Structure and tectonics of the southwestern continental margin of India

TECTONOPHYSICS ELSEVIER

Tectonophysics249 (1995) 267-282

Structure and tectonics of the southwestern continental margin of India V. Subrahmanyam a,., D. Gopala Rao a, M.V. Ramana a, K.S. Krishna G.P.S. Murty ", M. Gangadhara Rao b

a

a National Institute of Oceanography, Dona Paula-403004, Goa, India b Andhra University, Visakhapatnam-530003, India

Received 15 February 1994;accepted 9 February 1995

Abstract

A NNW-SSE-trending graben in the inner shelf (40-60 m isobaths), a mid-shelf basement ridge (60-150 m isobaths), a shelf-margin basin (150-1000 m isobaths) and the Prathap Ridge Complex in the slope (1000-2000 m isobaths) extend as linear features along the southwestern continental margin of India between 12°20'N and 15°00'N. The basic structural framework of the margin seems to be influenced by the pre-existing Precambrian structural fabric of the continent. The mid-shelf basement ridge and the Prathap Ridge Complex are interpreted as manifestation of the rift-phase volcanism when India separated from Madagascar around 84 Ma.

1. Introduction

The Arabian Sea evolved as a result of seafloor spreading following the intracontinental break-up of India, Madagascar and the Seychelles (McKenzie and Sclater, 1971; Whitmarsh, 1974; Norton and Sclater, 1979; Naini and Talwani, 1982; Chaubey et al., 1993; Bhattacharya et al., 1994). The Laxmi Ridge and the Chagos-Laccadive Ridge system (Fig. la) form approximately the boundary between the western and eastern basins of the eastern Arabian Sea (Naini, 1980). The eastern basin consists of the Prathap Ridge Complex, the mid-shelf basement ridge and various sediment-filled basins (Fig. lb). Magnetic anomalies relating to seafloor spreading have recently been identified in the Laxmi Basin, * Corresponding author.

east of the Laxmi Ridge (Bhattacharya et al., 1994). Naini (1980) has traced the northward extension of the Prathap Ridge Complex up to 17°N. The Chagos-Laccadive Ridge system and the Prathap Ridge Complex seem to join at this latitude. The western continental margin of India is interpreted as having been rifted along the dominant pre-existing N N W SSE and N W - S E Precambrian basement tectonic trends (Biswas, 1987). Subrahmanyam et al. (1989, 1991, 1993a, b, 1994) have delineated structural features such as the Prathap Ridge Complex, the shelf-margin basin, the mid-shelf basement ridge and the inner-shelf graben within water depths of 30-2000 m on the southwestern continental margin of India. Though the presence of several structural features had been inferred on the southwestern margin of India, the regional extent of the Prathap Ridge Complex and the mid-shelf base-

0040-1951/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI 0040- 195 1(95)00020-8

V. Subrahnzanyam et al. / Tectonophysics 249 (1995) 267-282

268

m e n t r i d g e and their origin had not b e e n traced nor e x p l a i n e d c o n c l u s i v e l y . Earlier the m i d - s h e l f b a s e -

( S u b r a h m a n y a m et al., 1993a) largely b e c a u s e o f its p r o x i m i t y to o t h e r P r e c a m b r i a n b a s e m e n t structures on the w e s t e r n shelf. K r i s h n a et al. ( 1 9 9 2 , 1 9 9 4 )

m e n t r i d g e had b e e n a s s i g n e d a P r e c a m b r i a n age

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Fig. 1. (a) (Jeneralised map of the Arabian Sea, showing magnetic lineations and main structural features (compiled from Naini and Talwani, 1982; Schlich, 1982; Chaubey et al., 1993; Ramana et al., 1993). 1 = Mid-Ocean ridge crest; 2 = fracture zones; 3 = magnetic lineations; SMP = Seychelles-Mascarene Plateau. (b) Location of the study area. Simplified bathymetry of the adjacent areas, structural features of the western continental margin (Ramaswamy and Rao, 1980; Naini and Talwani, 1982; Subrahmanyam et al., 1993a,b,1994) and tectonics of adjoining landmass (Ray, 1977). 1 = Chagos-Laccadive Ridge system; 11 = Prathap Ridge Complex; 111 = major and minor faults; IV = mid-shelf basement ridge.

