution in the Bay of Bengal and northern-central Indian Ocean. Source: Sandwell and Smith [15]. Contour interval 10 mGal. White dots indicate trace of the 85ºE ridge. A-N seamount D Afanasy-Nikitin seamount.
the P unconformity and a younger phase marking the post-collision depositional activity related to the erosion of Himalaya and deposition of the Bengal fan sediments [6]. The line sections were used to constrain the gravity modeling to the basement level.
3. Analysis and interpretation 3.1. 83ºE Ridge — extension and structure The satellite-derived gravity anomaly map (Fig. 2) brings out a negative anomaly associated with the
ridge quite prominently between 5º and 15ºN. The peak amplitude of about 80 mGal occurs at about 14ºN where the anomaly is also at its widest. Progressing towards the north this negative trend seems to merge with the shelf edge anomaly along the ECMI, but the connectivity is reflected in the form of a southward ‘nosing’ of gravity contours. The present gravity map indicates the 85ºE ridge abutting the east coast of India at about 18º–19ºN. Further extension of the ridge parallel to the coast is not visible in the gravity contours. To establish that the negative gravity anomaly is attributable to the presence of the ridge and its topographic relief, single
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Fig. 3. Gravity anomaly map of the Bay of Bengal from ship-borne gravity data. Contour interval 10 mGal. MAN01 and MAN03 are multichannel seismic profiles (Fig. 4) [9,16] along which gravity modeling was carried out (Fig. 6).
channel analog seismic records from the Bay of Bengal were examined in the longitudinal range of 84.5º–86ºE. These sections stacked in a latitudinal manner are shown in Fig. 5. In most sections the relief of the ridge could be easily traced. However, in a few cases, the presence of the ridge could only be indirectly inferred from the updip of the sediments probably underlain by the ridge (e.g. section at 10ºN in Fig. 5). The ridge could be traced from 8ºN to 17º–18ºN, conforming with the negative gravity trend and it is at its widest at 14.7ºN, correlating well with the peak negative gravity amplitudes observed in this region. At about 5ºN, the gravity contours take a curvilinear trend and appear to merge with the E–W-trending gravity highs and lows of the deformed blocks of the central Indian Ocean [17]. At these latitudes the connectivity of the ridge structure can be established through bathymetry. The Afanasy-Nikitin seamount
has been linked to the 85ºE ridge by several earlier workers, e.g. [6]. However, what is interesting to note is that this seamount is characterized by a strong positive gravity anomaly, quite unlike the negative anomaly associated with the 85ºE ridge further north. It appears natural to link the two structures mainly because of their topographic relief, the northern segment buried under thick piles of sediments while the southern part is exposed as a strong bathymetric feature in the central Indian Ocean. This contrasting tectonic setting could be the reason for the change in the gravity anomaly, from negative in the north to positive in the south. It should be mentioned here that Liu et al. [18] have interpreted the negative anomaly of the 85ºE ridge in terms of a two-stage deformation of the Bay of Bengal lithosphere, the first at the time of loading of the ridge over a weak and young BoB lithosphere, and the other, caused by the load of sediments over a lithosphere gaining
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Fig. 4. Line sections of two multichannel seismic sections MAN01 and MAN03. Locations shown in Fig. 3.
strength with time. Further, Ramana et al. [13] have established a positive magnetic anomaly trend for the entire length of the 85ºE ridge including the northern segment of the Afanasy-Nikitin seamount. Two multichannel seismic sections [9,16] across the Bay of Bengal bring out the relief of the 85ºE ridge and the distribution of the overlying sedimentary sequences in a clear manner. Line drawings of the two sections are shown in Fig. 4. The ridge has a steeply dipping western flank while the eastern flank has gentle slopes. Further, the ridge appears to be wider in the northern sections (MAN 01, Fig. 4a) and associated with a trough-like feature in the middle. The overall relief of the ridge appears to exceed 2 km at some places. Ship-borne gravity anomalies (Fig. 3) were used to interpret the negative anomaly associated with the 85ºE ridge as well as those over the BoB in general. This is done along the two multichannel seismic sections MAN01 and MAN03, which give the advantage of constraining the model with information from the seafloor to the basement. The inferred models for the ridge and the adjacent BoB ocean floor are shown in Fig. 6. In this exercise the
free air anomaly was first corrected for water depth and sediment thickness and a crust of uniform thickness (6 km). Further, it is assumed that the relief of the 85ºE ridge above the oceanic basement is isostatically compensated by crustal roots, an assumption supported by the inference [18] that the ridge is emplaced over a weak and young BoB lithosphere (and hence compensated by crustal thickening). Based on this, crustal roots beneath the ridge were estimated from the relief of the ridge (that above the basement level). Simple forward-modelling methods were used to obtain the combined effect of all the layers. For profile MAN01 it was noticed that the roots beneath the ridge extend to considerable depths exceeding 30 km, a situation that is abnormally high for an oceanic crustal environment and hence the root density was increased from an initial 2.9 g=cm3 to a 3.1 g=cm3 . For the MAN03 profile this requirement was not necessary because the calculated crustal thickness beneath the ridge at 18 km does not appear to be large. Over the adjacent parts, along both the profiles, there seem to be no large-scale variations either in crustal thickness or density, the only exception being the segment west of the 85ºE ridge in MAN01
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where the crust initially appears to thin out followed by thickening as we move towards the coast. This is the region where the Krishna–Godavari delta progrades into the sea and the somewhat thicker crust may represent the region of initial rifting followed by crustal extension and stretching. 3.2. The eastern continental margin of India Plate reconstructions for the periods prior to the breakup of eastern Gondwanaland show that the east coast of India is juxtaposed against the Enderbyland of East Antarctica [1,2,4,10]. Magnetic lineations of Mesozoic age were recently inferred for the Bay of Bengal ocean floor while for its conjugate part, the Enderby basin, such information is still awaited. Identified magnetic lineations in the central and northern parts of the Bay of Bengal [8] are found to be aligned parallel to the east coast of India, particularly north of 13ºN, suggesting a pullapart nature of the margin. South of this latitude, no magnetic lineations close to the east coast of India have been identified. To examine the nature of the gravity anomalies characterizing the ECMI, an enlarged satellite-derived gravity map of the ECMI is prepared and shown in Fig. 7, from which it is clear that gravity anomalies display a contrasting style north and south of 14.5ºN. Towards the north, where the spreading-type magnetic lineations were inferred and where the Krishna–Godavari basin (see Fig. 7) has developed into a prograded delta, the contours are well-distributed with a well-developed shelf edge anomaly and large negative gravity anomalies characterizing the depressions in the offshore delta. South of 14.5ºN, the gravity contours are tightly packed, displaying narrow bands of gravity highs and lows running parallel to the coast in a N–S direction. The almost linear band of negative anomalies is disrupted by a large localized ENE– WSW-trending positive gravity anomaly associated with the Karaikal ridge in the Cauvery offshore basin [3]. Bathymetry (Fig. 1b) of the ECMI shows good correspondence with gravity, characterized by a narrow shelf and steep slopes south of 14.5ºN, widening gradually further north. To demonstrate this, stacked bathymetric profiles across the margin are shown in Fig. 8. Profiles to the south display steep slopes characteristic of transform margins [10,19] while
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further north they display gentle slopes characteristic of pull-apart type of margins. The contrast in the two segments of the ECMI is also considerably displayed in the structural style and tectonism of the sedimentary basins that have developed along the margin from north to south. The seismic stratigraphic sections in the Krishna–Godavari basin to the north indicate deposition of sediments along a prograding deltaic front with gentle slopes [3]. Quite in contrast, sediments in the Cauvery basin (onshore only, no offshore seismic sections are available) seem to be confined to narrow and deep troughs. Also, structures (ridges and depressions) in the K-G and Mahanadi basins have trends parallel to the coast, while those in the Cauvery basin seem to be trending somewhat oblique to the coastline [3]. Thus, the satellite-derived gravity data, supported by bathymetry and seismic stratigraphic sections, suggest that the ECMI can be broadly divided into a northern pullapart segment and a southern sheared or transform segment.
