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Analyses of multichannel seismic reection, gravity andmagnetic data along a regional prole across

the central-western continental margin of India

A.K. Chaubey , D. Gopala Rao, K. Srinivas, T. Ramprasad, M.V. Ramana,V. Subrahmanyam

National Institute of Oceanography, Dona Paula, Goa 403 004, India

Received 26 July 2001; accepted 2 August 2001

Abstract

Analyses of multichannel seismic reflection, gravity, magnetic and bathymetry data along a regional profile

across the central-western continental margin of India have revealed the depositional pattern of sediments, crustal

structure and tectonics. The four most distinct and varied crustal regions of the margin are palaeo-shelf edges, shelf

margin basin, Prathap and Laccadive Ridges and the Arabian Basin. The shelf margin basin is carpeted by V4.5 km

maximum thick aggraded and prograded Paleocene to Holocene sediments. Six major seismic sequences of the

sediments of the margin are identified and their ages are assigned on correlation with drill-well results. Developmentof the sequence boundaries is attributed to the events of rifting of western India, eustatic sea-level changes, Indian and

Eurasian plate collision and Himalayan orogeny. Tilted fault blocks (half-grabens) located almost equi-distance from

the igneous construct of the Prathap Ridge in the shelf margin basin suggest a failed rift associated with stretched

continental crust of the basin. 2-D model studies of gravity and magnetic anomalies, constrained by the seismic

results, have revealed 6 to 27 km thick crust across the margin. The Laccadive Ridge crust limited by two volcanic

intrusives and a steep scarp at its western end is V16 km thick. It gradually thins towards offshore and juxtaposed

with early Tertiary normal oceanic crust V6 km thick of the Arabian Basin. The crustal thickness and velocity and

density structure of the ridge are comparable to that of the Laxmi Ridge, a continental sliver. The inferences and

abrupt change in magnetic and gravity anomaly signatures across the western end of the Laccadive Ridge mark the

zone of transition from continental to oceanic crust. 2002 Elsevier Science B.V. All rights reserved.

Keywords: Indian continental margin; Laccadive Ridge; multichannel seismic reection; seismic sequence; gravity; magnetic;tectonics

1. Introduction

Complex structural features such as horsts and

grabens, igneous intrusives and volcanic ows of

the western continental margin of India are con-

cealed under thick sediments. Seismic imaging

and modelling from potential eld data of the

margin lead to decipher the nature and structure

of the crust and the present-day continental mar-

gin conguration. So far, detailed seismic reec-

0025-3227 / 02 / $ ^ see front matter 2002 Elsevier Science B.V. All rights reserved.

PII : S 0 0 2 5 - 3 2 2 7 ( 0 1 ) 0 0 2 4 1 - 9

* Corresponding author.

E-mail address:[email protected] (A.K. Chaubey).

Marine Geology 182 (2002) 303^323

www.elsevier.com/locate/margeo



tion investigations of the margin are mostly con-

ned to the shelf region for hydrocarbon explora-

tion by the Indian oil industries. Seismic refrac-

tion measurements by Naini and Talwani (1983)

have provided velocity and crustal structure of themargin. An understanding of the evolution of the

entire margin would, however, require critical as-

sessment of the crustal structure and tectonics of

the margin and abyssal plain. Thus a detail study

of the shelf, shelf margin basin, Laccadive Ridge

and Arabian Basin is essential. The present study

is aimed to identify the sedimentary sequences,

sedimentation history, crustal structure and con-

tinent^ocean boundary/transition in the study

area. In this paper we present results of detailed

analyses of stacked seismic reection data along aprole across the central-western continental mar-

gin of India and forward model studies of free-air

gravity and magnetic anomalies constrained by

seismic results.

2. Tectonic setting

The major crustal regions of the western continen-

tal margin of India are the shelf, shelf margin basin,

Prathap and Laxmi^Laccadive Ridges and ArabianBasin (Fig. 1). The margin comprises a wide shelf

(V300 km) and narrow slope in the north, i.e. north

of 17N, and a narrow shelf (V50 km) and wide

slope in the south. The shelf margin basin in the

south comprises several basement rises with steep

slopes near the shelf edge (Harbison and Bassinger,

1973; Ramaswamy and Rao, 1980). The study of

Naini and Talwani (1983) revealed that some of the

basement rises constitute as a linear feature called

Prathap Ridge between 8N and 17N and transi-

tional crust between the Laccadive Ridge in the west

and the western continental slope of India in the

east. The ridge, mostly buried below the Tertiary

sediments and exposed at places, occurs as single

and multiple peaks along its length associated with

free-air gravity and magnetic anomalies. The origin

of the ridge is attributed either to volcanic intrusives

during the initial phase of rifting (Naini and Talwa-

ni, 1983; Subrahmanyam et al., 1995) or volcanic

emplacement from the Reunion hotspot when the

Indian plate moved over it (Krishna et al., 1992).

