Origin of the Ninety East Ridge from Studies Near the Equator

VOL. ?8, NO. 26 JOURNAL OF GEOPHYSICAL RESEARCH SEPTEMBER 10, 1973

Originof the NinetyEast Ridge from Studies near the EquatorI

CARL BOWEN

WoodsHole OceanographicInstitution, WoodsHole, Massachusetts02543

A partof the NinetyEast ridge near the equatorwas examined in 1971by seismicprofiling and gravityand magneticobservations. In the areaexamined, the topographyof the ridge consistsof blocklikeor en echelonmountainous masses. A fracturezone trending north-south parallelto the overalltrend was found alongthe easternmargin of the ridge topography. This fracture zone probably marks the principal boundary between the central Indian Ocean plate and the Wharton basinplate. The free air gravity anomaliesassociated with the Ninety East ridge are small, and thus the mass of the ridge must in some way be compensatedat depth. The Ninety East ridge may have originated as a result of emplacementof gabbro and serpentinizedperidotire beneath normal oceanic crustal layers. The lower density of the gabbro and serpentinized peridotire with respect to normal mantle at equivalent depths provides for both the uplift of the ridge and its compensationat depth.

The Ninety East ridge is remarkablystraight east of the ridge at 4ø0.2'S, 90ø46.9'E. The and extendssome 5000 km along a north-south geophysicalmeasurements include 3.5-kHz echo trend in the eastern Indian Ocean. Its bearing sounding,seismic profiling with a 40-in? air gun is approximatelyNSøE, and it intersectslongi- and an 80,000-joule sparker as sound sources tude 90øE near the equator. Laughton et al. (a slight modification of the system described [1970], who summarizedstudies of the Ninety by Knott and Bunce [1968]), a vibrating- East ridge to that date, noted that its great string accelerometergravity meter [Bowin et extent was first shown on a bathymetric chart a/., 1972], and a Varian proton precession of Stocks [19'60] and that it was a major new magnetometer.The data from this study, to- feature revealed in the Indian Ocean by the gether with information from other parts of International Indian OceanExpedition, a multi- the ridge (Figures 1, 2, and 3), are interpreted nation program. Over its entire length it ap- in an attempt to explain the topography of the pearsto be a singleridge rising generally1500- ridge. Previous interpretations have suggested 2000 meters above the surrounding sea floor. that the Ninety East ridge is a horst-type However, south of about 10øS, a complex of structure [Francis and Raitt, 1967; Laughton ridges and troughs trending parallel to the et al., 1970] or the result of overriding of one Ninety East ridge occursin the basin area to oceanic crustal plate by another [Le Pichon the east [Heezen and Tharp, 1964, 1966; R. L. and Heirtzler, 1968]. McKenzie and Sclater Fisher, unpublishedbathymetric chart, 1968]. [1971] refer to it as marking a fracture zone In May 1971 a study of the Ninety East without specifyingits structure. Veevers et al. ridge betweenthe equator and 5øS (Figures 1, [1971] proposed that the ridge had been a 2, and 3) was conductedaboard RV Chain of center of sea floor spreading.Morgan [1972] theWoods Holb Oceanographic Institution dur- classifiedit as having been produced by a ing leg 6 of cruise 100. Continuous seismic plume from the deep mantle. profiling and bathymetric, gravity, and mag- DEEP-SEA DRILLING netic observations were conducted on a series of crossingsof the ridge, and one dredge haul of Deep-Sea Drilling Project (DSDP) results weathered basalt was obtained from a scarp have indicated that reasonably complete sec- tions from Upper Cretaceousor Paleoceneto the Recent occur both on the Ninety East ridge • Contribution 3023 of the Woods Hole Oceano- and in the basins to both the east and the west graphic Institution. [von der Botch et al., 1972]. In the basinsthe Copyright ¸ 1973 by the American Geophysical Union. Mioceneto Pleistocenesection thins greatly from

6029 75 ø 80" 85" 90" 95" 100"

10" 217 ß 218 ß

3o o

21:5 - 10" - 25 214 • LSD-55 I I ..r' LSD-55(•• 15 ø LSD-54 _

LSD-52

212 ß 20"

25 ø

50 ø

55 ø I I I Fig. 1. Chart of the Ninety East ridge. The part of the ridge shownin Figures2, 4, and 5 is outlined. The location of the inferred fracture zone to the east of the ridge is indicated by the line trending nearly north-southin the easternpart of the area between0 ø and 5•S. The dashed line to the south is the inferred southward continuation of the fracture zone. The dots with underlined numbers indicate DSDP drill hole locations from yon der Botch et al. [1972]; the• dots labeled LSD indicate seismic refraction profiles from Francis a,nd Raitt [1967]. The locations of magnetic anomalies 23, 25, and 30 and transform faults west of the ridge are from McKenzie and Sclater [1971, Figure 21.] BOWIN' ORIGIN OF NINETY EAST RIDGE 6031

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i 85 ø Fig. 2. Locationchart for gravity and magneticanomaly and continuousseismic reflection profilesover the Ninety East ridge shownin Figure 3. Ship's track is that of RV Clia.in cruise 100, leg 6, May-June 1971.The short ticks crossingthe track lines indicate the location of scarpsoccurring at the easternmargin of the ridge, or irregular subbottomtopography in the caseof profile 2. The heavy dashednorth-south line indicatesthe inferred location of the fracture zone (transform fault) between the Indian Ocean and Wharton basiri plates.

