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The relationship between rifting andmagmatism in the northeasternArabian SeaTIMOTHY A. MINSHULL1*, CHRISTINE I. LANE1, JENNY S. COLLIER2 AND ROBERT B. WHITMARSH1

1National Oceanography Centre, Southampton, University of Southampton, European Way, Southampton SO14 3ZH, UK2Department of Earth Science and Engineering, Imperial College London, Exhibition Road, London SW7 2AZ, UK*e-mail: tmin@noc.soton.ac.uk

Published online: 22 June 2008; doi:10.1038/ngeo228

The causal mechanisms linking continental flood basalts,lithospheric extension and mantle plumes, as well as therelative timing of extension and volcanism, are controversial1–4.The eruption of the Deccan flood basalts was approximatelycontemporaneous with the separation of the Seychellesmicrocontinent from India. However, between these continentalblocks lies the enigmatic Laxmi Ridge, and the sequence ofextensional events that formed these various tectonic elementsis poorly understood. Here we present wide-angle seismic dataalong a profile across Laxmi Ridge that permit delineation ofoffshore igneous bodies associated with the Deccan magmatism;these bodies are similar to those associated with flood-basaltvolcanism and rifting in the Atlantic region and elsewhere1. Fromthe geometry of these bodies, we infer that there were two periodsof extension. The first phase, which involved extension betweenLaxmi Ridge and the Indian subcontinent, was accompaniedby significant Deccan-related magmatism. Full development ofa continental margin was achieved during the second phaseof weakly magmatic extension between Laxmi Ridge and theSeychelles. We suggest that between these rifting events theregion passed beyond the reach of lateral flow from the sourceregion of the Deccan flood basalts.

Voluminous magmatism at rifted margins is expressed in theform of lenses of high-velocity material at the base of the crust,interpreted as resulting from magmatic intrusion (underplating),and in seismic reflection profiles by arcuate seaward-dippingreflectors, interpreted as basalt flows. The detection of the productsof extensive magmatism beneath distal continental margins hasbeen inferred to indicate a direct link between some flood-basaltprovinces and continental breakup1. The Deccan traps form aflood-basalt province adjacent to the west Indian margin with anestimated volume of 2–4 × 106 km3, most of which is inferred5 tohave erupted in a period of less than 1 Myr. Several authors haveconcluded that the main phase of tholeiitic flood-basalt eruptionpreceded significant lithospheric extension2. This conclusion hasled to a model in which the Seychelles microcontinent becameisolated by weakening of the lithosphere owing to the presenceof a mantle plume, and this process resulted in a shift in thelocus of extension landward from the Mascarene Basin into theproto-Arabian Basin6 (Fig. 1).

However, the debate regarding the origin and relative timingof magmatism and extension at this margin has been based largelyon observations made onshore. Understanding of the offshore is

handicapped by thick Indus Fan sediments and the presence ofseveral enigmatic basement ridges and troughs. The Laxmi Ridgeis one such basement high, covered by up to 4 km of sediment. Itcoincides with a gravity low, indicating thick crust, and is separatedfrom the Indian subcontinent by a region of deep basement thatin our study area is termed the Gop Rift7 (Fig. 1). Laxmi Ridge hasbeen interpreted as either continental or thickened oceanic crust,and the Gop Rift as either oceanic crust or thinned continentalcrust7–9. Although some of the first observations of seaward-dipping reflectors were made on the western margin of India10, theseaward-dipping reflector packages observed so far are thin, weaklyexpressed and relatively rare10–12. The presence of underplatedmagmatic rocks has been inferred from wide-angle seismic data inadjacent regions onshore13, but could not be resolved by previouswide-angle seismic data offshore14, although these data did indicatethe presence of lower-crustal velocities greater than 7.0 km s−1.Underplated bodies have been incorporated in gravity modelsof Laxmi Ridge11,15, but their presence is not required, becausesuch models suffer from trade-offs between layer thicknessesand densities.

We acquired wide-angle seismic data in 2003 along a 470 kmprofile across the Gop Rift and the east–west-trending portion ofLaxmi Ridge (Fig. 1), where the most recent reconstruction16 showsthat it is conjugate to the Seychelles plateau. The profile was locatedaway from known offsets between seafloor spreading magneticanomalies in the Arabian Basin and oriented perpendicularto anomaly 27. The ocean-bottom seismometer/hydrophone(OBS/H) data are of high quality (Fig. 2 and SupplementaryInformation, Fig. S1) and show clear refracted arrivals from thesedimentary cover, which is 2–4 km thick along most of the profile,from the crust (Pg) and from the uppermost mantle (Pn). Wide-angle reflections from the base of the crust (PmP) are also visibleon most record sections. Critical to our interpretation is theidentification of a thick, high-velocity subcrustal layer beneath theLaxmi Ridge, with prominent wide-angle reflections marking itstop (for example phase PtP at ∼25 km offset north of OBS11) andbase (for example phase PbP at >35 km offset south of OBS11)(Fig. 2). An analysis of the travel-times of observed phases17 (see theMethods section) led to a final velocity model that is constrained bydense ray coverage in the sediments and crust, but sparser coveragein the subcrustal layer and in the mantle (Fig. 2).