The Laccadive Ridge, northern part of the Cha-

gos^Laccadive Ridge (C-L-R) parallel to the west-

ern Indian margin, is the most prominent bathy-

metric feature seawards of the southwestern

continental margin of India. The ridge is associ-ated with relative positive free-air gravity anoma-

lies in the southwest and high-amplitude magnetic

anomalies at places (McKenzie and Sclater, 1971;

Kahle and Talwani, 1973; Naini and Talwani,

1983). The origin of the ridge has been attributed

to various dierent processes, such as transform

faulting (Fisher et al., 1971; McKenzie and Sclat-

er, 1971), micro-continent tectonics (Avraham

and Bunce, 1977) and a hotspot trace (Dietz

and Holden, 1970; Morgan, 1972; Whitmarsh,

1974). Detrick et al. (1977) have suggested a sub-areal volcanic origin of the ridge that has sub-

sided to V2075 m. Drilling results (DSDP and

ODP) suggest that the Deccan Trap province of

the Indian Peninsula and the Chagos^Laccadive

Ridge formed by volcanic outpouring during

northward motion of the Indian plate over the

Reunion hotspot (Shipboard Scientic Party,

1988; Richards et al., 1989; Duncan, 1990). These

ndings have led to a considerable debate on the

nature and origin of the Laccadive Ridge. The

crust of the Arabian Basin, west of the LaccadiveRidge, is oceanic and formed during the early

Tertiary (Chaubey et al., 1998; Dyment, 1998).

3. Data acquisition and processing

Multichannel seismic (MCS) reection, gravity,

magnetic and bathymetry data along prole

SK12-07 across the central-western continental

margin of India were collected during 1984^1985

onboard the ORV Sagar Kanya (Fig. 1). Positions

along the prole were obtained by an integrated

navigation system using a dual channel satellite

receiver as primary navigational aid. The MCS

data were acquired using a 24-channel seismic

streamer with 32 hydrophones per group spaced

at 25 m intervals. A recording duration of 8 s, a

sampling interval of 4 ms and a shot interval of

25 m were chosen with a ship speed of 4^5 knots

to achieve a 12-fold coverage of data. The seismic

source consisted of a D-type array combination of

A.K. Chaubey et al. / Marine Geology 182 (2002) 303^323304



Fig. 1. Generalized tectonic map of the eastern Arabian Sea, showing magnetic lineations (28Ny^18Ny), fracture zones (dashed

lines) and main structural features (compiled from Bhattacharya et al., 1994; Chaubey et al., 1998). Diagonal osets of magnetic

lineations are pseudofaults. Location of DSDP Sites (Whitmarsh et al., 1974) and industrial drill-well KR-1 (Pandey and Dave,

1998) are shown as solid circles and star, respectively. Bathymetric contours are in metres. Refraction stations (Naini and Talwa-

ni, 1983) are shown as solid triangles. Multichannel seismic reection prole (SK12-07) is shown as solid line with open circles.

LB = Laxmi Basin, L1^L4 = Laxmi Basin magnetic lineations, PB = Padua Bank.

A.K. Chaubey et al. / Marine Geology 182 (2002) 303^323 305



Table 1

Summary of seismic sequence stratigraphy in the study area

Seismic

sequence

Outer shelf Shelf margin basin Seismic

sequence

Laccadiv

Thicknessb Seismic

character

Likely

lithology

Inferred ages Thicknessb Seismic

character

Likely

lithology

Inferred ages Thicknes

H6 0.1^0.74 Low

amplitude,

nearly

reection free

Clay L. P le istoce ne^

Recent

0.2 0.48 Continuous

and parallel

reectors

Clay L. Pleistocene

^Recent

L6 0.11^0.2

H5 0.05 1.22 Near-parallel

reectors with

good reection

continuity

Clay M. Pliocene^

L. Pleistocene

0.09^0.6 Low-amplitude

and continuous

reectors

Clay M. Pliocene^

L. Pleistocene

L5 0.07^0.1

H4 Claystone M. Miocene^

M. Pliocene

0.18^0.65 High-amplitude,

low-frequency

and

discontinuous

reectors

Claystone M. Miocene^

M. Pliocene

L4 0.09^0.2

H3 0.2^0.7 Varied

amplitude near

parallel,

good to fairly

good reection

continuity

Intervening

limestones

and shales

L. Oligocene^

M. Miocene

0.1 0.68 Very good to

fairly good

reection

continuity,

high-amplitude

and near-

parallel

reectors

Intervening

limestones

and shales

L. Oligocene^

M. Miocene

L3 0.15^0.5

H2 0.1^0.21 Top of H2 is

uneven, high-

amplitude,

discontinuous

to continuous

reector

Dolomitic

limestone

with minor

shale

L. Eocene^

L. Oligocene

0.1 0.9 Discontinuous,

low-frequency,

high-amplitude

reectors

Dolomitic

limestone

with minor

shale

L. Eocene^

L. Oligocene

L2 0.06^0.2

H1 ^ Near-parallel

reectors

Dolomitic

limestone

with

minor

shale

Paleocene^

L. Eocene

0.1^0.9 Dolomitic

limestone

with minor

shale

Paleocene^

L. Eocene

L1 ^

a The sequence boundaries are correlated to the litho-logs of DSDP Site 219. Chert layer is the key reector over the Laccb Values measured in seconds.

Note: thickness and seismic character of sequences H5 and H4 of the outer shelf are given together. Seismic character of s

same as that of L6.