269

V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

suggested that the Prathap Ridge Complex and the mid-shelf basement ridge formed when the Indian plate moved northward on the Reunion hotspot during the early Tertiary.

In the present study we have reexamined and synthesized all available offshore geophysical data for the area between Marmagao and Kasaragod within water depths of 3 0 - 2 0 0 0 m (Fig. 2) and have at~0 I

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Area of Eastern Ghats folding (Ca. 2 6 0 0 m y )

Fig. 2. Map showing the cruise tracks and bathymetryof the study area. Thin lines with dots indicatethe total magneticintensity traverses, thick lines with dots indicate gravity traverses. Thick solid lines SK-A, SK-B, RM-01 are multichannelseismic reflection traverses and RVG-A is a shallow seismic reflection (sparker) traverse.

270

V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

tempted to explain the origin of mid-shelf basement ridge and the Prathap Ridge Complex in the context of the separation of India from Madagascar.

of gravity and 600 line km of multichannel seismic reflection data (Fig. 2) collected aboard the R.V.s Gaveshani, Sagar Kanya, Farnella and Skandi Surveyor. Acquisition details may be found in Subrahmanyam (1992) and Subrahmanyam et al. (1991). The magnetic anomaly profiles (Fig. 3), residual contour map (Fig. 4), gravity anomaly maps (Fig. 5a, b) and seismic reflection records (Fig. 6) were used for interpretation. In addition, these interpretations

2. Data The geophysical data comprise 7944 line km of bathymetric, 5744 line km of magnetic, 2200 line km ,~" E

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Fig. 3. Total magnetic intensity anomaly profiles.

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V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

are supplemented by seismic refraction records (Rao, 1976), well-log data (Ramaswamy and Rao, 1980) and dredged sample results (Siddiquie et al., 1987).

characterized by an even topography between water depths of 20 and 100 m with a gentle gradient of 1:870. The shelf break with a steep gradient of 1:20 coincides with the 200 m depth contour. Beyond the shelf break, the area is characterized by a number of localized shoals. There are two shoals between Karwar and Honavar in water depths of 800-1000 m with a relief of about 400 m. A system of shoals with

3. Results The bathymetry varies from 20 m in the nearshore to 2000 m in the offshore (Fig. 2). The seabed is

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V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

273

Fig. 6. Multichannel seismic reflection records SK-A, SK-B, RM-OI and shallow seismic reflection record RVG-A. Location of the traverses shown in Fig. 2.

V. Subrahmanyam el al. / Tectonol~hysics 249 (1995) 267-282

274

mid-shelf basement ridge, the shelf-margin basin and the Prathap Ridge Complex (Subrahmanyam ct al., 1993a, 1994) are seen continuing as regional features all along the shelf in the study area. The free-air gravity anomaly pattern (Fig. 5) shows the same trends as the magnetic anomaly map. NNW-SSE-trending structural features with sediment thickness varying from 0.5 to 4.0 km are deduced from model studies (Subrahmanyam et al., 1991). A maximum of thickness about 4.0 km of sediments is estimated in the region between 150 and

a relief of about 300 m from a 1000 m water depth is also observed beyond the slope between Malpe and New Mangalore (Fig. 2). These shoals are approximately aligned in a N N W - S S E direction and thought to be linear IEatures. NNW-SSE, N W - S E , E N E - W S W and E - W magnetic trends (Figs. 3 and 4) dominate the southwestern continental margin. The depth to the magnetic basement (Fig. 7) varies between 0.5 and 6.0 km (Subrahmanyam, 1992). The previously identified features like a graben in the inner shelf, the

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Fig. 7. Map showing the depth to the top of the basement in kilometers (deduced fronl magnetic data).

