Physicsof theEarth andPlanetary Interiors, 73 (1992)189—198 189ElsevierSciencePublishersB.V., Amsterdam

Seismicheterogeneitiesin the Indianlithosphere

G. Mohan,S.S. Rai and G.F. Panza1

National GeophysicalResearchInstitute, Uppal Road,Hyderabad—500007, India

(Received27 March1992; accepted1 April 1992)

ABSTRACT

Mohan,G., Rai, S.S.andPanza,G.F., 1992. Seismicheterogeneitiesin theIndian lithosphere.Phys.EarthPlanet. Inter., 73:189—198.

Rayleighwave groupvelocity distributions over theIndian regionwereobtained,by the Backus—Gilbertmethod, for timeperiods15, 25, 40, 50 and 60 s. The observedvelocity variationsreveal sharp lithosphericheterogeneitiesin this region.SouthernpeninsularIndia is markedby high velocities in contrastto the northern Indo-Gangeticplain and the Himalaya.Highestsurfacewave velocities areobservedover the southernpart of the Deccantrapsand the Dharwarcraton,whereasthemetamorphicterrainsandrift zonesexhibit anomalouslow velocities.TheNepal—Himalayaand Indo-Burmesecollisionzonesarecharacterisedby contrastingvelocities,which reflect their diverseorogenesis.The Bay of Bengal is markedby ananomalous6—12%highervelocity as comparedwith the Indian continentalregion,for the periods25—50 s.

1. Introduction davari and Kaladgi-Bhima terrains, apart fromthe post-rifting sedimentarydepositsthat arere-

The Indian subcontinenthas a complex geol- strictedto thecontinentalmargins.The southern-ogy (Fig. 1) consistingof rockassemblagesof ages most and easternmargin of peninsular Indiaranging from 3.8 by to the present.The region comprisesan exhumed Precambrianmetamor-can be divided into threemajor subregions:(1) phic mobile belt once conjugate with easternthe peninsular Indian craton; (2) the Indo- Antarctica in Gondwanaland. Volcanic rocksGangeticplain; (3) the Himalayanmountainsys- cover a large part of peninsularIndia, with thetern. Peninsular India has evolved through the Deccan traps constituting the largest volcanicaccretion of Archean cratons—the Dharwar, flow, which eruptedaround65 my ago,andcoy-Singhbhum,Bastarand Aravalli cratons;with in- ered half a million squarekilometres in west—terveningmobilebelts(Granulite terrains,Delhi, central India and possibly an equal areain theSatpuraand Godavari orogenies). The general ArabianSea.The processandplace of extrusionpattern,nature anddeepstructuresof thesemo- of the Deccanbasaltsis a subjectof controversybile beltsremaincontroversialanduncertain.The centredaroundhypothesesrelatedto the reunionPrecambrianterrain is also coveredin manyareas hotspot,rifting andextraterrestrialeffects.by sedimentaryandvolcanicrocks. The sedimen- The northern part of the Indian peninsulatary rocks were depositedduring the Paleozoic comprisesthe Vindhyan (1400—1600 my) sedi-and Mesozoic while India was still a part of mentarybasin, formed as a result of sedimenta-Gondwanaland,and include the Cuddapah,Go- tion in a pre-existingsea. This basin continues

underneaththe Indo-Gangeticplain andpossiblyCorrespondenceto: G. Mohan, National GeophysicalRe- into the Himalaya. To the north-east of thesearchInstitute, Uppal Road,Hyderabad—500007, India.

Presentaddress:Instituto di Geodesiae Geofisica,1-34100 peninsulalies the Shillong uplandplateau,com-