7 air guns with a total capacity of 7.98 l. A stan-

dard processing package NORSEIS of GECO,

Norway was used on an ND-570 computer at

the National Institute of Oceanography, Goa to

obtain stacked sections. The gravity data wereacquired using the Bo denseewerk sea gravimeter

system KSS-30. The free-air gravity anomalies

were computed using the IGA 1967 international

gravity formula and by applying the Eo tvo s cor-

rection. Total magnetic intensity data were col-

lected using the Proton Precession Magnetometer

and the prole plot was made after being duly

corrected for the IGRF. Bathymetry data were

collected using a Honeywell Elac narrow beam

echosounder. The measured depth values were

corrected for variation of the sound velocity usingMatthews tables (Carter, 1980).

4. Analyses of the data

4.1. Seismic reection

The 720 km long seismic section, along SK12-

07 in 80^4300 m water depth across the central-

western continental margin depicts seismic se-

quences and basement images. The sequence

boundaries and facies changes are identied and

ages are assigned. The seismic character, inferred

lithology, thicknesses (in two-way travel time,TWT, in seconds) and their ages are presented

in Table 1.

4.1.1. Outer shelf

Seismic sequences H2, H3, H4+H5 and H6 of

V3.0 s thick are identied on the shelf in 80^200

m water depth (shot points, sps, 29 000^27 000;

Fig. 2). The H2 top occurs at V0.53 s subsurface

depth (sp 28 600). Sequence H1 and base of the

sequence H2 could not be identied due to the

presence of multiples. The sequences H4 and H5are marked together as they are not easily discern-

able individually. The sequence H2 top is very

uneven especially towards the coast (sps 28 900^

28 100). The lower parasequences of the H3 lap

onto it. The sequences H4+H5 and H6 together

indicate a typical sigmoidal reection pattern in

the vicinity of the shelf break. A marked change

in the gradient of sequences H2 (top) and H3

(top) reectors occurs at shot points 27 300 and

Fig. 2. Multichannel seismic reection record and interpreted line drawing across the outer shelf show seismic sequences H1^H6,

present and palaeo-shelf breaks and Late Oligocene erosional unconformity. Location of prole is shown in Fig. 1.




27 350, respectively, indicating palaeo-shelf edges.

The palaeo-shelf edge marked by the H3 top re-

ector is V8 km behind the present shelf break.

4.1.2. Shelf margin basin

The basin in 200 to 2100 m water depth, paral-

lel to the shelf edge between the Laccadive Ridge

and shelf (sps 27000^22 400; Figs. 3 and 4), is

about 115 km wide. It is divided into two parts:

the eastern and western basins separated by a at-

topped structural high (sps 25 850^25 300). The

high bounded by near-vertical faults is 14 km

wide, carpeted by sediments ofV1.3 s maximum

thickness. It rises by V550 and V1700 m from

the adjacent seaoor of the eastern and western

basins, respectively.

The eastern basin, carpeted by sediments of

about 3.5 s maximum thickness in V1000 m

water depth, is 30 km wide and consists of se-

quences H1^H6 (Fig. 3). The base reector of

the sequence H1 is high-amplitude, uneven and

deepens towards the centre of the basin and is

marked as acoustic basement (ACB). The western

basin carpeted by sediments of about 2.3 s max-

imum thickness in V2000 m water depth is 72 km

wide (Fig. 4). Here the base of sequence H1 dis-

plays hyperbolic reections. In the centre of the

basin (sps 24 500^23 500, Fig. 4) the reections

with vertices at varied depths constitute a base-

ment high at 0.5 s subsurface depth with a total

relief of V1.6 s from the adjoining areas. To-

wards the margins of the basin, short-segmented

H1 top reectors are bounded by faults and are

tilted away from the centre of the basin (Fig. 4).

Fig. 3. Multichannel seismic reection record and interpreted line drawing across the shelf margin basin (eastern) show seismicsequences H1^H6, present and palaeo-shelf breaks and shelf margin high. ACB= acoustic basement. Location of the prole is

shown in Fig. 1.




Fig. 4. Multichannel seismic reection record and the interpreted line drawing across the shelf margin basin (western) show

(tilted fault blocks), eastern ank of the Laccadive Ridge and seismic sequences H1^H6. Location of prole is shown in Fig.



4.1.3. Laccadive Ridge

The Laccadive Ridge, V255 km wide in 1650^

3700 m water depth, is overlain by V0.9 s thick

sediments and is bounded on the east and west bysteep scarps at sps 22 400 and 12 200, respectively

(Figs. 4 and 6). Two basement highs, character-

ized by hyperbolic reections, occur near the

western end of the ridge at sps 14 000 and

12300 and are V4.5 km apart with V1.0 km

relief. A steep fall ofV560 m in seaoor topog-

raphy occurs at the western end of the ridge. The

seismic sequence L1 top reector is high-ampli-

tude, and discontinuously associated with numer-

ous diraction hyperbolae (Fig. 5). The underly-

ing reectors are less distinct and near parallel

(sps 20 000^19 000). Further deep reections of

crystalline basement could not be identied. The

sequences L2 and L3 reectors are near parallel.