4. Discussion and conclusions The origin, structure and extension of the 85ºE ridge, particularly its northward extension, seem to have gained importance as is evident from recent studies based on a variety of data sets, e.g. [13]. The trace of the ridge towards south, on the basis of the present study as well as those earlier, appears to take a curvilinear trend around Sri Lanka and to merge with the Afanasy-Nikitin seamount in the central Indian Ocean. This is clear from both the satellite-derived gravity anomalies and bathymetry, and from seismic reflection surveys [11]. The only contrast seems to be in the switch-over from a negative gravity anomaly in the north, where the ridge is buried under thick sediments, to a positive anomaly in the south, where the volcanic ridge is directly exposed over the seafloor. The extension of the ridge towards the north on land and its connectivity with the Rajmahal Traps is more difficult to establish. Occurrences of volcanic rocks were reported from both the Mahanadi offshore basin [20] and the Bengal basin [21,22]. The Rajmahal and Sylhet Traps, dated at 117 Ma, are known to extend eastward beneath the sediments of
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Fig. 5. Analog single-channel seismic sections across the 85ºE ridge from 8ºN in the south, east of Sri Lanka, to 18ºN near the Mahanadi basin in the north along the east coast of India. Where the ridge could not be marked directly on the section, updipping sedimentary sequences are taken as indicators for the existence of the ridge. See text for explanation.
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Fig. 5 (continued).
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Fig. 6. Gravity interpretation of the structures along the MAN01 and MAN03 seismic lines across the Bay of Bengal. Note the inferred crustal underplating beneath the 85ºE ridge in MAN01 and its absence in MAN03. Crustal thinning between the east coast of India and the 85ºE ridge can also be noticed. Figures in brackets denote densities in gm=cm3 .
the Bengal basin. A deep onland well in the Cuttack depression near Cuttack in the Mahanadi basin brings out two volcanic layers, L1 and L2, both belonging to the Early Cretaceous but separated by about 700-m-thick sediments [23]. Ages of 107 Ma and 114–116 Ma are assigned to layers L1 and L2, respectively. In addition to these, a dyke intrusion near Cuttack yields a K–Ar age of 111 Ma [24]. Further, lamprophyres were reported to have K–Ar ages between 105 and 121 Ma [25] and a Rb–Sr age of 117C2 Ma [26] southwest of the Rajmahal Traps. These data indicate large-scale volcanic occurrences in eastern India, both exposed (Rahmahal and Sylhet Traps and others) and buried under sediments (Mahanadi and Bengal basins) and extending into the deep-sea to the Afanasy-Nikitin seamount, all of which seem to be closely related to 85ºE ridge vol-
canism. Fig. 1a reflects the large volcanic province in eastern India envisaged earlier [21,22], with a closely related volcanic hotspot trace in the form of the 85ºE ridge extending into the deep ocean. This scenario is quite similar to the Deccan volcanic province in western India and the Reunion hotspot trace in the form of the Chagos–Laccadive ridge [27]. The age progression from 117 Ma for the Rajmahal Traps to the Cretaceous age of the Afanasy-Nikitin seamount[28] is clear. In the Mahanadi basin the age of 114–116 Ma for layer L2 (older volcanics) is probably related to the Rajmahal Trap volcanism while the younger age of 107 Ma for layer L1 (younger volcanics) relates to that of the hotspot trace. This suggests the initiation of the emplacement of the 85ºE Ridge in the Bay of Bengal at about 107 Ma at 20ºN in the Mahanadi basin and continuing to Creta-
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Fig. 7. Satellite-derived free air gravity contours along the Eastern Continental Margin of India (ECMI). Contour interval 10 mGal. Note the tightly packed narrow band of gravity contours over the southern sheared segment contrasting with the well spread-out gravity contours of the rifted segment in the north.
ceous times at equatorial latitudes (Afanasy-Nikitin seamount) in the central Indian Ocean. Considering that the hotspot trace traversed a distance of about 2200 km (¾20º of latitude) in a N–S direction in a span of about 40 m.y. a spreading rate of about 5.5 cm=yr for the ocean floor of the BoB and northern central Indian Ocean can be inferred. This is close to the average of the rates of 3.4 to 10.0 cm=yr predicted for the Mesozoic anomalies in the Bay of Bengal [8] and is almost the same as the rates of 5 cm=yr predicted for the period 70–80 Ma for the central Indian Ocean [2].
Spreading magnetic lineations in the Bay of Bengal have a NE–SW orientation, particularly in the northern and central parts. With the 85ºE ridge having a N–S orientation in the BoB, somewhat oblique to the direction of spreading, a tectonic setting in which the emplacement of the ridge on a young, but already evolved ocean floor can be inferred. Gravity modelling favors the emplacement of the ridge on a young oceanic lithosphere (¾15 Ma old) [18]. Similarly, the evolution of the Afanasy-Nikitin seamount at a ridge–transform intersection has been proposed on the basis of gravity–bathymetry correlations [28].
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Fig. 8. Bathymetry profiles to reflect the contrast between the sheared and rifted segments of the ECMI. Inset shows the locations of the profiles.
Emplacement of the ridge due to outpouring of volcanic material along a crack in the ocean floor has been suggested, but the curvilinear trend of the ridge east and south of Sri Lanka poses problems as it is difficult to understand the tectonics leading to the development of such cracks on the ocean floor with curvilinear trends. The present study favours the hotspot model for the origin of the 85ºE. To evolve a more complete picture of the early breakup history between India and East Antarctica and the plume tectonism in the northern Indian Ocean, there is need for identification of seafloor spreading-type magnetic lineations of probable Mesozoic age from the conjugate Enderby basin, together with plate reconstruction models for the Mesozoic period. The crustal structure beneath the 85ºE ridge inferred in the present study suggests development of
thick roots beneath the ridge, probably indicating a crustal underplating process. This interpretation leans heavily on the results of seismic surveys over hotspot traces like the Marquesas Islands in the Pacific Ocean and also the gravity modeling of the Ninetyeast ridge in the Indian Ocean where thick and high-density roots were inferred [29,30]. Inference of thick roots beneath volcanic hotspot traces is not uncommon. For the Chagos–Laccadive ridge in the northwestern Indian Ocean thick roots beneath the central parts were inferred on the basis of admittance analysis of gravity–bathymetry profiles [27]. Similarly, seismic surveys reveal thick crustal roots beneath the Hawaiian seamount chain and the Kerguelen plateau [31,32]. Nur and Ben-Avraham [33] show crustal models for several oceanic plateaus underlain by thick crustal roots. The underplating
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Fig. 9. Schematic diagram of India–East Antarctica plate reconstruction [37].