Trieste,Italy. posed of Archean gneisses and Proterozoic

0031-9201/92/$05.00© 1992 — Elsevier SciencePublishersB.V. All rights reserved

190 G. MOHAN AND SS. RAI

meta-sediments.The mechanismof uplift of the and the nature of the Ninety East Ridge. Theplateau remains uncertain. The northern and detailedgeological frameworkof the Indian re-easternextremitiesof the Indian peninsulacon- gion(Fig. 1) hasbeenpresentedby Currayet al.sist of the high mountainchainsof the Himalaya, (1982), Valdiya (1984), Naqyi and Rogers(1987),a consequenceof protracted collision of India Molnar (1988)andVerma(1991).The space—timewith Asia over the past 38 my. This led to the variability of thegeotectonichistoryof the Indiansubduction of the Indian plate in the region forms an important set of problems inHindukush—Tibet—Burmaregions. Apart from earth sciences.It is a consequenceof past andthe abovecontinentalregionsof the Indian plate, continuinggeodynarnicprocesses,the unravellingthe study areaalso includes the Bay of Bengal, of which needsdetailedexplorationof the litho-which has the thickest young sedimentaryde- sphericarchitecture.posits.This hasimportantaseismicridges, at 85° Surface-wavetomographyhas emergedin theand 90°E. Recentcontroversyabout the Bay of lastdecadeas a powerful tool in 3-D imagingofBengal is centred around the explanation for the Earth. It has been extremely successfulinefficient shearwave transmissionthroughits crust studyingglobal structure(Nakanishiand Ander-

70 75 80 85 90 95 00I ~ I

3~- - 35

GEOLOGICAL FRAMEWORK OFTHE INDIAN REGION

-30

GETIC25 ... PLAIN ~ - 25

~ VINO HYAN ~ IBASIN ‘~‘~ I! r—~

‘~l~20 - DECCAN 4.. ~ rn - 20

TRAPS ~ B EN GAL ~

FAN / IKB ~ ~‘ / /

ARABIAN 7 C I:

INDIAN OCEAN ~j ~/ ~S / (n~ Q -5

SP -Shillong platiou ~ eCB-Cuddapoh basIn ~‘ IKB-Kalodgi BhImo bos,nG8-GodovarI bosin

01 I ~tio70 75 80 65 90 95 100

Fig. 1. Geologicalframework of the Indian region andlocation of seismologicalstations.

191SEISMIC HETEROGENEITIES IN INDIAN LITHOSPHERE

son, 1982, 1983, 1984; Nataf et al., 1984; Wood- TABLE 1houseand Dziewonski, 1984;Tanimoto, 1986a,b; Seismicstationsandtheir geographiclocations

MontagnerandTanimoto, 1990;ZhangandTani- Station Coordinate

moto, 1991) andregionalstructure(Yanovaskaya, Lat.(°N) Long(°E)

1982, 1984; Suetsuguand Nakanishi, 1985; Mon- New Delhi 28.680 77.220

tagner, 1986; Zhang and Tanimoto, 1989; Ha- Hyderabad 17.416 78.553

diouche et al., 1991). We investigatehere the Kodaikanal 10.230 77.466lateral heterogeneityin the Indian region from Shillong 25.560 91.880

Rayleighwave groupvelocity distributionsat van- Poona 18.530 73.850Bokaro 23.780 85.880

ous time periods(15—60 s) and correlateit with Vishakhapatnam 17.700 83.300the regionaltectonics. ChiangMai 18.790 98.977

2. Method andGilbert, 1970; Yanovaskaya,1982,1984;Tan-imoto, 1986b). We compute the lateral distribu-

The horizontal distribution of surface wave tion of Rayleigh wave group velocities over thevelocities V(x, y) correspondingto a time period Indian region adopting the B—G formalism de-T is computedfrom the velocity data for given scribedby Keilis-Borok (1989).pathscontainingsource—receiveror receiver—re-ceiverpairs.The lateralvelocity distribution V(x,y) can be written as V(x, y) = V0 + ~V(x, ~, 3. Datawhere 5V(x, y) is the velocity perturbationoveran averagevelocity V0 in the region. The aim is We usedthefundamentalmodeRayleighwaveto computeöV(x, Y). groupvelocitiesfor periodsrangingfrom 15 to 60

The travel time betweenpoints ~ y~)and s for source—receiverpairs in the Indian region.(x1~,y1~)is Details of the seismologicalstationsused in this

study are presentedin Table 1. Coordinatesof

ti=fLds/V(x, y)

where j = 1, N (N is the numberof pathsin the N50

region). The difference between observed and .—~

theoreticaltravel timesfor a path j is given by40-

~~tJ fLJ( 1 1 \———JdsV0+~V(x,y) Vol 30

NOl

=f m(x, y)V~ds 20 B

L~ P0IS H

wherem(x, y)= —6V(x, y)/V0. 10 ~ H

Computationof m(x, y) from a finite set ofdata ~t has beenattemptedthrough model dis-cretisation (Toksoz and Ben-Menahem,1963; 0 • Epec.ntr.