The sequence L4 reectors are continuous, paral-

lel and low-amplitude. Two low-relief graben-like

features, bounded by vertical faults, are noted on

the crest of the ridge. Fig. 5 shows one of the

grabens between sps 18 800 and 18 600. The se-

quence L5 could not be traced over 25^30 km

distance (sps 20 250^19 800) and here the L6 re-

ectors overlie directly the sequence H4 reectors

and thus form two lens-shaped structures (sps

20600^20 000, and 19900^19 100; Fig. 5). The

seaoor topography here also shows a bathymet-ric low ofV200 m and V8 km wide. It perhaps

marks erosion in the geological past and is most

probably due to palaeo-contour currents paral-

leling the ridge.

4.1.4. Arabian Basin

The Arabian Basin, to the west of the Lacca-

dive Ridge in 4200 to 4300 m water depth, is

carpeted by sediments ofV1.2 s maximum thick-

ness (Figs. 6 and 7). The sediments consist of two

well-dened seismic sequences. The reectors of

the top sequence are discontinuous, near parallel

and some of them lap onto the top reector of the

underlying sequence. Their continuity is often ob-

literated by short-distance oblique reections cen-

tred by depressions. They mark cut- and ll-type

sediment reectors typical of turbidites. The re-

ectors of the lower sequence are continuous

and near parallel. The reector separating the

two sequences is medium- to high-amplitude,

which we denote as R. This reector marks the

Fig. 5. Multichannel seismic reection record and interpreted line drawing over the Laccadive Ridge show seismic sequences L1^

L6, graben-like feature and prominent Early^Middle Eocene chert reector. Location of the prole is shown in Fig. 1.




Fig. 6. Multichannel seismic reection record and interpreted line drawing towards seaward edge of the Laccadive Ridg

prominent Early^Middle Eocene chert reector, volcanic intrusives and inferred ocean^continent boundary/transition. Locati


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boundary between two distinct phases of sedimen-

tation. The basal reectors (sps 12 100^10 500) at

0.8 s subsurface depth are discontinuous and

high-amplitude associated with diractions.

4.2. The free-air gravity and magnetic anomalies

The free-air gravity anomalies along the prole

SK12-07 (Fig. 8) display three distinct patterns.

High-amplitude (50^95 mGal) and short-wave-

length (50^60 km) anomalies occur in the shelf

and shelf margin basin. A relative positive broad

anomaly superimposed by short-wavelength low-

amplitude anomalies characterises the Laccadive

Ridge. The anomalies of the Arabian Basin are

subdued and show a regional trend at higher lev-

el. The amplitude and wavelength of magnetic

anomalies of the Arabian Basin are distinct and

dier from the rest of the area. The marked dier-

ences in the anomalies occur in the close vicinity

of the intrusive volcanic body (sp 12 300) at the

western end of the Laccadive Ridge indicating

basic dierences in the crust and density structure.

4.3. Refraction velocities and crustal structure

The published refraction investigations (Fig. 9,

location shown in Fig. 1) close to the seismic line

SK12-07 at sites 73V, 84C, 59V and 72V of the

Arabian Basin, and L12 and L08 of the Laccadive

Ridge are considered to constrain the crustal

structure of the margin. The velocities 1.7^3.8

km/s, 5.3^5.7 km/s, 6.4^6.6 km/s of the Arabian

Basin are comparable to the velocities of layer 1

(sediments), layers 2 (extrusive basaltic lava) and3 (gabbroic rocks), respectively, of the normal

oceanic crust. The velocity structure of the Lacca-

dive Ridge consists of 1.65^2.12 km/s, 4.2^4.4

km/s, 5.6^5.7 km/s, 6.3 km/s and 7.2^7.3 km/s,

while the representative velocity structure of the

Laxmi Ridge shows 2.04, 4.46, 5.43, 6.2 and 7.15

km/s. The Laccadive Ridge crustal layer velocities

are comparable to the Laxmi Ridge, which is

widely accepted as a continental sliver by Naini

and Talwani (1983), Miles et al. (1998), Talwani

and Reif (1998) and Todal and Eldholm (1998).

Two-dimensional model studies, constrained by

seismic results, of the free-air gravity and mag-

netic anomalies of prole SK12-07 are carried

out to determine the crustal structure of the mar-

gin (Fig. 8). The densities are obtained using pub-

lished refraction velocities and the velocity^den-

sity curve of Nafe and Drake (1963). Densities

of 1.03 and 3.3 g/cm3 are assumed for seawater

and upper mantle, respectively. A reasonable t

between the computed and the observed gravity

Fig. 7. Multichannel seismic reection record and interpreted line drawing across the Arabian Basin show oceanic basement, pe-

lagic and turbidite sediments separated by reector R of Middle^Late Oligocene age. Location of prole is shown in Fig. 1.




anomalies is obtained with a mist of individual

points generally less than 5% of the observed val-

ues. The best-t model revealed V27, V16

and V6 km thick crusts of the outer shelf,

Laccadive Ridge and Arabian Basin, respectively.