V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

1000 m water depth. The crust-mantle boundary was computed to be at a depth of around 30 km in the nearshore region and gradually shallows further offshore (Subrahmanyam, 1992). Seismic reflection records (near-trace monitor record, Fig. 6) show the presence of a paleo-shelf

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with the progradation of sediments forming presentday shelf. A positive topographic feature is noticed beyond the shelf break in profiles SK-A and RVG-A, while this topographic high appears as a subsurtace feature in profiles SK-B and RM-01. The feature is N N W - S S E trending and is interpreted as a linear

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Fig. 8. Map showing the structural features in the study area as inferred from (a) magnetic data, (b) gravity data and (c) seismic data; (d) composite map.

276

V. Subrahmanyam el all/Tecmnot~hysics 249 (1995) 2 6 7 - 2 8 2

feature by Naini and Talwani (1982) who named it the Prathap Ridge Complex (Fig. lb). In places the ridge may have three peaks. A pile of sediments, about 4 km thick, fills a basin-like depression between the Prathap Ridge Complex and the paleoshell. This basin seems to be the continuation of the shelf-margin basin earlier indentified in the north (Ramaswamy and Rao, 1980). Farther west, another basin with a 2.3-km-thick pile of sediments is also noticed between the Prathap Ridge Complex and the Chagos-Laccadive Ridge system. The inner-shelf graben, mid-shelf basement ridge, shelf-margin basin and the Prathap Ridge Complex (Fig. 8a, b and c) have been interpreted independently from gravity, magnetic and seismic reflection data (Subrahmanyam, 1992). The Prathap Ridge is reflected well on the seismic records. The ridge stands out as a discrete feature on the different profiles (Fig. 8c). The location of the mid-shelf basement ridge inferred from the magnetic data matches well with the location of the basement high on the seismic records (Fig. 8c). Its western flank

72 °

74 °

T6 °

mergers with the paleo-shelf, however, the eastern boundary of the ridge could not be mapped on the shelf due to inadequate data. The western boundary of mid-shelf basement ridge (basement high) towards the offshore is also evident as a discrete feature in Fig. 8c. In this paper we show, using gravity, magnetic and seismic data (Fig. 8d), that these features extend linearly parallel to the southwestern continental margin of India.

4. Discussion 4.1. Tectonics

The composite map (Fig. 8d) depicts the grabenlike feature in the inner shelf (40-60 m isobaths), the mid-shelf basement ridge (60-150 m isobaths), the shelf-margin basin (150-1000 m isobaths) and the Prathap Ridge Complex (1000-2000 m isobaths). The integrated analysis of all the geophysical data clearly indicates that the Prathap Ridge Complex and

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Fig. 9. (a) Map showing the extension of the Prathap Ridge Complex and the mid-shelf basement ridge up to the southern tip of the Indian peninsula. (b) Gravity, magnetic and seismic signatures over the southern part of the Prathap Ridge Complex. Source of the geophysical data: Naini and Talwani (1982); Gopala Raoet al. (1987).

V. Subrahmanyamet al. / Tectonophysics 249 (1995) 267-282 mid-shelf basement ridge extend parallel to the southwest coast, right to the southern tip of the Indian peninsula (Fig. 9a). Though the Prathap Ridge Complex and mid-shelf basement ridge are seen as continuous on a regional scale in the study area, they are offset along pre-existing ENE-WSW-trending Precambrian faults. A recent study by Subrahmanyam et al. (1993b) suggested that some of these faults were reactivated with the commencement of seafloor spreading at the Cartsberg Ridge.

L a t e Cretaceous (,~- 8 4

277

4.2. Origin of the Prathap Ridge and the mid-she!f basement ridge The break-up of eastern Gondwanaland and the dispersion of the continents since the Early Cretaceous resulted in the formation of various features, including the Indian Ocean, several aseismic ridges, and plateaus. The initial juxtaposition of India and Madagascar was traced roughly along the 1000 m isobath (Katz and Premoli, 1979), and around 84 Ma

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65 Ma)

(Rift ReLated Ridges,"~,84Ma)

SMB- SheLf Margin Basin Fig. ]0. Schematic mode] showing the various stages of development of the southwestern continental margin of India li'om the Late Cretaceous to Present.