Seaward of Laxmi Ridge, the oceanic crust is thin (5–6 km;Fig. 2b). Rays turning at ∼10 km depth correspond to a

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70˚ 75˚

65.0

Deccan

Chagos – Laccadive

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Laxmi Ridge

Gop Rift

Arabian Basin

Murray R

idge

60.9

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–140 –120 –100 –80 –60 –40 –20 0 20 40 60 80

Gravity

mGal

Arabian Basin

S

MB

East SomaliBasin

15˚

20˚

25˚

Figure 1 Location of wide-angle seismic experiment with gravity anomalies27.The black region marks the onshore outcrop of Deccan basalts. The black circlemarks the ocean-bottom seismometer (OBS) for which data are shown in Fig. 2. Thered lines mark magnetic anomaly picks15. The white circles, labelled with ages inMyr, mark the estimated locations of the Deccan plume centre16. Inset: Regionalsetting. The thin lines mark the 2,000m isobath, the thicker line marks themid-ocean ridge, S marks the Seychelles Plateau and MB marks theMascarene Basin.

high-amplitude arrival probably turning in the lower part ofoceanic Layer 2. Signals turning deeper in the crust were weakand difficult to pick (see Supplementary Information, Fig. S1),as commonly observed in oceanic Layer 3, but PmP travel-timesindicate a mean velocity of ∼7.2 km s−1 in this part of the crust.

Beneath Laxmi Ridge, the thickness and velocity of the lowercrust change, and the crustal thickness increases to ∼10 km. Atthe base of the crust lies a high-velocity body with a maximumthickness of 9 km and velocities reaching ∼7.6 km s−1 (Fig. 3).Such velocities may be interpreted as due to the presence ofpartially serpentinized mantle rocks, or due to underplatingby fractionated, Mg-rich magmatic rocks. We prefer the latterinterpretation because the body thickens beneath thicker crust,where seawater penetration into the mantle would have been moredifficult. Reflections from the base of this body disappear on bothsides of Laxmi Ridge, and refracted upper-mantle arrivals withhigher velocities are observed instead (Fig. 2). The presence ofan abrupt velocity discontinuity at the top of the body indicatesthat the ∼10-km-thick crust existed before the emplacement ofthe underplate.

Beneath the Gop Rift, the crustal thickness varies between7 and 11 km and velocities range from 4.7 to 7.4 km s−1, with

some lateral variation. Although velocities differ little from thoseobserved at Laxmi Ridge and further landward, the lower crustshows a low-velocity gradient characteristic of oceanic Layer 3 andvelocities match well those observed for thick oceanic crust adjacentto rifted margins in the northern North Atlantic (Fig. 3). Thestrong PmP reflection (see Supplementary Information, Fig. S1)also suggests that oceanic crust is present, and the presence ofhigh-amplitude, linear magnetic anomalies7,15 is consistent withthis interpretation. The absence of underplating argues againstthe presence of continental crust thinned by rifting after orcontemporaneously with the Laxmi Ridge crust.

At the northern end of the profile, the crust thickensfurther, and crustal velocities are reduced significantly, with the6.6 km s−1 contour occurring several kilometres deeper. Hereagain, a high-velocity subcrustal body is observed (Fig. 3), alsoconstrained by wide-angle reflections from its top and base (Fig. 2and Supplementary Information, Fig. S1). The body reaches amaximum thickness of ∼12 km and has a velocity of 7.4 km s−1.The northern limit of this body is not constrained by our data.

The simplest interpretation of our observations is that thevoluminous oceanic magmatism in Gop Rift is a consequenceof the same mantle-melting event as led to the formation ofthe underplated bodies. In this interpretation, Laxmi Ridge iscomposed either of thinned, possibly highly intruded, continentalcrust or of pre-existing oceanic crust related to an older extensionevent; both interpretations are consistent with magnetic-anomalydata (see Supplementary Information, Fig. S2). Underplated bodiesat rifted margins are very difficult to date, and a much earlierorigin of the bodies, unrelated to the Deccan or to Laxmi Ridgerifting, cannot be excluded. Our observations cannot be explainedby a model in which magmatic underplating resulted from thepresence of a mantle thermal anomaly during breakup betweenthe Seychelles and Laxmi Ridge. In such models4, the first-formed oceanic crust is anomalously thick (>10 km) and decaysto normal thickness (6–7 km) over a period of several Myr, yetimmediately south of Laxmi Ridge the oceanic crustal thicknessis not anomalously thick. In addition, the presence of a thermalanomaly at Seychelles–Laxmi Ridge breakup time cannot explainthe presence of separate underplated bodies beneath the twomargins of Gop Rift.