process seems to be resulting from massive intrusions related to hotspot volcanism [29]. The Eastern Continental Margin of India can be segmented into two blocks, a transform margin in the south and a pull-apart one further north, based on gravity and bathymetry data. This implies an unusual spreading pattern for the southern segment of the ECMI which, with our present knowledge of the plate kinematics of the region, is somewhat difficult to understand. In some plate reconstructions this segment is shown as a system of closely spaced transforms and ridge segments, e.g. [1,4]. The transform nature of some segments of passive margins was inferred for the Ghana coast in northwestern Africa, Grand Banks margin off the eastern Canadian margin and several others [34]. These sheared or transform margins appear to be invariably connected to fracture zones in the deep ocean. Admittance analysis of gravity and bathymetry data seems to bring out the contrast between sheared margins and rifted or pullapart margins [35]. These results suggest that sheared or transform margins may be isostatically compensated in a local manner compared to the rifted margins compensated regionally. For a more complete interpretation there is need for information on the seismic crustal structure of this segment of the ECMI.
The tectonic setting of the ECMI emerges more clearly with East Antarctica juxtaposed against the east coast of India [1]. Disposition of the 2000-m isobath and evidence from existing seismic profiles have helped in demarcating the Continent–Ocean Boundary (COB) for East Antarctica and the conjugate east coast of India and Sri Lanka. The southern transform segment of the ECMI, termed ‘Coromandel coast’, displays a remarkably linear bathymetry, similar to that of the Krons Prins Olav Kyst region of East Antarctica. The conjugate nature of East Antarctica with respect to the ECMI is further substantiated with the presence of alkalic igneous intrusives (110 Ma) in the Prince Charles Mountains adjacent to the Lambert graben [36,37], similar to the Layer L1 volcanic rocks found in the Mahanadi basin of India. This brings into close alignment the Mahanadi and the Lambert grabens in a pre-breakup tectonic setting of eastern Gondwanaland (Fig. 9).
Acknowledgements We thank the Director, National Geophysical Research Institute, for encouragement and guidance, and his kind permission to publish this paper. We
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thank Prof. J.R. Curray and an anonymous reviewer for their critical reviews of the manuscript. We are grateful to Prof. Curray for a correction on the age of the A-N seamount and also for his helpful comments. CS acknowledges many fruitful discussions with M. Radha Krishna and Anil Kumar. [MK]
[14]
[15]
References [1] C.McA., Powell, S.R. Roots, J.J. Veevers, Pre-breakup continental extension in East Gondwanaland and the early opening of the eastern Indian Ocean, Tectonophysics 155 (1988), 261–283. [2] J.Y. Royer, D.T. Sandwell, Evolution of the eastern Indian Ocean since the Late Cretaceous: constraints from Geosat altimetry, J. Geophys. Res. 94 (1989) 13755–13782. [3] V.V. Sastri, B.S. Venkatachala, V. Narayanan, The evolution of the east coast of India, Palaeogeogr., Palaeoclimatol., Palaeoecol. 36 (1981) 23–54. [4] J.R. Curray, F.J. Emmel, D.G. Moore, R.W. Raitt, Structure, tectonics, and geological history of the northeastern Indian ocean, in: A.E.M. Nairn, F.G. Stehli (Eds.), The Ocean Basins and Margins, Plenum, New York, 1982, pp. 399– 450. [5] M. Mukhopadhyay, M.R. Krishna, Gravity field and deep structure of the Bengal fan and its surrounding continental margins, northeast Indian Ocean, Tectonophysics 186 (1991) 365–386. [6] J.R. Curray, Possible greenschist metamorphism at the base of a 22-km sedimentary section, Bay of Bengal, Geology 19 (1991) 1097–1100. [7] J.N. Brune, J.R. Curray, L. Dorman, R. Raitt, A proposed super-thick sedimentary basin, Bay of Bengal, Geophys. Res. Lett. 19 (1992) 565–568. [8] M.V. Ramana, R.R. Nair, K.V.L.N.S. Sarma, T. Ramprasad, K.S. Krishna, V. Subrahmanyam, M. D’Cruz, C. Subrahmanyam, J. Paul, A.S. Subrahmanyam, D.V. Chandra Sekhar, Mesozoic anomalies in the Bay of Bengal, Earth Planet. Sci. Lett. 121 (1994) 469–475. [9] D. Gopala Rao, K.S. Krishna, D. Sar, Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous, J. Geophys. Res. 102 (1997) 17747–17768. [10] L.A. Lawver, J.Y. Royer, D.T. Sandwell, C.R. Scotese, Crustal development: Gondwana breakup, in: M.R.A. Thomson, J.A. Crame, J.W. Thomson (Eds.), Geological Evolution of Antarctica, Cambridge Univ. Press, Cambridge, 1991, pp. 533–539. [11] J.R. Curray, T. Munasinghe, Origin of the Rajmahal Traps and the 85ºE ridge: preliminary reconstructions of the trace of the Crozet hotspot, Geology 19 (1991) 1237–1240. [12] R.D. Mueller, J.Y. Royer, L.A. Lawver, Revised plate motions relative to the hotspots from combined Atlantic and Indian ocean hotspot tracks, Geology 21 (1993) 275–278. [13] M.V. Ramana, V. Subrahmanyam, A.K. Chaubey, T. Ramprasad, K.V.L.N.S. Sarma, K.S. Krishna, M. Desa, G.P.S.