The crustal densities of the Arabian Basin consist

of three layers of 2.3, 2.75 and 2.85 g/cm3 repre-

senting sediments, and layers 2 and 3 of the oce-

anic crust, respectively, whereas the crustal struc-

tures of the shelf, shelf margin basin and

Laccadive Ridge consist of 2.3, 2.6 and 2.9 g/

cm3, representing sediments, upper and lower

continental crust, respectively. A density of 2.8

g/cm3 is assigned to the intrusive bodies of the

Laccadive Ridge.

The magnetic anomalies across the continental

margin are interpreted as several two-dimensional

bodies of various magnetization intensities (1^5

A/m), while alternate normal and reverse magnet-

izations, similar to seaoor spreading type crust,

are considered to explain the magnetic anomalies

of the Arabian Basin. The best t between syn-

Fig. 8. Two-dimensional gravity and magnetic model and interpreted crustal structure along the seismic line SK12-07. OCB=

ocean continent boundary/transition, SMB = shelf margin basin.




thetic and the observed magnetic anomalies of the

Laccadive Ridge has been simulated using either

densely spaced igneous intrusive-type magnetic

sources, or both ows and intrusions. We prefer

the later model to explain the anomalies consid-

ering the geologic situation of the area. The

marked dierence between the oceanic and conti-

nental crust is clearly discernable from the calcu-

lated anomalies and crustal structure at the west-

ern end of the Laccadive Ridge.

5. Interpretation

5.1. Ages of the seismic sequences

5.1.1. Seismic sequences H1^H6 of the shelf and

shelf margin basin

The ages of seismic sequences H1^H6 of the

shelf and shelf margin basin are interpreted as

Paleocene to Late Eocene, Late Eocene to Middle

Oligocene, Middle Oligocene to Middle Miocene,

Fig. 9. Compilation of velocity structure (after Naini and Talwani, 1983) along line SK12-07 from refraction stations of the Ara-

bian Basin (73V, 84C, 59V and 72V), Laccadive Ridge (L12 and L08) and continental shelf (85C). LR represents average crustal

column of the Laxmi Ridge. P-wave velocities (km/s) are shown inside the crustal column. Location of refraction stations are

also shown in Fig. 1.




Fig. 10. Correlation of boundaries of seismic sequences H1^H5 (top) of the outer shelf and shelf margin basin with the lithostra-

tigraphy of drill-well KR-1 (Singh and Lal, 1993) and eustatic sea-level curve (Haq et al., 1987). Location of prole is shown in

Fig. 1.




Middle Miocene to Middle Pliocene, Middle Plio-

cene to Late Pleistocene and Late Pleistocene to

Holocene, respectively, on correlation with drill-

well results of KR-1 (Singh and Lal, 1993; Pan-

dey and Dave, 1998), global sea-level curve (Haqet al., 1987) and published results (Ramaswamy

and Rao, 1980; Biswas and Singh, 1988).

In the shelf region, top of H3 is a key horizon

and correlates well with the Middle Miocene re-

ector of the KR-1 drill-well (Fig. 10). We there-

fore assign a Middle Miocene age to the H3 top.

The sequence H2 top (Fig. 2) is another promi-

nent reector which lies V550 m below seaoor.

This reector is very uneven and overlying para-

sequences of H3 lap onto it. The velocity structure

from a nearby refraction station, 85C (Fig. 9, lo-cation in Fig. 1), has revealed V1.6 km/s velocity

of the sequence indicating absence of any vol-

canics at that depth. Haq et al. (1987) have inter-

preted a major drop in eustatic sea-level (V350 m)

at the end of the Early Oligocene (Fig. 10). Whit-

ing et al. (1994) inferred a characteristic Late Oli-

gocene to Early Miocene rapid increase in subsi-

dence rate of the western continental margin of

India. The interplay between eustatic sea-level and

tectonics during the Early Oligocene have caused

the sequence to remain within the wave base orsub-aerial erosion resulting in the observed reec-

tion pattern. Subsequent rise of the sea-level fol-

lowed by sediment deposition had given rise to

onlapping sequences of H3 onto the H2 top. We

therefore infer that this boundary is an erosional

unconformity of Early Oligocene age.

The upper reectors of sequences H1 and H2 of

the shelf margin basin (Fig. 3) are at 2.7 s and 1.6

s subsurface depths, respectively, and are corre-

lated to the unconformities of sequence II identi-

ed by Rao and Srivastava (1984). Based on in-

terval velocities, they opined that sequence II

might represent carbonate sediments developed

during stable platform conditions. Singh and Lal

(1993) have inferred that the end of the Middle

Eocene and the Early Oligocene marked regional

hiatuses. Thus, the sequences H1 and H2 tops

mark the Middle Eocene and Middle Oligocene

unconformities, respectively, within the carbo-

nates.

The near-parallel and acoustically transparent

reectors of the seismic sequences H4 and H5 ofthe shelf margin basin (Figs. 3 and 4) reect a

change in facies of the sediments. Middle Pliocene

and Late Pleistocene ages are assigned to the se-

quences H4 and H5 tops, respectively (Fig. 10).

Two distinct sediment inux maxima in the Late

Miocene (9^6 Ma) and Middle Pliocene (4^2 Ma)

have been also reported in the northern Indian

Ocean (Rea, 1992). The overlying H6 sequence

represents Holocene sediments.