Santo and Sato, 1966; Dziewonski, 1971; Mills, ~ ,tatIon

1978;Leveque,1980;ZhangandTanimoto, 1989),Sbseriesexpansion(Sato and Santo, 1969; Nakan- 50 60 70 80 90 bOOE

ishi andAnderson,1982,1983; Natafet at., 1986) Fig. 2. Earthquakesource, recording and paths used for

and the Backus—Gilbert (BG) method (Backus Rayleighwave regionalisation.

192 G. MOHAN AND S.S.RAI

TABLE 2Detailsof earthquakeeventsandrecordingstations

Event Date Event Station ReferenceflO~ Lat. (°N) Long.(°E)

1 13 Oct. 1985 40.30 69.82 SHL2 22 Feb. 1984 39.40 54.02 SHL, CHG3 12July1986 29.95 51.51 CHG4 29 Oct. 1985 36.72 54.80 CHG5 06 May 1985 30.88 70.24 CHG Presentstudy6 01 Feb. 1984 34.68 70.54 CHG7 16 July 1986 30.94 77.87 CHG8 06 July 1986 34.42 80.07 CHG9 19 Feb. 1984 49.88 78.78 CHG

10 30May 1971 25.20 96.41 NDI11 17 July 1971 26.41 93.41 HYB12 16 Jan.1973 33.29 75.83 HYB13 31 May 1973 24.31 93.52 ND!14 24 Mar. 1974 27.66 86.00 HYB15 19 June1975 32.39 78.50 KOD16 29 July 1975 32.57 78.49 FIYB17 10 Dec. 1975 32.95 76.10 HYB18 7 Jan.1976 32.97 76.12 HYB Singh(1987)19 10 May 1976 29.33 81.46 HYB20 23 June1976 21.18 88.62 HYB21 8 Sep. 1976 32.03 78.76 HYB22 13 Nov. 1977 26.51 93.00 HYB23 23 Dec. 1977 23.71 92.31 HYB24 10 Feb.1978 28.03 84.70 HYB25 19 June1979 26.74 87.48 HYB

26 5 Nov. 1966 28.22 83.87 KOD27 3 Dec. 1965 —47.4 99.90 KOD-POO28 16 Dec. 1965 —47.53 100.11 KOD-POO29 6 Apr. 1966 —45.81 96.06 KOD-POO30 7 Jan. 1967 —48.80 112.76 KOD-POO Bhattacharya(1974)31 27 June1967 —48.38 96.03 KOD-POO32 21 Feb. 1967 33.65 75.44 KOD33 27 May 1967 36.07 77.66 KOD

34 13 Sep.1967 17.40 73.40 NDI35 20 Sep.1980 17.24 73.87 NDI, VIS Bhattacharya(1991)36 20Sep. 1980 17.26 73.64 NDI, VIS37 11 Dec. 1967 17.20 73.72 BOK38 12 Dec. 1967 17.30 73.85 BOK39 24Dec. 1967 17.34 73.96 SHL40 29 Oct. 1968 17.41 73.84 BOK, SHL41 3 Nov. 1969 17.51 73.92 BOK, SHL Bhattacharya(1981)42 15 Sep. 1967 27.42 91.86 P0043 18 Aug. 1968 26.42 90.62 P0044 25 July 1970 25.72 88.58 P00

45 28 Jan.1980 —3.39 88.87 HYB46 21 Aug. 1983 3.23 87.47 HYB47 17 Sep. 1983 7.90 93.16 HYB, VIS BruneandSlngh(1986)48 16 Dec. 1982 11.69 92.96 HYB, V!S