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
[30] [31]
Murty, Structure and origin of the 85ºE ridge, J. Geophys. Res. 102 (1997) 17995–18012. A.S. Subrahmanyam, K.S.R. Murthy, S. Lakshminarayana, M.M. Malleswara Rao, K. Venkateswarlu, T.C.S. Rao, Magnetic expression of some major lineaments and Cretaceous quiet zone in the Bay of Bengal, Geo-Mar. Lett. 17 (1997) 202–206. D.T. Sandwell, W.H.F. Smith, Marine gravity anomaly from Geosat and ERS 1 satellite altimetry, J. Geophys. Res. 102 (1997) 10039–10054. M.L. Pateria, M.K. Rangaraju, V. Raiverman, A note on the structure and stratigraphy of Bay of Bengal sediments, Geol. Surv. India Spec. Publ. 29 (1992) 21–23. C.A. Stein, S. Cloetingh, M. Wortel, SEASAT-derived gravity constraints on stress and deformation in the northeastern Indian Ocean, Geophys. Res. Lett. 16 (1989) 823–826. C. Liu, D.T. Sandwell, J.R. Curray, The negative gravity field over the 85ºE ridge, J. Geophys. Res. 87 (1982) 7673– 7686. J.P. Kennet, Marine Geology, Prentice-Hall, Englewood Cliffs, NJ. R.C. Fuloria, Geology and hydrocarbon prospects of Mahanadi basin, India, Proc. 2nd Sem. Petroliferous Basins of India 1 (1993) 355–369. A.K. Baksi, T.R. Barmann, D.K. Paul, E. Farrar, Widespread early Cretaceous flood basalt volcanism in eastern India: geochemical data from Rajmahal–Bengal–Sylhet Traps, Chem. Geol. 63 (1987) 133–141. A.K. Baksi, Petrogenesis and timing of volcanism in the Rajmahal Flood Basalt Province, northeastern India, Chem. Geol. 121 (1995) 73–90. H.C. Tewari, The effect of thin high velocity layers on seismic refraction data: an example from Mahanadi basin, India, Pure Appl. Geophys. 151 (1998) 63–79. J.K. Agarwal, Rama, Chronology of Mesozoic volcanics of India, Proc. Ind. Acad. Sci. 84A (1976) 157–179. Sarkar et al., Lamprophyres from Indian Gondwanas — K–Ar ages and chemistry, J. Geol. Soc. India 21 (1980) 188–193. A. Kumar, A.M. Dayal, V.M. Padma Kumari, K. Gopalan, Rb–Sr age of Jharia Lamprophyre: part of the Kerguelen=Crozet hotspot trace? (in prep.). B. Ashalatha, C. Subrahmanyam, R.N. Singh, Origin and compensation of Chagos–Laccadive ridge, Indian ocean, from admittance analysis of gravity and bathymetry data, Earth Planet. Sci. Lett. 105 (1991) 47–54. J. Paul, R.N. Singh, C. Subrahmanyam, R.K. Drolia, Emplacement of Afanasy-Nikitin seamount based on transfer function analysis of gravity and bathymetry data, Earth Planet. Sci. Lett. 96 (1990) 419–426. D.W. Caress, M.K. McNutt, R.S. Detrick, J.C. Mutter, Seismic imaging of hotspot-related crustal underplating beneath the Marquesas Islands, Nature 373 (1995) 600–603. C. Bowin, Origin of the Ninety East Ridge from studies near the equator, J. Geophys. Res. 78 (1973) 6029–6043. A.B. Watts, U.S. ten Brink, P. Buhl, T.M. Brocher, A multichannel seismic study of lithospheric flexure across
C. Subrahmanyam et al. / Earth and Planetary Science Letters 171 (1999) 237–251 the Hawaiian-Emperor seamount chain, Nature 315 (1985) 105–111. [32] M. Recq, D. Brefort, J. Malod, J. Veinante, The Kerguelen Isles (southern Indian ocean): new results on deep structure from refraction profiles, Tectonophysics 182 (1990) 227– 248. [33] A. Nur, Z. Ben-Avraham, Oceanic plateaus, the fragmentation of continents and mountain building, J. Geophys. Res. 87 (1982) 3644–3661. [34] R.A. Scrutton, Continental breakup and deep crustal structure at the margins of southern Africa, An. Acad. Bras.
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Cienc. 48 (1976) 275–286. [35] J. Verhoeff, H.R. Jackson, Admittance signatures of rifted and transform margins: examples from eastern Canada, Geophys. J. Int. 105 (1991) 229–239. [36] J.W. Sheraton, Geochemistry of mafic igneous rocks of the northern Prince Charles Mountains, Antarctica, J. Geol. Soc. Aust. 30 (1983) 295–304. [37] A. Cooper, H. Stagg, E. Geist, Seismic stratigraphy and structure of Prydz Bay, Antarctica: implications from Leg 119 drilling, Proc. ODP 119 (1991) 5–25.
Earth and Planetary Science Letters 171 (1999) 237–251 www.elsevier.com/locate/epsl
Tectonics of the Bay of Bengal: new insights from satellite-gravity and ship-borne geophysical data C. Subrahmanyam a,Ł , N.K. Thakur a , T. Gangadhara Rao a , Ramesh Khanna a , M.V. Ramana b , V. Subrahmanyam b a National b
Geophysical Research Institute, Hyderabad 500 007, India National Institute of Oceanography, Dona Paula, Gao 404 004, India Received 10 December 1998; accepted 16 June 1999
Abstract Recently released satellite-derived free air gravity anomalies and the existing ship-board geophysical data provide new insights into the tectonics of the Bay of Bengal with respect to the structure and regional extension of the buried 85ºE ridge and the tectonics of the Eastern Continental Margin of India (ECMI). The 85ºE ridge can be visualized extending inland via the Mahanadi basin volcanics to the Rajmahal Traps. A large volcanic province in eastern India encompassing the Rajmahal and Sylhet Traps and volcanics in the Bengal and Mahanadi basins, almost on the scale of the Deccan volcanic province along the west coast, can be envisaged taking into account the occurrences of intrusive and extrusive rocks around the age of 117 Ma. The 85ºE ridge represents the deep-ocean volcanic trace of this magmatic activity. Towards the south, the ridge continues in an arcuate manner to the Afanasy–Nikitin seamount at equatorial latitudes in the central Indian Ocean. Gravity models of the ridge are indicative of hotspot-related crustal underplating processes beneath the ridge. The ECMI can be divided into a southern transform and northern rifted segments on the basis of gravity and bathymetry data, which bear similarities with the conjugate East Antarctica margin. 1999 Elsevier Science B.V. All rights reserved. Keywords: Bay of Bengal; tectonics; satellite methods; geophysical surveys
1. Introduction The evolution of the Bay of Bengal (BoB) marks the early breakup history of eastern Gondwanaland in the northeastern Indian Ocean [1,2]. The breakup between the east coast of India and East Antarctica led to the development of the Eastern Continental Margin of India (ECMI) associated with subsidence and sediment deposition in the marginal basins [3– 5]. Subsequently, the uplift of the Himalayas and Ł Corresponding
author. Fax: C91 40 7171 564; E-mail: [email protected]
their erosion resulted in the development of the Bengal fan over the Bay of Bengal ocean floor where the thickest pile of sediments are deposited and this activity continues even today [6,7]. Three contentious issues restrict our understanding of the structure, tectonics and evolution of the Bay of Bengal ocean floor in a clear manner. The first concerns the time of breakup between India and East Antarctica and the evolution of BoB ocean floor. Most of the plate reconstructions attempted for this segment of the Indian ocean lithosphere have remained conjectural [1,2] mainly for the lack of identification of magnetic lineations of probable
0012-821X/99/$ – see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 2 - 8 2 1 X ( 9 9 ) 0 0 1 4 8 - X
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Mesozoic age both in the Bay of Bengal and its conjugate Enderby basin. A first attempt of identifying Mesozoic magnetic lineations in BoB was made recently based on newly acquired marine magnetic data [8]. On the basis of several NW–SE-trending ship track data, anomalies ranging from M11 to M0 were inferred with half spreading rates of 3.6 to 3.4 cm=yr during anomalies M11–M9 and M4–M0 and a faster spreading rate of 10.0 cm=yr during anomalies M9–M4. This puts the time of evolution of the Bay of Bengal ocean floor between 132 and 118 m.y. Several fracture zones disturbing the continuity of the magnetic lineations from track to track were also inferred [8]. In contrast to this view, there is an inference that the entire BoB ocean floor has evolved during the Cretaceous Magnetic Quiet Period, viz. during anomalies M0 to A34 [9], an inference based on two E–W-trending tracklines. Lawver et al. [10] observed that the pull apart of India from East Antarctica cannot be placed as older than anomaly M10 (130 Ma) due to space problems and argued that the separation cannot be determined to closer than anomalies M5 to M0 times (127–118 Ma). In general, the arguments favoring the evolution of the Bay of Bengal ocean floor prior to anomaly M0 appear to be stronger. The second aspect relates to the mode of emplacement of the 85ºE ridge, a longitudinal ridge running the length of the Bengal fan and completely buried under the fan sediments (Fig. 1). One view projects the ridge as a hotspot trace on the ocean floor, connecting it through the Afanasy-Nikitin seamount in the central Indian Ocean, to either the Crozet hotspot [11] or the Conrad Rise [12] in the southern Indian Ocean. Another inference [13] based on interpretation of marine magnetic data, considers this ridge as outpouring of volcanic material through a crack in the ocean floor. Ship-borne gravity data were interpreted as the ridge being composed of a homogeneous volcanic material of density 2.9 g=cm3 [13]. The main issue appears to be whether the ridge is a hotspot trace or not. The northward extension of the ridge is also not clearly resolved, whether it is connected to the Rajmahal Traps observed in northeastern India via the Bengal basin [11,13] or the ridge abuts the east coast of India, further south at about 20ºN in the proximity of the Mahanadi basin [14], both inferences based on marine magnetic data.