The oldest, undisputed seafloor spreading anomaly in theArabian and East Somali Basins16 is 27n (61.7–62.0 Myr; timescaleof Gradstein et al.18), although 28n (63.1–64.1 Myr) is alsorecognized off the western tip of the Seychelles plateau19. Initialseafloor spreading in these basins was three to five times fasterthan during breakup in the North Atlantic (50–60 mm yr−1 half-rate15,16). Generally, it is inferred that the Gop Rift formed beforethe Seychelles–Laxmi Ridge breakup, because plate reconstructionsto anomaly 27ny time fit well without the need for any rotationbetween Laxmi Ridge and India16. Unfortunately, the magneticanomaly sequence is too short for unambiguous identificationas seafloor spreading anomalies8,19. These anomalies may haveresulted from fan-shaped extension in Gop Rift (faster in thenorthwest than in the southeast) associated with fan-shapedspreading in the Mascarene Basin, either between anomaly 29and 27 times20 or starting at anomaly 31 time21. Boreholes onthe adjacent continental margin do not penetrate pre-Deccanstrata, but data from exploration wells at 16–23◦ N on the westernIndian margin22 suggest that there was significant subsidence, andtherefore extension, before 60 Myr.

Isotope geochronology23 shows that the bulk of Deccan lavaswere erupted in an interval of less than 1 Myr, close to 65.5 Myr,whereas palaeomagnetic data6 suggest that most eruptionsoccurred during Chron 29r (65.1–65.9 Myr), with smaller volumesduring 30n (65.9–67.7 Myr) and 29n (64.4–65.1 Myr). Seafloor

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Redu

ced

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obs11

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MantleUnderplate

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Sediments

11

0

5

10

15

20

25

Figure 2 Seismic velocity model. a, Example record section from OBS11 deployed on Laxmi Ridge (Fig. 1). White circles mark predicted travel-times for every tenth pick.The reduction velocity is 6 km s−1. b, Best-fitting P-wave velocity model, with velocities contoured at 0.5 km s−1 intervals below 6 km s−1 and at 0.2 km s−1 intervals at6.0–7.8 km s−1. Inverted triangles mark receiver locations. c, Ray diagram showing every tenth ray within the basement. Red rays turn within the crust (Pg); pink rays reflectfrom the base of the crust (PmP)/top of high-velocity bodies (PtP); green rays reflect from the bottom of the high-velocity bodies (PbP); blue rays turn within the subcrustalhigh-velocity body; orange rays turn within the mantle (Pn).

spreading in the Arabian Basin therefore post-dated the Deccan,probably by at least 2.0 Myr (the interval between the end of 29rand the end of 28n). The linear magnetic anomalies in Gop Rift

can be modelled as due to basement relief on a single reversedpolarity block about 100 km wide and edge effects at the marginsof such a block (see Supplementary Information, Fig. S2). With a

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Dept

h in

bas

emen

t (km

)

Velocity (km s–1)

0

5

10

15

20

4 6 8

Figure 3 Comparison of velocity–depth profiles with those observed at NorthAtlantic margins. Models shown are for Laxmi Ridge (200 km model distance inFig. 2; red), Gop Rift (330 km model distance; dark blue) and thinned Indiancontinental crust (420 km model distance; pale blue). The dashed line marksvelocities for thickened oceanic crust off East Greenland at 350 km model distancein the model of ref. 28. The dotted line marks velocities for thickened oceanic crustoff Hatton Bank on the northwest European margin (expanded spread profile ESPH (ref. 29)).

full spreading rate of 100–120 mm yr−1, as in the initial openingof the Arabian Basin15,16, the presence of such a block is consistentwith the formation of the Gop Rift oceanic crust almost entirelywithin Chron 29r, which has a duration of 0.8 Myr, and thereforecontemporaneously with the main phase of Deccan activity. Inthis scenario, the underplating represents the furthest extent ofDeccan magmatism, transported far from its source by lateral floweither of hot asthenosphere24 or of melt within the lithosphere(Fig. 4). Although the Arabian Basin opened only a short timelater, the Deccan source was then still further from the locus ofrifting (Fig. 1), the mantle was already depleted and voluminousDeccan melts did not reach this rift. Alternatively, it is possiblethat Gop Rift and the associated underplated bodies formed duringan earlier reversed polarity interval such as 31r (68.7–71.0 Myr),perhaps associated with pre-Deccan magmatism19.