5.1.2. Seismic sequences L1^L6 of the LaccadiveRidge

Seismic sequences stratigraphy of the Laccadive

Ridge (Fig. 11) has been determined based on

correlation with the lithologs of the DSDP site

219 (Whitmarsh et al., 1974). The top of seismic

sequence L1 is a very prominent reector and is

condently identied across the ridge (Figs. 5 and

6). The DSDP Site 219 results have revealed the

presence of a chert layer, V4.0 km/s P-wave ve-

locity, of Early and Middle Eocene age. The ve-

locity structure from the nearby refraction stationalso revealed a layer with a velocity ofV4.2 km/s

overlain by a sediment layer with a velocity of 2.0

km/s (Fig. 9). The reection character of the chert

layer (Whitmarsh et al., 1974) is strikingly similar

to that of the L1 top. Considering the reection

pattern and refraction velocities, we interpret

upper sequences of the L1 as a chert layer and

assign an Early^Middle Eocene age. The sequen-

ces L2 and L3 are thin and their upper boundaries

are also prominent reectors. Lower sedimenta-

tion rates during the Middle Oligocene to Middle

Miocene period and an unconformity at the end

of the Early Miocene were reported at the DSDP

site 219. Therefore, we assign an Early Miocene

and a Late Oligocene age for the top reectors of

L3 and L2, respectively. The post-Early Miocene

seismic sequences L4 and L5 may represent the

Fig. 11. Correlation of boundaries of seismic sequences L1^L5 (top) of the Laccadive Ridge with the lithostratigraphy of DSDP

Site 219 (Whitmarsh et al., 1974). The location of prole is shown in Fig. 1.




Fig. 11.




Middle Pliocene and Late Pleistocene ages, re-

spectively, similar to the shelf margin basin se-

quences.

5.1.3. Reector R of the Arabian BasinThe sedimentary column in the Arabian Basin

is divided into two sequences separated by a re-

ector R which marks a major unconformity

surface (Fig. 7). According to the results of the

DSDP Site 221, illite-rich sediments characteristic

of Indus Fan sediments started depositing in the

distal Arabian Sea by the Middle^Late Oligocene

(Weser, 1974). Considering the Late Paleocene^

Early Eocene age of the oceanic basement (Chau-

bey et al., 1998), the onlapping nature of the

upper sequence onto reector R and the natureof the upper seismic sequence possibly represent-

ing Fan sedimentation, a Middle^Late Oligocene

age is suggested for reector R.

Fig. 12 presents crustal regions, interpreted seis-

mic sequence boundaries and structural elements

along transect SK12-07 across the central-western

continental margin of India.

5.2. Nature and sedimentation history

In the shelf and shelf margin basin (sequenceH1) and over the Laccadive Ridge (sequence L1),

the sedimentation has started since the Paleocene.

The Eocene through Middle Miocene is dominat-

ed by deposition of carbonates and carbonates

interspersed with shales in the shelf and shelf mar-

gin basin (sequences H2 and H3). The widespread

carbonate sedimentation under stable platform

conditions (Aubert and Droxler, 1996) during

the Eocene to Middle Miocene are reported on

the shelf (Nair et al., 1992; Rao and Srivastava,

1984). The sequences L2 and L3 of about the

same period over the Laccadive Ridge also show

carbonate sedimentation. However, they are very

thin compared to the corresponding shelf margin

basin sequences. The upper boundary of sequen-

ces H3 and L3 show remarkable change in the

sedimentation pattern. In the shelf, the upper

boundary of sequence H3 also coincides with

the Middle-Miocene shelf edge. The onset of the

intense Indian monsoon, as a result of Indian^

Eurasian plate collision and the build-up of the

Himalayas during the post-Middle Miocene,

caused rapid erosion and deposition of terrige-

nous clastics onto the shelf and shelf margin basin

and cessation of carbonate deposition (Rao and

Srivastava, 1984; Nair et al., 1992; Singh and Lal,1993; Whiting et al., 1994), whereas the Laccadive

Ridge continued to receive less sediments, which

is clearly evident from the thickness of sequences

and their internal reectors. Due to high sedimen-

tation on the shelf, the palaeo-shelf edge (upper

boundary of sequence H3) has prograded sea-

wards by V8 km to form the present shelf

edge. Furthermore, the sequences H4, H5 and

H6 clearly depict the sigmoidal reection pattern.

An aggraded phase of sedimentation is interpreted

between the Middle Miocene and Middle Oligo-cene, as the Oligocene shelf edge (top of H2 se-

quence) lies below the Middle Miocene shelf edge.

The seismic records of the continental shelf and

slope region (Fig. 2) thus show three distinct shelf

edges: Middle Oligocene, Middle Miocene and

Present.

The reection pattern of the sediments of the

Arabian Basin suggests that the lower sequence

diers markedly from the upper sequence in de-

positional environment. The two sequences ap-

pear to be conformable along the reector R.We consider the upper sequences due to fan sed-

imentation, and the lower sequences to pelagic

sedimentation.