The third important point, which relates to the evolution of the continental margin (ECMI), is whether the margin is a total pull-apart margin or there is a segment, which possibly could have evolved as a sheared or transform margin. The development of coastal sedimentary basins along the ECMI [3] and the development of prograding deltas in the offshore regions give the impression of a pullapart nature of the margin in its entirety. However, the disposition of the coastline north and south of 15ºN, with a NE–SW trend in the north and a N–S trend in the south, casts some doubt on the total pullapart nature of the entire ECMI. Inferences based on examination of bathymetry contours from the conjugate margins of the Coromandel coast of eastern India (southern segment of ECMI) and the Kron Prins Olav Kyst region of Antarctica suggest that the margins could initially be of the transform-type allowing a transform motion of about 87 km [1]. In this study we have examined four different data sets from the Bay of Bengal, viz. satellite-derived free air anomalies [15] and shipborne gravity [13], line drawings of multichannel seismic reflection data [16], analog seismic sections obtained from the National Geophysical Data Center, USA, and bathymetry profiles across the ECMI, to address some of the problems stated above, namely structure and extension of the 85ºE ridge and the nature of the ECMI.
2. Data presentation The satellite-derived free air anomalies, along with single channel analog seismic records, were examined to outline the N–S extension of the 85ºE ridge and its probable connectivity with the AfanasyNikitin seamount in the central Indian Ocean basin. From the 20 ð 20 free air anomaly gridded database [15] a contour map is prepared (Fig. 2). This map was also examined to study the probable distinction between the northern and southern segments of the ECMI. Shipborne gravity data were used to model the density distributions beneath the 85ºE ridge. The gravity contour map prepared from these data, are shown in Fig. 3. Two sets of data were used here, one obtained from the National Geophysical Data Center, Boulder, USA, comprising all available data
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Fig. 1. Bathymetry, tectonics and sedimentation in the Bay of Bengal and adjacent central Indian Ocean. (a) Tectonic map after Curray [6]. Sediment thickness contours shown at a 2-km interval. Hachured circle in the north indicates the probable extent of hotspot-related volcanism in eastern India. See text for explanation. (b) Bathymetry contours in the northeastern Indian Ocean (from Curray et al [4]). Contour interval 100 m.
from cruises of international agencies, while the second set comprises the data obtained on board ORV Sagar Kanya during the 1992–93 cruises [8,13]. Line sections drawn from two processed multichannel reflection seismic sections [16] are shown in Fig. 4
and their locations are shown in Fig. 3. These sections have clear information from the seafloor to the basement. Sedimentation in the BoB can be categorized into two distinct phases, an older phase prior to collision along the Himalayan front, marked by
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Fig. 2. Satellite-derived free air anomaly distribution in the Bay of Bengal and northern-central Indian Ocean. Source: Sandwell and Smith [15]. Contour interval 10 mGal. White dots indicate trace of the 85ºE ridge. A-N seamount D Afanasy-Nikitin seamount.
the P unconformity and a younger phase marking the post-collision depositional activity related to the erosion of Himalaya and deposition of the Bengal fan sediments [6]. The line sections were used to constrain the gravity modeling to the basement level.
3. Analysis and interpretation 3.1. 83ºE Ridge — extension and structure The satellite-derived gravity anomaly map (Fig. 2) brings out a negative anomaly associated with the
ridge quite prominently between 5º and 15ºN. The peak amplitude of about 80 mGal occurs at about 14ºN where the anomaly is also at its widest. Progressing towards the north this negative trend seems to merge with the shelf edge anomaly along the ECMI, but the connectivity is reflected in the form of a southward ‘nosing’ of gravity contours. The present gravity map indicates the 85ºE ridge abutting the east coast of India at about 18º–19ºN. Further extension of the ridge parallel to the coast is not visible in the gravity contours. To establish that the negative gravity anomaly is attributable to the presence of the ridge and its topographic relief, single
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Fig. 3. Gravity anomaly map of the Bay of Bengal from ship-borne gravity data. Contour interval 10 mGal. MAN01 and MAN03 are multichannel seismic profiles (Fig. 4) [9,16] along which gravity modeling was carried out (Fig. 6).
channel analog seismic records from the Bay of Bengal were examined in the longitudinal range of 84.5º–86ºE. These sections stacked in a latitudinal manner are shown in Fig. 5. In most sections the relief of the ridge could be easily traced. However, in a few cases, the presence of the ridge could only be indirectly inferred from the updip of the sediments probably underlain by the ridge (e.g. section at 10ºN in Fig. 5). The ridge could be traced from 8ºN to 17º–18ºN, conforming with the negative gravity trend and it is at its widest at 14.7ºN, correlating well with the peak negative gravity amplitudes observed in this region. At about 5ºN, the gravity contours take a curvilinear trend and appear to merge with the E–W-trending gravity highs and lows of the deformed blocks of the central Indian Ocean [17]. At these latitudes the connectivity of the ridge structure can be established through bathymetry. The Afanasy-Nikitin seamount
has been linked to the 85ºE ridge by several earlier workers, e.g. [6]. However, what is interesting to note is that this seamount is characterized by a strong positive gravity anomaly, quite unlike the negative anomaly associated with the 85ºE ridge further north. It appears natural to link the two structures mainly because of their topographic relief, the northern segment buried under thick piles of sediments while the southern part is exposed as a strong bathymetric feature in the central Indian Ocean. This contrasting tectonic setting could be the reason for the change in the gravity anomaly, from negative in the north to positive in the south. It should be mentioned here that Liu et al. [18] have interpreted the negative anomaly of the 85ºE ridge in terms of a two-stage deformation of the Bay of Bengal lithosphere, the first at the time of loading of the ridge over a weak and young BoB lithosphere, and the other, caused by the load of sediments over a lithosphere gaining
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Fig. 4. Line sections of two multichannel seismic sections MAN01 and MAN03. Locations shown in Fig. 3.