SEISMIC HETEROGENEITIES IN INDIAN LITHOSPHERE 193

TABLE 2 (continued)

Event Date Event Station ReferenceflOP Lat. (°N) Long. (°E)

49 26 Nov. 1968 —3.15 86.41 SHL50 14 Dec. 1968 —3.03 85.53 SHL Souriau(1981)51 14 Dec. 1972 —1.34 89.17 SHL, CHG52 19 June1963 25.00 92.10 ND!53 27 Feb. 1964 21.70 94.40 NDI Chaudhury(1965)54 28 Feb. 1964 18.20 94.30 NDI55 24 Jan. 1966 29.92 69.62 ND! \ Ch (198656 19 June1963 24.97 92.06 NDI J un

57 3 May 1971 30.79 84.33 NDI ChunandYoshii (1977)58 12 Sep.1965 —6.5 70.80 P0059 22 Sep.1969 —5.6 68.20 P0060 15 Aug. 1966 3.8 64.00 P00 5~ h 198861 3 Nov. 1966 6.6 60.40 P00 ing62 29 Mar. 1969 10.4 56.80 P0063 14 Sep. 1966 14.5 56.40 P00

eventsused,togetherwith recordingstationsand rameters,0 = 0.01, 0.05 and 0.1. Decreasein 0sourcesof data, are listed in Table 2. The data gives a sharpersolution regionandan increaseinset includes publishedgroupvelocity dispersion solution error, whereasincreasein 0 leads tocurves for 54 paths in addition to the data for smoothingof the solution spacewith decreaseinnine new paths to CHG and SHL computedby error. We present the regional Rayleigh groupus. Most of the datasets contain group velocity variations(Fig. 3), radius of smoothing (Fig. 4)dispersionfrom a singlestation, exceptfor a few andsolution error(Fig. 5) for time periods15, 25,where two-station (KOD—POO) data for events 40, 50 and 60 s correspondingto a smoothing(27—31) are used.As seenfrom Fig. 2, coverage parameterof 0.05. The resolutionradius of 500—of paths is not uniform throughoutthe region. 600 km covers a major part of peninsularIndia,The Shillong plateau, Indo-Gangeticplain, and with a velocity error of 0.05—0.08 km s’. Thecentral and south—centralIndia havebetterpath velocity solutions are contoured at 0.1 km s~coveragethanthewesternandsouthernmostparts intervals.This resultprovidesbetter lateral reso-of India. The Bay of Bengalhas a bettercoverage lution over the Indian regioncomparedwith thethan the Arabian Sea.We processedthe data existing global and regionalstudies.At a certainfrom stations CHG and SHL using frequency period the surfacewave groupvelocity is an inte-time analysis (FTAN) to obtain group velocity gral of velocities to a correspondingdepth. Thedispersioncurves for paths that crossthe axis of depth of penetrationof surfacewavesincreasesthe Himalaya. with increasingperiod. In view of this, we draw

somebroadgeophysicalinferencesaboutthe sur-facewave heterogeneitiesin the Indian region.

4. Velocity distribution and geologicalcorrelation Up to a time period T= 50 s, the Indiancontinental plate can be broadly divided into

Velocity perturbation, smoothing radius and southernandnorthern,characteriseddistinctly bysolution accuracywere computed at nodesthat high andlow Rayleighwave groupvelocities.Theare 1°apartin a 35°X 40°grid coveringan area southernblock in generalexhibits10—20% higherin the Indian region betweenlatitudes5°Sand velocities than the northern block. This may be35°Nandbetweenlongitudes65°and 100°E.Cal- critically related to the origin of continentalcrustculations were made for several smoothing pa- in India. SouthernIndia has an Archean—Pre-

194 G. MOHAN AND S.5. RAI

cambrian (3.4—1.8 by) crustal orogeny,whereas tons. This crustal feature does not appear tothe northerncrustalblock evolved at closeto 1.4 continueinto the uppermantle.The moststableby. At shorter periods (15—25 s) a high-velocity part of the Indian continentalregion, the Dhar-(3.4 km

5_i) north—southelongatedfeature is war craton, is characterisedby the highestS-waveobservedover central India, at the contactzone velocities,which range from 3.5 to 3.8 km s~inbetweenthe Dharwar, Bastar and Aravalli cra- the periodrange25—60 s. The southernmostpart

T~I5Sec T5OSec35N

25

~ ~ ~, ~0’JJ)J

~ I 00e~~~\\~ L~I I I _________

______ IS

• T-25Sec T~6OSec35N ~

~ ___25l5 -o• 25 -I M~I/foI 0• Ii 01 00 RJiS 65E 75 85 95• T~4OSec

35N

~ ~ob 0 0.