278

V. Subrahmanyam et al. / Tectomq~hysics 249 (I 995) 2 6 7 - 2 8 2

India separated from Madagascar (Norton and Sclater, 1979; Agarwal et al., 1992) to begin its northward journey. The initiation of seafloor spreading around 84 Ma might have induced a rift phase of volcanism between India and Madagascar, as is evident from the St. Mary volcanic islands (Valsangkar ct al., 1981) on the western shelf, the extensive volcanism (dykes and lava flows) along eastern Madagascar (Norton and Sclater, 1979), and the volcanic banks off the eastern margin of Madagascar (Admiralty charts nos. 758, 759 and 760, published at the Admiralty, London, 1966). Kennett (1982) summarized that rifting in general may be preceded by a period of regional domal uplift and possibly simultaneous volcanism and that the uplifts are pre-

sumed to be of thermal origin. Subsequent erosion of the uplifted area and thermal metamorphism of the lower crust may cause crustal thinning. As the area of uplift expands, the crust breaks to form a subparallel fault pattern. The magnetic m a g m a most probably intruded along these faults. A schematic model (Fig. 10) representing the various stages of development of the southwestern continental margin of India from the Late Cretaceous to Present is proposed based on the above-mentioned geodynamic processes. The lbrmation of the Prathap Ridge Complex and the mid-shelf basement ridge seems to be manifestations of these rifting processes. Magnetic model studies reveal that these ridges are comprised of mafic rocks (Subrahmanyam et al., 1989,1993a,

(a) (b)

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Fig. 1I. (a) The traces of the Marion, Reunion and Kerguelen hotspots with ages (Courtillot et al., 1986; Beckman et al., 1988). Dots with numbers indicate ODP sites. C L R = Chagos-Laccadive Ridge; NR = Ninety-east Ridge; LR = Laxmi Ridge; B H = basement high; PR = Prathap Ridge; M B R = mid-shelf basement ridge. (b) Hotspot traces and present-day distribution of aseismic ridges in the Indian and South Atlantic oceans (Emerick, 1985). R = Reunion; C = Comores; K = Kerguelen; B = Bouvet; M = Marion; T = Tristan hotspots.

V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

1994). We are therefore of the view that the N N W SSE-trending mid-shelf basement ridge and the Prathap Ridge Complex may have evolved parallel to the southwest coast of India during the initial break-up of India from Madagascar. The trend of

these features strangely coincides with reported Precambrian trends suggesting that the alignment of these structures was influenced by the pre-existing structural fabric of the continent as mentioned by Kennett (1982).

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Fig. 12. Map showing the Chagos-Laccadive Ridge system, Prathap Ridge Complex, mid-shelf basement ridge and the trace of the Reunion hotspot.