strength with time. Further, Ramana et al. [13] have established a positive magnetic anomaly trend for the entire length of the 85ºE ridge including the northern segment of the Afanasy-Nikitin seamount. Two multichannel seismic sections [9,16] across the Bay of Bengal bring out the relief of the 85ºE ridge and the distribution of the overlying sedimentary sequences in a clear manner. Line drawings of the two sections are shown in Fig. 4. The ridge has a steeply dipping western flank while the eastern flank has gentle slopes. Further, the ridge appears to be wider in the northern sections (MAN 01, Fig. 4a) and associated with a trough-like feature in the middle. The overall relief of the ridge appears to exceed 2 km at some places. Ship-borne gravity anomalies (Fig. 3) were used to interpret the negative anomaly associated with the 85ºE ridge as well as those over the BoB in general. This is done along the two multichannel seismic sections MAN01 and MAN03, which give the advantage of constraining the model with information from the seafloor to the basement. The inferred models for the ridge and the adjacent BoB ocean floor are shown in Fig. 6. In this exercise the
free air anomaly was first corrected for water depth and sediment thickness and a crust of uniform thickness (6 km). Further, it is assumed that the relief of the 85ºE ridge above the oceanic basement is isostatically compensated by crustal roots, an assumption supported by the inference [18] that the ridge is emplaced over a weak and young BoB lithosphere (and hence compensated by crustal thickening). Based on this, crustal roots beneath the ridge were estimated from the relief of the ridge (that above the basement level). Simple forward-modelling methods were used to obtain the combined effect of all the layers. For profile MAN01 it was noticed that the roots beneath the ridge extend to considerable depths exceeding 30 km, a situation that is abnormally high for an oceanic crustal environment and hence the root density was increased from an initial 2.9 g=cm3 to a 3.1 g=cm3 . For the MAN03 profile this requirement was not necessary because the calculated crustal thickness beneath the ridge at 18 km does not appear to be large. Over the adjacent parts, along both the profiles, there seem to be no large-scale variations either in crustal thickness or density, the only exception being the segment west of the 85ºE ridge in MAN01
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where the crust initially appears to thin out followed by thickening as we move towards the coast. This is the region where the Krishna–Godavari delta progrades into the sea and the somewhat thicker crust may represent the region of initial rifting followed by crustal extension and stretching. 3.2. The eastern continental margin of India Plate reconstructions for the periods prior to the breakup of eastern Gondwanaland show that the east coast of India is juxtaposed against the Enderbyland of East Antarctica [1,2,4,10]. Magnetic lineations of Mesozoic age were recently inferred for the Bay of Bengal ocean floor while for its conjugate part, the Enderby basin, such information is still awaited. Identified magnetic lineations in the central and northern parts of the Bay of Bengal [8] are found to be aligned parallel to the east coast of India, particularly north of 13ºN, suggesting a pullapart nature of the margin. South of this latitude, no magnetic lineations close to the east coast of India have been identified. To examine the nature of the gravity anomalies characterizing the ECMI, an enlarged satellite-derived gravity map of the ECMI is prepared and shown in Fig. 7, from which it is clear that gravity anomalies display a contrasting style north and south of 14.5ºN. Towards the north, where the spreading-type magnetic lineations were inferred and where the Krishna–Godavari basin (see Fig. 7) has developed into a prograded delta, the contours are well-distributed with a well-developed shelf edge anomaly and large negative gravity anomalies characterizing the depressions in the offshore delta. South of 14.5ºN, the gravity contours are tightly packed, displaying narrow bands of gravity highs and lows running parallel to the coast in a N–S direction. The almost linear band of negative anomalies is disrupted by a large localized ENE– WSW-trending positive gravity anomaly associated with the Karaikal ridge in the Cauvery offshore basin [3]. Bathymetry (Fig. 1b) of the ECMI shows good correspondence with gravity, characterized by a narrow shelf and steep slopes south of 14.5ºN, widening gradually further north. To demonstrate this, stacked bathymetric profiles across the margin are shown in Fig. 8. Profiles to the south display steep slopes characteristic of transform margins [10,19] while
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further north they display gentle slopes characteristic of pull-apart type of margins. The contrast in the two segments of the ECMI is also considerably displayed in the structural style and tectonism of the sedimentary basins that have developed along the margin from north to south. The seismic stratigraphic sections in the Krishna–Godavari basin to the north indicate deposition of sediments along a prograding deltaic front with gentle slopes [3]. Quite in contrast, sediments in the Cauvery basin (onshore only, no offshore seismic sections are available) seem to be confined to narrow and deep troughs. Also, structures (ridges and depressions) in the K-G and Mahanadi basins have trends parallel to the coast, while those in the Cauvery basin seem to be trending somewhat oblique to the coastline [3]. Thus, the satellite-derived gravity data, supported by bathymetry and seismic stratigraphic sections, suggest that the ECMI can be broadly divided into a northern pullapart segment and a southern sheared or transform segment.
4. Discussion and conclusions The origin, structure and extension of the 85ºE ridge, particularly its northward extension, seem to have gained importance as is evident from recent studies based on a variety of data sets, e.g. [13]. The trace of the ridge towards south, on the basis of the present study as well as those earlier, appears to take a curvilinear trend around Sri Lanka and to merge with the Afanasy-Nikitin seamount in the central Indian Ocean. This is clear from both the satellite-derived gravity anomalies and bathymetry, and from seismic reflection surveys [11]. The only contrast seems to be in the switch-over from a negative gravity anomaly in the north, where the ridge is buried under thick sediments, to a positive anomaly in the south, where the volcanic ridge is directly exposed over the seafloor. The extension of the ridge towards the north on land and its connectivity with the Rajmahal Traps is more difficult to establish. Occurrences of volcanic rocks were reported from both the Mahanadi offshore basin [20] and the Bengal basin [21,22]. The Rajmahal and Sylhet Traps, dated at 117 Ma, are known to extend eastward beneath the sediments of
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Fig. 5. Analog single-channel seismic sections across the 85ºE ridge from 8ºN in the south, east of Sri Lanka, to 18ºN near the Mahanadi basin in the north along the east coast of India. Where the ridge could not be marked directly on the section, updipping sedimentary sequences are taken as indicators for the existence of the ridge. See text for explanation.
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Fig. 5 (continued).
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Fig. 6. Gravity interpretation of the structures along the MAN01 and MAN03 seismic lines across the Bay of Bengal. Note the inferred crustal underplating beneath the 85ºE ridge in MAN01 and its absence in MAN03. Crustal thinning between the east coast of India and the 85ºE ridge can also be noticed. Figures in brackets denote densities in gm=cm3 .