Contour Value in ,—.,

Km / Sec ) 1006)/ °~~T~

S 65E 75 85 95

Fig. 3. Rayleighwave group velocity variationsin theIndian region at time periods15, 25, 40, 50 and60 s.

SEISMIC HETEROGENEITIES IN INDIAN LITHOSPHERE 195

T.l5Sec T~5OSec35~

ci I I ______________ o

S• T~25Sec T-6OSec35

~-<~ ~ 0025N

0 ~ v

I I I I I I I I I I

S 65E 75 85 95• T~4OSec

35N

25

C7

Contour Value in Km (~~)

0• -

5 I I I IS 65E 75~ 85 95

Fig. 4. Radiusof smoothingof the solution(time periodsas in Fig. 3).

of the Deccantraps shows an anomaloushigh- but probably reflects the Precambrianbasementvelocity (3.9—4.1 km ~I) signatureat time pen- tectonicsbeneath the Deccantraps. At shorterods40—50 s. As a result of progressivethinning of period (T = 15 s) the granulite terrains of thethe basaltflow from northto southin the Deccan southand easternIndian margin show distinctlytrap region, this anomaly,which is concentrated lower velocities (2.8—3.0 km s~)in contrasttoin the southernpart of the Deccantrap, is un- 3.1—3.2 km s~ observed over the adjoininglikely to be related to the Deccanbasaltgenesis, Dharwarcraton.The Godavari rift, at the junc-

196 G. MOHAN AND S.S. RAI

tion of the DharwarandBastarcratons,showsan gravity(—90 mgal) anomaly(Qureshyet al., 1968).anomalouslow velocity at time periods 15—60 s. In NE India, the high Shillongplateauexhibits aIn contrastto the Dharwarcraton, the Godavari possibly colder lower crust—uppermantle,as re-grabenexhibits a 5—10% slower S-velocity. This flectedby high S-wavevelocitiesat periods25—60region is also characterisedby a significant low s. This is in conformitywith the presenceof deep

• T~I5Sec T~50Sec35

25N ~

I5 ~

0

• o~3~~//

- 0o~fl L-/~0~I I I __________

_______ I

ST~25Sec T•6OSec

35.

2 5• ~

N ~ ~~00,/

I ~

c~(j) O~ /0 oe~ ~ - I

S 65E 75 85 95• T~4OSec

35N /~~O0c~)CO

Contour Value in(Km/Sec) 0-06~ ~0-o~ Q~ i

I \ ~S 65E 75 85 95

Fig. 5. Errorsassociatedwith the Rayleighgroup velocity computations(time periodsas in Fig. 3).

SEISMIC HETEROGENEITIES IN INDIAN LITHOSPHERE 197

lithospheric earthquakesand the inference of The oceaniccrust beneaththe Arabian Sea ap-colderlithospherein the Shillong plateauregion pearsto resemblethat beneaththe Bay of Ben-(Chen and Molnar, 1990). gal, as evidencedby the velocity characteristics.