280

V. Subrahmanyam et al. / Tectomq~hysics 249 (1995) 267 282

On the basis of above inferences, the spreading axis between India and Madagascar should lie west of the Prathap Ridge Complex, i.e., in the oceanic crust. The interpretation of the gravity data across the southwestern continental margin of India (Subrahmanyam, 1992) shows the presence of thick (around 30 kin) continental crust in the nearshore areas which gradually thins out west of Prathap Ridge Complex and farther offshore, perhaps indicating a transition from continental to oceanic crust. The oldest oceanic crust that lies west of the Prathap Ridge thus may be younger than 84 Ma and closer to Anomaly 28 (or 68 Ma) crust as the separation of India and Madagascar seems to have taken place in more than one episode as evidenced by ridge jumps around Anomaly 28 time on the eastern side of Madagascar (Schlich, 1982). The Mascarene-Chagos-Laccadive volcanic lineament is a major aseismic ridge system that connects the young volcanic activity of the Reunion hotspot (Beckman et al., 1988). This lineament parallels the remarkable Ninety-east Ridge which is also a trace of a hotspot. Both these N-S-trending ridges record the northward motion of the Indian sub-continent away from stationary hotspots near Reunion and Kerguelen Islands, respectively (Fig. l la). All hotspot traces (Emerick, 1985) in the Indian and South Atlantic oceans (Fig. l lb) lie along definite paths that depict the directions of plate motion. The approximate N - S direction of the Chagos-Laccadive Ridge system and the Ninety-east Ridge differ considerably from that of the NNW-SSE-trending Prathap Ridge and mid-shelf basement ridge, which both extend right to the southern tip of the Indian landmass. Furthermore, the ridges of hotspot origin do not show a consistent pattern of magnetic anomalies all along their length, whereas the Prathap and mid-shelf ridge are characterised by magnetic and gravity highs (Figs. 3, 4, 9a and 9b). The subdued magnetic anomaly signatures at some places are due to the ridges being buried to various depths. The shallowest depth observed over the Prathap Ridge in the study area (profiles SK-A and RVG-A) is around 300-400 m from the chart datum and here the ridge is a flat-topped feature. The northern portion of the Prathap Ridge Complex and the trace of the Reunion hotspot seem to join around 17°N. We believe that

thermal melting/metamorphism may have distorted the magnetic field considerably and given rise to ill-defined magnetic anomalies. It is rather difficult to ascertain whether the negative magnetic anomalies observed by Krishna et al. (1992) north of Goa are due to the Prathap Ridge Complex, the Chagos-Laccadive Ridge system (i.e., the trace of the Reunion hotspot) or due to the culmination of these two ridges. In our studies we have noticed that the Prathap Ridge Complex and the mid-shelf basement ridge have well-developed associated magnetic and gravity highs of ~ 300 and 200 nT, and ~ 70 and 60 reGal, respectively, all along their length. We, therelbre, consider that the Prathap Ridge Complex and the mid-shelf basement ridge are different from the Chagos-Laccadive Ridge system. Their N N W - S S E orientation also differs considerably fiom that of the N-S-trending Chagos-Laccadive Ridge system (Fig. 12). It is thus rather unconvincing to consider that the Prathap Ridge Complex and the mid-shelf basement ridge in terms of composition and structure should resemble the Chagos-Laccadive Ridge system. We are, therefore, hesitant to suggest a genetic link between the Prathap Ridge Complex and the mid-shelf basement ridge, and the Chagos-Laccadive Ridge system. A few selected deep cores from the Prathap Ridge Complex and the mid-shelf basemerit ridge would resolve the question whether all these features are genetically linked or not and also confirm the ages of the crust. Further detailed geophysical studies are suggested west of the Prathap Ridge Complex for deciphering the continent ocean boundary (COB) since the Chagos-Laccadive Ridge system considerably affected the adjacent crust during its evolution. 5. Conclusions

(1) The NNW-SSE-trending graben in the inner shelf, the mid-shelf basement ridge, the shelf-margin basin and the Prathap Ridge Complex extend as linear features along the southwestern continental margin of India between 12°20 'N and 15000'N. (2) The Prathap Ridge Complex and the mid-shelf basement ridge are rift-related ridges formed during the separation of India from Madagascar around 84 Ma and they followed the pre-existing trends of the Precambrian basement fabric.

V. Subrahmanyam et al. / Tectonophysics 249 (1995) 267-282

Acknowledgements We

are grateful

to Dr.

B.N.

D e s a i , Director,

National Institute o f O c e a n o g r a p h y and Shri R.R. Nair, P r o j e c t C o o r d i n a t o r , for their interest and enc o u r a g e m e n t . T h e a u t h o r s t h a n k Dr. E. Banda, Dr. B a r b a r a S h e f f e l s and t w o a n o n y m o u s r e v i e w e r s for their critical r e v i e w and c o n s t r u c t i v e c o m m e n t s , w h i c h h e l p e d to i m p r o v e the paper. W e also gratefully a c k n o w l e d g e the D e p a r t m e n t o f O c e a n D e v e l o p m e n t ( D O D ) o f I n d i a for p r o v i d i n g ship time.

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