the Bengal basin. A deep onland well in the Cuttack depression near Cuttack in the Mahanadi basin brings out two volcanic layers, L1 and L2, both belonging to the Early Cretaceous but separated by about 700-m-thick sediments [23]. Ages of 107 Ma and 114–116 Ma are assigned to layers L1 and L2, respectively. In addition to these, a dyke intrusion near Cuttack yields a K–Ar age of 111 Ma [24]. Further, lamprophyres were reported to have K–Ar ages between 105 and 121 Ma [25] and a Rb–Sr age of 117C2 Ma [26] southwest of the Rajmahal Traps. These data indicate large-scale volcanic occurrences in eastern India, both exposed (Rahmahal and Sylhet Traps and others) and buried under sediments (Mahanadi and Bengal basins) and extending into the deep-sea to the Afanasy-Nikitin seamount, all of which seem to be closely related to 85ºE ridge vol-
canism. Fig. 1a reflects the large volcanic province in eastern India envisaged earlier [21,22], with a closely related volcanic hotspot trace in the form of the 85ºE ridge extending into the deep ocean. This scenario is quite similar to the Deccan volcanic province in western India and the Reunion hotspot trace in the form of the Chagos–Laccadive ridge [27]. The age progression from 117 Ma for the Rajmahal Traps to the Cretaceous age of the Afanasy-Nikitin seamount[28] is clear. In the Mahanadi basin the age of 114–116 Ma for layer L2 (older volcanics) is probably related to the Rajmahal Trap volcanism while the younger age of 107 Ma for layer L1 (younger volcanics) relates to that of the hotspot trace. This suggests the initiation of the emplacement of the 85ºE Ridge in the Bay of Bengal at about 107 Ma at 20ºN in the Mahanadi basin and continuing to Creta-
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Fig. 7. Satellite-derived free air gravity contours along the Eastern Continental Margin of India (ECMI). Contour interval 10 mGal. Note the tightly packed narrow band of gravity contours over the southern sheared segment contrasting with the well spread-out gravity contours of the rifted segment in the north.
ceous times at equatorial latitudes (Afanasy-Nikitin seamount) in the central Indian Ocean. Considering that the hotspot trace traversed a distance of about 2200 km (¾20º of latitude) in a N–S direction in a span of about 40 m.y. a spreading rate of about 5.5 cm=yr for the ocean floor of the BoB and northern central Indian Ocean can be inferred. This is close to the average of the rates of 3.4 to 10.0 cm=yr predicted for the Mesozoic anomalies in the Bay of Bengal [8] and is almost the same as the rates of 5 cm=yr predicted for the period 70–80 Ma for the central Indian Ocean [2].
Spreading magnetic lineations in the Bay of Bengal have a NE–SW orientation, particularly in the northern and central parts. With the 85ºE ridge having a N–S orientation in the BoB, somewhat oblique to the direction of spreading, a tectonic setting in which the emplacement of the ridge on a young, but already evolved ocean floor can be inferred. Gravity modelling favors the emplacement of the ridge on a young oceanic lithosphere (¾15 Ma old) [18]. Similarly, the evolution of the Afanasy-Nikitin seamount at a ridge–transform intersection has been proposed on the basis of gravity–bathymetry correlations [28].
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Fig. 8. Bathymetry profiles to reflect the contrast between the sheared and rifted segments of the ECMI. Inset shows the locations of the profiles.
Emplacement of the ridge due to outpouring of volcanic material along a crack in the ocean floor has been suggested, but the curvilinear trend of the ridge east and south of Sri Lanka poses problems as it is difficult to understand the tectonics leading to the development of such cracks on the ocean floor with curvilinear trends. The present study favours the hotspot model for the origin of the 85ºE. To evolve a more complete picture of the early breakup history between India and East Antarctica and the plume tectonism in the northern Indian Ocean, there is need for identification of seafloor spreading-type magnetic lineations of probable Mesozoic age from the conjugate Enderby basin, together with plate reconstruction models for the Mesozoic period. The crustal structure beneath the 85ºE ridge inferred in the present study suggests development of
thick roots beneath the ridge, probably indicating a crustal underplating process. This interpretation leans heavily on the results of seismic surveys over hotspot traces like the Marquesas Islands in the Pacific Ocean and also the gravity modeling of the Ninetyeast ridge in the Indian Ocean where thick and high-density roots were inferred [29,30]. Inference of thick roots beneath volcanic hotspot traces is not uncommon. For the Chagos–Laccadive ridge in the northwestern Indian Ocean thick roots beneath the central parts were inferred on the basis of admittance analysis of gravity–bathymetry profiles [27]. Similarly, seismic surveys reveal thick crustal roots beneath the Hawaiian seamount chain and the Kerguelen plateau [31,32]. Nur and Ben-Avraham [33] show crustal models for several oceanic plateaus underlain by thick crustal roots. The underplating
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Fig. 9. Schematic diagram of India–East Antarctica plate reconstruction [37].
process seems to be resulting from massive intrusions related to hotspot volcanism [29]. The Eastern Continental Margin of India can be segmented into two blocks, a transform margin in the south and a pull-apart one further north, based on gravity and bathymetry data. This implies an unusual spreading pattern for the southern segment of the ECMI which, with our present knowledge of the plate kinematics of the region, is somewhat difficult to understand. In some plate reconstructions this segment is shown as a system of closely spaced transforms and ridge segments, e.g. [1,4]. The transform nature of some segments of passive margins was inferred for the Ghana coast in northwestern Africa, Grand Banks margin off the eastern Canadian margin and several others [34]. These sheared or transform margins appear to be invariably connected to fracture zones in the deep ocean. Admittance analysis of gravity and bathymetry data seems to bring out the contrast between sheared margins and rifted or pullapart margins [35]. These results suggest that sheared or transform margins may be isostatically compensated in a local manner compared to the rifted margins compensated regionally. For a more complete interpretation there is need for information on the seismic crustal structure of this segment of the ECMI.
The tectonic setting of the ECMI emerges more clearly with East Antarctica juxtaposed against the east coast of India [1]. Disposition of the 2000-m isobath and evidence from existing seismic profiles have helped in demarcating the Continent–Ocean Boundary (COB) for East Antarctica and the conjugate east coast of India and Sri Lanka. The southern transform segment of the ECMI, termed ‘Coromandel coast’, displays a remarkably linear bathymetry, similar to that of the Krons Prins Olav Kyst region of East Antarctica. The conjugate nature of East Antarctica with respect to the ECMI is further substantiated with the presence of alkalic igneous intrusives (110 Ma) in the Prince Charles Mountains adjacent to the Lambert graben [36,37], similar to the Layer L1 volcanic rocks found in the Mahanadi basin of India. This brings into close alignment the Mahanadi and the Lambert grabens in a pre-breakup tectonic setting of eastern Gondwanaland (Fig. 9).