We conclude that extension in the Gop Rift resulted in theformation of thick oceanic crust and significant underplating atits margins, and that this magmatism is Deccan or pre-Deccanin origin. However, this rifting event eventually failed, and a fullydeveloped continental margin that separated the Seychelles blockfrom the Indian subcontinent was formed later with subduedmagmatism. This final breakup seems to have occurred at a locationwhere the magmatic underplate was relatively thin, althoughthis body may have been thinned by prebreakup extension. Thegeometry is similar to that observed in the Main Ethiopian Rift,where rifting occurs today at the southeastern edge of a regionunderlain by a pre-existing ∼15-km-thick magmatic underplate25.Our observations are consistent with models in which extensivemelting depletes the asthenosphere and thus leads to reducedmagmatism during any subsequent rifting26. Our proposed rifting

Seychelles

Seychelles

Seychelles

LaxmiCrust

Underplate

UnderplateUnderplate

India

India

India

Gop Rift

GopRift

Mantle

Deccananomaly

Laxmi Ridge

Carlsberg Ridge

LaxmiRidge

a

b

c

lithosphere

Mantle lithosphere

Figure 4 Cartoon illustrating a model of events through time that is consistentwith data. a, Deccan (or pre-Deccan) magmatism and lateral melt transport lead toinitiation of underplating across a broad extending region. b, The underplate is splitby the formation of oceanic crust in Gop Rift. c, Extension has ceased in Gop Rift andseafloor spreading has commenced at the proto-Carlsberg Ridge. The horizontal andvertical scales are smaller in a than in b and c.

history (Fig. 4), in which the final rift jump is towards the landwardedge of a rifted margin but in a direction away from the Deccan,requires some refinement of models relating mantle plumes tomicrocontinent formation6.

METHODS

During wide-angle seismic work, shots were fired from a large (111 L) tunedairgun source at 60 s intervals and recorded simultaneously on 26 OBH/Ssand on a 2.4 km, 96-channel hydrophone streamer. Part of the profile wasre-shot with a higher-frequency airgun array fired at 30 s intervals to providehigh-resolution imaging of the top basement. For display (Fig. 2a), OBS/H datawere bandpass filtered at 2–25 Hz and reduced at a velocity of 6 km s−1, andan offset-dependent gain was applied. Travel-times of refracted and reflectedphases were modelled using a two-dimensional ray-trace forward-modellingand inversion approach17. A total of 16,184 travel-time picks were modelled;these picks were assigned range- and phase-dependent uncertainties of10–160 ms. To derive the simplest velocity model consistent with the data, thecrystalline crust was divided into two layers that were continuous across themodel; signals from the upper layer were obscured by those from the overlyingsedimentary layers and the layer boundary has no geological significance.The model was adjusted until the observed travel-times for most phases werematched within their uncertainties; some phases had higher final misfits,perhaps owing to unmodelled three-dimensional effects or to local structureunresolved by the parameterization. The final model had a root-mean-squaremisfit of 120 ms, with values for individual phases ranging from 97 ms forenergy refracted in the sediment column to 206 ms for energy refracted withinthe high-velocity subcrustal layer. Model uncertainties were examined by aperturbation analysis; this analysis indicated that crustal velocities have anuncertainty of ∼0.1 km s−1 in well-constrained parts of the model, velocities inthe subcrustal layer have an uncertainty of ∼0.3 km s−1, and the boundaries ofthis layer have depth uncertainties of ∼0.3–0.8 km.

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Received 17 August 2007; accepted 20 May 2008; published 22 June 2008.

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Supplementary Information accompanies this paper on www.nature.com/naturegeoscience.

AcknowledgementsThis work was supported by UK Natural Environment Research Council (NERC) grantsNER/A/S/2000/01332, 01390 and 01391, and by an NERC studentship to C.I.L. OBS/OBH deploymentwas supported by the EU Large Scale Facility at Geomar, Germany. We thank the officers, crew andtechnical and scientific staff of RRS Charles Darwin cruise 144 for their support, and G. Karner andC. Ebinger for constructive comments on earlier versions of this paper.

Author contributionsJ.S.C., T.A.M. and R.B.W. planned and led the experiment. C.I.L. conducted the wide-angle seismicdata analysis. All authors contributed to the magnetic modelling. T.A.M. wrote the paper.

Author informationReprints and permission information is available online at http://npg.nature.com/reprintsandpermissions.Correspondence and requests for materials should be addressed to T.A.M.

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