The Laccadive Ridge remained elusively a phys-

iographic high either since the Paleocene or earlier

and received only pelagic or hemi-pelagic sedi-

ments. It is evident from the abutting nature of

the H1 sequence reectors of the shelf margin

basin, dierence in thickness between the sequen-

ces over the Laccadive Ridge and the sequences of

the shelf margin basin, the near-vertical scarps in

the west and east disassociating the ridge from

oceanic crust of the Arabian Basin and the shelf

margin basin, respectively. The chert layer across

the ridge may owe its origin either to pelagic sed-

imentation rich in siliceous oozes (due to high

biogenic productivity) over a long duration of

time (few millions of years), or to extrusive vol-

canism. Deposition of siliceous-rich sediments by

these processes is possible because of (1) proved

upwelling on the western margin of India, and (2)




Fig. 12. Interpreted line drawing of the seismic section along track SK12-07. ACB = acoustic basement, SMH= shelf ma

PRC = Prathap Ridge complex.



volcanism related to rifting and/or passage of the

Indian plate over the Reunion hotspot.

5.3. Nature of the crust of the Laccadive Ridge

The origin of the ridge is equivocal and many

workers suggested its origin as due to the mantle

plume trace of the Reunion hotspot formed dur-

ing the northward motion of the Indian plate over

this hotspot (Shipboard Scientic Party, 1988;

Richards et al., 1989; Duncan, 1990). Hotspot

related volcanism can occur in a large variety of

plate tectonic settings ranging from continental to

oceanic lithosphere. In the continental setting,

magma is erupted through thick and old litho-

sphere, whereas in an oceanic lithosphere settingespecially near the spreading ridge, high exten-

sional factors allow the mantle plume to generate

a substantial amount of melt. We present two

plate tectonic settings: (1) on-spreading-ridge hot-

spot volcanism, and (2) o-spreading-ridge hot-

spot volcanism. They are discussed in the light

of published results as well as results of the

present study.

The mantle plume can generate a thick igneous

crust if it lies directly beneath the spreading

centres. The seismic investigations of parts ofthe aseismic Kerguelen and Madagascar Ridges,

formed due to a mantle plume lying beneath the

spreading centre, have revealed a maximum crus-

tal thickness of 20 1.3 km (Sinha et al., 1981;

Recq et al., 1990; White et al., 1992). The vol-

canic record of the Reunion hotspot and palaeo-

geographic reconstruction of the western Indian

Ocean indicate that the Reunion hotspot was

never centred directly below a spreading centre

at least up to V47 Ma (Duncan, 1990). By the

time the hotspot was away from the Indian sub-

continent and was lying below the Northern Cha-

gos Bank. Further, in case of sub-aerially em-

placed on-spreading axis hotspot volcanic con-

structs of the ridge and/or subjected to shallow

submarine conditions over a considerable geologic

period, as was the case with the Laccadive Ridge,

a thick layer 2 (2A and 2B) would develop due to

weathering. Krishna et al. (2001) have reported an

excessively thick (4^5 km) upper crustal layer (2A

and 2B) of the Ninetyeast Ridge, a plume em-

placed construct. The refraction results over the

Laccadive Ridge have revealed a comparatively

less thick (6 2 km) crustal layer 2 which lies be-

neath the chert layer of V4.2 km/s velocity (Fig.

9). Therefore, it is dicult to believe that theLaccadive Ridge is a solely volcanic build-up

due to the mantle plume lying directly beneath

the spreading centre.

Mantle plumes such as Hawaii, Iceland and the

Cape Verdes in an oceanic crust setting (o-

spreading ridge) generated a region of anoma-

lously hot asthenospheric mantle extending up

to 1000 km away from the central plume and

caused variations in crustal structure (Courtney

and White, 1986; Watson and McKenzie, 1991).

The reported igneous crustal thicknesses fromseismic measurements and rare-earth element in-

version of such regions are 10.3 1.7 km and

10.7 1.6 km, respectively (White et al., 1992).

The estimated crustal thickness of V16 km over

the Laccadive Ridge from the present study is

higher than the crustal thickness for o-spread-

ing-ridge hotspot volcanism.

The thickness, velocity and density structure,

structural (volcanic intrusives and graben) and

physiographic features of the Laccadive Ridge

do not support a volcanic construct either dueto on-spreading-axis or o-spreading-axis hotspot

volcanism. Moreover, the velocity and density

structure and crustal thicknesses are comparable

with the Laxmi Ridge, which is widely accepted as

a continental sliver of the northwestern Arabian

Sea. The Laxmi Ridge is associated with a pro-

nounced gravity low in a geographic and tectonic

setting similar to that of the Laccadive Ridge.

Interestingly the Laccadive Ridge is not so well

reected in the free-air gravity anomaly in spite of

its large dimensions (V2 km relief from present

seaoor and V255 km width). The lack of an

appreciable gravity anomaly over the ridge per-

haps suggests the isostatically compensated nature

of the ridge. These results and observations point

to thinned/extended continental crust of inter-

mediate thickness intruded by volcanics. The in-

dustry drill-well of the Oil and Natural Gas Cor-

poration Limited of India on the Padua Bank,

northern part of the Laccadive Ridge (Fig. 1),

was terminated in continental basalts (Murty et




al., 1999). Based on results of the drill-well and

vertical seismic proling, they suggested the exis-

tence of continental crust beneath the bank and

even further west of it. Their inferences further

support our contention that the crust of Lacca-dive Ridge is a stretched continental crust.