Acknowledgements We thank the Director, National Geophysical Research Institute, for encouragement and guidance, and his kind permission to publish this paper. We
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thank Prof. J.R. Curray and an anonymous reviewer for their critical reviews of the manuscript. We are grateful to Prof. Curray for a correction on the age of the A-N seamount and also for his helpful comments. CS acknowledges many fruitful discussions with M. Radha Krishna and Anil Kumar. [MK]
[14]
[15]
References [1] C.McA., Powell, S.R. Roots, J.J. Veevers, Pre-breakup continental extension in East Gondwanaland and the early opening of the eastern Indian Ocean, Tectonophysics 155 (1988), 261–283. [2] J.Y. Royer, D.T. Sandwell, Evolution of the eastern Indian Ocean since the Late Cretaceous: constraints from Geosat altimetry, J. Geophys. Res. 94 (1989) 13755–13782. [3] V.V. Sastri, B.S. Venkatachala, V. Narayanan, The evolution of the east coast of India, Palaeogeogr., Palaeoclimatol., Palaeoecol. 36 (1981) 23–54. [4] J.R. Curray, F.J. Emmel, D.G. Moore, R.W. Raitt, Structure, tectonics, and geological history of the northeastern Indian ocean, in: A.E.M. Nairn, F.G. Stehli (Eds.), The Ocean Basins and Margins, Plenum, New York, 1982, pp. 399– 450. [5] M. Mukhopadhyay, M.R. Krishna, Gravity field and deep structure of the Bengal fan and its surrounding continental margins, northeast Indian Ocean, Tectonophysics 186 (1991) 365–386. [6] J.R. Curray, Possible greenschist metamorphism at the base of a 22-km sedimentary section, Bay of Bengal, Geology 19 (1991) 1097–1100. [7] J.N. Brune, J.R. Curray, L. Dorman, R. Raitt, A proposed super-thick sedimentary basin, Bay of Bengal, Geophys. Res. Lett. 19 (1992) 565–568. [8] M.V. Ramana, R.R. Nair, K.V.L.N.S. Sarma, T. Ramprasad, K.S. Krishna, V. Subrahmanyam, M. D’Cruz, C. Subrahmanyam, J. Paul, A.S. Subrahmanyam, D.V. Chandra Sekhar, Mesozoic anomalies in the Bay of Bengal, Earth Planet. Sci. Lett. 121 (1994) 469–475. [9] D. Gopala Rao, K.S. Krishna, D. Sar, Crustal evolution and sedimentation history of the Bay of Bengal since the Cretaceous, J. Geophys. Res. 102 (1997) 17747–17768. [10] L.A. Lawver, J.Y. Royer, D.T. Sandwell, C.R. Scotese, Crustal development: Gondwana breakup, in: M.R.A. Thomson, J.A. Crame, J.W. Thomson (Eds.), Geological Evolution of Antarctica, Cambridge Univ. Press, Cambridge, 1991, pp. 533–539. [11] J.R. Curray, T. Munasinghe, Origin of the Rajmahal Traps and the 85ºE ridge: preliminary reconstructions of the trace of the Crozet hotspot, Geology 19 (1991) 1237–1240. [12] R.D. Mueller, J.Y. Royer, L.A. Lawver, Revised plate motions relative to the hotspots from combined Atlantic and Indian ocean hotspot tracks, Geology 21 (1993) 275–278. [13] M.V. Ramana, V. Subrahmanyam, A.K. Chaubey, T. Ramprasad, K.V.L.N.S. Sarma, K.S. Krishna, M. Desa, G.P.S.
[16]
[17]
[18]
[19] [20]
[21]
[22]
[23]
[24] [25]
[26]
[27]
[28]
[29]
[30] [31]
Murty, Structure and origin of the 85ºE ridge, J. Geophys. Res. 102 (1997) 17995–18012. A.S. Subrahmanyam, K.S.R. Murthy, S. Lakshminarayana, M.M. Malleswara Rao, K. Venkateswarlu, T.C.S. Rao, Magnetic expression of some major lineaments and Cretaceous quiet zone in the Bay of Bengal, Geo-Mar. Lett. 17 (1997) 202–206. D.T. Sandwell, W.H.F. Smith, Marine gravity anomaly from Geosat and ERS 1 satellite altimetry, J. Geophys. Res. 102 (1997) 10039–10054. M.L. Pateria, M.K. Rangaraju, V. Raiverman, A note on the structure and stratigraphy of Bay of Bengal sediments, Geol. Surv. India Spec. Publ. 29 (1992) 21–23. C.A. Stein, S. Cloetingh, M. Wortel, SEASAT-derived gravity constraints on stress and deformation in the northeastern Indian Ocean, Geophys. Res. Lett. 16 (1989) 823–826. C. Liu, D.T. Sandwell, J.R. Curray, The negative gravity field over the 85ºE ridge, J. Geophys. Res. 87 (1982) 7673– 7686. J.P. Kennet, Marine Geology, Prentice-Hall, Englewood Cliffs, NJ. R.C. Fuloria, Geology and hydrocarbon prospects of Mahanadi basin, India, Proc. 2nd Sem. Petroliferous Basins of India 1 (1993) 355–369. A.K. Baksi, T.R. Barmann, D.K. Paul, E. Farrar, Widespread early Cretaceous flood basalt volcanism in eastern India: geochemical data from Rajmahal–Bengal–Sylhet Traps, Chem. Geol. 63 (1987) 133–141. A.K. Baksi, Petrogenesis and timing of volcanism in the Rajmahal Flood Basalt Province, northeastern India, Chem. Geol. 121 (1995) 73–90. H.C. Tewari, The effect of thin high velocity layers on seismic refraction data: an example from Mahanadi basin, India, Pure Appl. Geophys. 151 (1998) 63–79. J.K. Agarwal, Rama, Chronology of Mesozoic volcanics of India, Proc. Ind. Acad. Sci. 84A (1976) 157–179. Sarkar et al., Lamprophyres from Indian Gondwanas — K–Ar ages and chemistry, J. Geol. Soc. India 21 (1980) 188–193. A. Kumar, A.M. Dayal, V.M. Padma Kumari, K. Gopalan, Rb–Sr age of Jharia Lamprophyre: part of the Kerguelen=Crozet hotspot trace? (in prep.). B. Ashalatha, C. Subrahmanyam, R.N. Singh, Origin and compensation of Chagos–Laccadive ridge, Indian ocean, from admittance analysis of gravity and bathymetry data, Earth Planet. Sci. Lett. 105 (1991) 47–54. J. Paul, R.N. Singh, C. Subrahmanyam, R.K. Drolia, Emplacement of Afanasy-Nikitin seamount based on transfer function analysis of gravity and bathymetry data, Earth Planet. Sci. Lett. 96 (1990) 419–426. D.W. Caress, M.K. McNutt, R.S. Detrick, J.C. Mutter, Seismic imaging of hotspot-related crustal underplating beneath the Marquesas Islands, Nature 373 (1995) 600–603. C. Bowin, Origin of the Ninety East Ridge from studies near the equator, J. Geophys. Res. 78 (1973) 6029–6043. A.B. Watts, U.S. ten Brink, P. Buhl, T.M. Brocher, A multichannel seismic study of lithospheric flexure across
C. Subrahmanyam et al. / Earth and Planetary Science Letters 171 (1999) 237–251 the Hawaiian-Emperor seamount chain, Nature 315 (1985) 105–111. [32] M. Recq, D. Brefort, J. Malod, J. Veinante, The Kerguelen Isles (southern Indian ocean): new results on deep structure from refraction profiles, Tectonophysics 182 (1990) 227– 248. [33] A. Nur, Z. Ben-Avraham, Oceanic plateaus, the fragmentation of continents and mountain building, J. Geophys. Res. 87 (1982) 3644–3661. [34] R.A. Scrutton, Continental breakup and deep crustal structure at the margins of southern Africa, An. Acad. Bras.
251
Cienc. 48 (1976) 275–286. [35] J. Verhoeff, H.R. Jackson, Admittance signatures of rifted and transform margins: examples from eastern Canada, Geophys. J. Int. 105 (1991) 229–239. [36] J.W. Sheraton, Geochemistry of mafic igneous rocks of the northern Prince Charles Mountains, Antarctica, J. Geol. Soc. Aust. 30 (1983) 295–304. [37] A. Cooper, H. Stagg, E. Geist, Seismic stratigraphy and structure of Prydz Bay, Antarctica: implications from Leg 119 drilling, Proc. ODP 119 (1991) 5–25.