The Indian plate collision zonesin the north(Indo-Asian) and east (Indo-Burma) show dis-tinctly different S-velocity characterswith depth. AcknowledgementsFor a 15 s period, the Nepal—Himalayanzoneshows relatively higher S velocity (2.8—2.9 kms~)compared with 2.3—2.7 km ~_l over the This work was initiated by G. Mohan duringIndo-Burmeseregion. Such a contrastcould be his visit to the InternationalCentrefor Theoreti-due to the presenceof the sedimentarybasin cal Physics, Trieste, Italy, under the fellowshipapproximately13 km thicknessto the eastof the ‘Training and Researchin the Italian Laborato-Indo-Bunmese arc (Mitchell and McKerrow, nies’. He thanksthe Director, ICTP, for hospital-1975). At greater periods (25—60 s), the Indo- ity at ICTP. This paper is a contributionto theBurma collision zoneshowsmuch highervelocity activities of the ILP task group 11-4: Three di-(3.4—4.0 km s’) in contrastto the lower Sveloc- mensionalmodellingof the EarthsTectosphere.ity (3.0—3.3 km s1) over the northernIndo-Asiancollision zones.This velocity contrastis viewedasa reflection of the variedsubductionprocessesof Referencesthe Indian plate in these two zones. The highS-velocity lithospheresupportsthe Benioff sub- Backus, G. andGilbert, F., 1970. Uniquenessin theinversion

duction of Indian plate in the Indo-Burmesene- of inaccurategross earth data. Philos. Trans. R. Soc.

gion (Mitchell and McKerrow, 1975).This high- London,226: 123—192.

velocity, steeply subducting Indian lithosphere Bhattacharya,SN., 1974. The crust—mantlestructureof the

beneathBurma is responsiblefor the deep-focus Indian Peninsulafrom surfacewave dispersion.Geophys.J.R. Astron.Soc., 36: 273—283.

earthquakes(up to 180 km) in the region. Bhattacharya,S.N., 1981. Observationand inversion of

The Bay of Bengal is markedby severalinter- face wave groupvelocitiesacrosscentral India. Bull. Seis-

estingfeatures.At 15 s period, Rayleigh group mol. Soc. Am., 71: 1489—1501.velocities are anomalouslylow (2.4—2.6 km s~) Bhattacharya,S.N., 1991. Surfacewave andlithosphericstruc-

in contrastto 2.8—3.2km s~oven thecontinental lure acrossthe northwesternpartof the Indian peninsula.Pageophys.,135: 53—60.region. This could be due to an anomaloussedi- Brune, J.N. and Singh, D.D., 1986. Continentallike crustalmentarycolumn thickness,which increasespro- thickness beneaththe Bay of Bengal sediments. Bull.

gressivelyfrom 1 km in the southernpart(5°S)to Seismol.Soc. Am., 76: 191—203.13 km near the continentalshelf of Bangladesh. Chaudhury,H.M., 1966. Seismicsurfacewave dispersionand

At longer periods (25—40 s), the Bay of Bengal crust acrossthe Gangeticbasin.Indian J. Meteorol. Geo-phys., 17: 385—393.

shows a progressivevelocity decreasefrom south Chen, W.P. and Molnar, P., 1990. Source parametersof(4.1 km s I) to north (3.0 km s 1)~This is easily earthquakesand intraplatedeformation beneaththe shil-

explainedby the increasingsedimentarycolumn long plateau and the northern Indoburmon ranges.J.

thicknesstowardsthe north. Betweenlongitudes Geophys.Res., 95: 12527—12552.

85 and 95°Ethe velocity contoursare elongated Chun, K.Y., 1986. Crustal block of thewesternGangabasin:A fragmentof oceanaffinity? Bull. Seismol. Soc. Am., 76:

north—south,the maximabeing towardsthe cen- 16871698.

tre (4.1 km s I)~ The observedvelocity contours Chun, K.Y. and Yoshii, T., 1977. Crustal structureof thein fact closely follow the axis of the Ninety East Tibetanplateau:A surfacewave studyby a moving win-

Ridge (Curray et al., 1982). The Bay of Bengal, dow analysis.Bull. Seismol.Soc. Am., 67: 735—750.

surprisingly, exhibits much highervelocitiesthan Curray, R.J., Emmel, F.J., Moore, D.G. and Raitt, R.W.,1982. Structure, tectonics and geological history of the

the continentalIndia for the time periods25—50 northeasternIndian Ocean.In: E.M. Nairn andG.F. Stehlis. Theseobservationsare in conformity with the (Editors),TheOceanBasinsand Margins,Vol. 6. Plenum,

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