5.4. Nature of the crust of the shelf margin basin

The crustal features of the shelf margin basin

are buried under thick sediments. The Prathap

Ridge, 1000^3000 m water depth, parallel to the

shelf edge between 8 and 17N latitudes and as-

sociated with hyperbolic reections, lies close to

the centre of the basin and its structure is complex

with multi-peaks. It is bounded on either side bywell-developed rotated fault blocks. They repre-

sent half-grabens, tilted away from the ridge,

and are located almost equi-distance from the

ridge. These results lead us to suggest that the

basin represents a failed rift of extended continen-

tal crust and igneous emplacement in the middle

of the basin.

5.5. Oceanic continent transition

The volcanic intrusives and steep scarp at thewestern end of the Laccadive Ridge limit further

seaward extension of the ridge. Similar features of

the ridge have also been identied in the south by

Gopala Rao et al. (1987), and thus they appear to

be regional in extent. They are also associated

with relatively positive magnetic and gravity sig-

natures. Again, a marked dierence in the anoma-

lies occurs on the seaward side of the intrusives

with a steep gradient in free-air gravity anomaly.

Basement ridges of many rifted margins, e.g. the

Gabon^Cango region of West Africa (Belmonte

et al., 1965) and Norwegian margins (Talwani

and Eldholm, 1972), are reported. Widespread oc-

currences of the basement ridges at the continen-

tal margins are interpreted as due to initial rifting

and volcanic emplacement (Burk, 1968). We

therefore infer that the volcanic features at the

western end of the Laccadive Ridge mark the

boundary of the rifted crust and early rift-em-

placed volcanics. Major change in the basement

elevation normally occurs at the boundary be-

tween oceanic and continental crusts (Talwani

and Eldholm, 1973). The observed linear ridges,

steep scarps of the Laccadive Ridge, and the grav-

ity and magnetic signatures at the western end of

the Laccadive Ridge indicate the continent^oceancrust transition. Therefore, the ocean continent

boundary/transition must lie to the immediate

west of the Laccadive Ridge where thinned con-

tinental crust of the ridge is juxtaposed with the

known early Tertiary normal oceanic crust.

6. Summary

The study of the seismic reection, gravity,

magnetic and bathymetry data of the 720-km-long SK12-07 prole across the central-western

continental margin of India has revealed the fol-

lowing:

(1) The shelf, shelf margin basin, Laccadive

Ridge and Arabian Basin are carpeted by sedi-

ments of, respectively, 3.9, 4.5, 1.2 and 1.6 km

maximum thickness. Six major seismic sequences

of sediments of Paleocene to Holocene age are

identied on the shelf, shelf-margin basin and

Laccadive Ridge. The pre- and post-Middle Mio-

cene periods were dominated by carbonate andclastic sedimentation, respectively. The seismic

section of the shelf and slope region displays three

distinct shelf edges: Late Oligocene, Middle Mio-

cene and Recent. The Middle Miocene shelf edge

prograded V8 km to form the present-day shelf

edge. The Late Oligocene to Middle Miocene sed-

imentation was dominantly aggradational. An

erosional unconformity of Late Oligocene age (de-

veloped during lowered sea-level) was interpreted

on the shelf and a chert layer of Early^Middle

Eocene age across the Laccadive Ridge is identi-

ed.

(2) The shelf margin basin is characterized by

the presence of rotated fault blocks at the margins

of the basin and emplaced volcanic construct

(Prathap ridge). They suggest failed rift and vol-

canism of the stretched continental regime of the

basin.

(3) The crustal and velocity structure, structural

elements (intrusions, ows, grabens, physio-

graphic features) and free-air gravity and mag-




netic signatures of the Laccadive Ridge indicate

thinned continental crust and associated volcan-

ism.

(4) The ocean^continent boundary/transition is

inferred to lie at the western end of the LaccadiveRidge in the vicinity of the western scarp face of

the ridge where as V16 km thick crust of the

ridge gradually thinned and juxtaposed to V6

km thick early Tertiary oceanic crust of the Ara-

bian Basin.

Acknowledgements

The authors are thankful to Dr Ehrlich Desa,

Director, National Institute of Oceanography forsupport and encouragement to carry out this

work. The authors are grateful to the Department

of Ocean Development (DOD), Government of

India for providing the ORV Sagar Kanya to

collect the data. Thanks are due to scientific and

technical colleagues, officers and crew of the ORV

Sagar Kanya (cruise 12) for their help during data

collection. Mr R. Uchil is thanked for neatly

drafting the figures. We are also grateful to

journal reviewers Lindsay Parsen and John

Sclater. This is NIO contribution 3706.

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analyses of multichannel seismic re£ection, gravity and magnetic data along a regional profile...

Documents

shelf margin basin

thick crust

laccadive ridge crust

western india

gravity andmagnetic

western end

prathap ridge

seismic imagingand modelling