8øN to 8øS, and the turbidire layers commonin tion also supportsthe suggestionmade by the northernpart do not extendto 8øS,whereas Ewing et al. [1969] that increaseddeposition of the ridge sedimentsare more calcareousthan calcareoussediments took place on the Ninety thoseof the basins.The depositionof Miocene East ridgebecause it wasabove the carbonate to Pleistocene sediments of the Bengal fan compensationdepth. [Curray and Moore, 1971] explains why the The scientificstaff of leg 22 [vo'n der Borch sedimentarysection is seen to be thicker be- et al., 1972] and leg 26 [Luyendyket al., 1973] neath the ocean basin on both the east and concluded that the basal sediments of the the west•sides of the ridgethan it is on the ridge Ninety East ridge increasein age northward, itself and why the horizontal layered sediments that early in its history the ridge reachedsea of the basins on both sides of the ridge thin level, and that it has subsidedas it migrated to the south. In the original recordsof profiles northward. 1, 2, 4, and 5 (Figure3) thereis goodindicaiion MAGNETIC ANOMALIES that most of the sedimentary section that occurs on the crest of the ridge is overlapped Magneticanomalies 23, 25, and 30 of the by the flat-lyingsediments on the Westernflank Heirtzler et al. [1968] time scale have been of the ridge,supporting the DSDP and Curray identified in the Indian Ocean basin to the west and Moore [1971] results.The DSDP informa- of the Ninety East ridge by McKenzieand 6032 BOWIN' ORIGIN OF NINETY EAST RIDGE BOWIN' ORIGIN OF NINETY EAST RIDGE 6033 $clater [1971]. They concluded that these anomalies are offset by two transform faults (Figure 1). The most compellingidentification is of anomaly 30 near 8øS, 83øE. To the east of Ninety East ridge, east-west-trendingmag- netic anomalies 25 and 26 have been identified near 10øS, 94øE by Sclater (reported by von der Botch et al. [1972]). The age of basal sediments (54-58 m.y.) in DSDP hole 213 at. that site suggeststhat the observedmagnetic anomalies there are younger than magnetic anomalies 25 and 26 (62-64 m.y.), that the age assign- ments are somewhat in error, or that there is a hiatus between the basal sediments and the underlying basalt.

TOPOGRAPHY AND FAULTING Le Pichon and Heirtzler [1968] concluded that the blocky profile of the ridge at 17øSand the steep east-facing scarp there suggestthat the western crustal plate overrode the eastern crustal plate. However, the eastern flank of the ridge does not always have the steeper scarp, nor is there everywhere a scarp. The profiles prepared by Hilde in about 1967 and reproducedby Laughton et al. [1970] illustrate the topographic variations from 10øN to 33øS. In many of these the eastern flank of the ridge doeshave a steeper scarp than the western one [Laughton et al., 1970, Figure 9, profiles C, E, H, I, J, K, N, and R]. But other profiles (B, I), M, O, and P) show the western flank to have a slightly steeperaverage slope. In the area between 0 ø and 5øS, scarps are most prominent on the eastern flank of the Ninety East ridge. They are particularly strik- ing in profiles 2 and 6 (Figure 3), where the trough between two blocklike sectionsof the ridge is.crossed. Profile 2 occurswhere the ridge is very narrow (Figure 2) and contains the only observedscarp that has a relief equivalent to that of the elevation of the ridge crest. These scarps suggest that here block faulting has contributed to the formation of the block topography, which is en echelonbetween 2øS and 5øN, although it is not thought to be the principal agent. On the western flank a fault scarp with a vertical separationof 0.65 sec (two-way travel time) is shown in profile 2 (Figure 3). The same or greater displacementmight occur on the scarp shownin profile 4 (near 1400 on May 6034 BowIN: ORIGIN OF NINETY EAST RIDGE 30, 1971). Some scarpswith considerablyless the Ninety East ridge itself does not. The relief are seenin profiles3, 4, and 5. For the correlative conclusion that the Ninety East most par•, however, the sedimentarysection ridge belongsto the Indian plate is supported simply slopes,with small irregularities, from by the sedimentsof the DSDP drill holes,which the crest of the ridge down the western flank, suggestthat the Ninety East ridge was part of as is observedparticularly in profile 1. Within the Indian plate in the late Cretaceous[yon der the well-stratified sediments, fault displace- Botch et al., 1972]. ments are seenin a few placeson the crestand SEISMIC REFRACTION AND GRAVITY alsoin placesin the upper well-stratifiedsection of the western basin. The limited evidence for Seismicrefraction profiles between 13øS and faulting on the westernflank also supportsthe 14øSfrom the Ninety East.ridge into the Whar- hypothesisthat block faulting is only a second- ton basin have been reported by Francis and ary agent in the elevationof the ridge. Raitt [1967]. They conclude from these data Away from the 0ø-5øS area, evidence for that the oceanic crustal layers have about the block faulting is similarly inconsistent.Seismic same thicknessbeneath the ridge and the basin reflection profiles across the ridge near 7øS, to the east, unlike the great thicknessof crustal 12øS, and 25øS show prominent scarps [Mc- material usually found under compensatedlarge Kenzie and $clater, 1971, Figure 48]. Faults volcanic island structures. Francis and Raitt with only small displacementare observedin [1967] suggestedthat the ridge is a horst-type the seismicreflection profiles to the north near feature rather than a volcanic accumulation 6øN and 7øN •Curray and Moore, 1971, Figure along a line of weaknessin the earth's crust 4]. Near 2øN a prominent scarp occurson the and noted that the large scarpsvisible on many east margin, but only minor faulting is observed bathymetric profiles of the ridge support the on the westslope [Ewing et al., 1969,Figure 4]. horst interpretation. Their refraction informa- In the profilesat 7øN, 6øN, 2øN, 7øS, 12øS, tion further indicates that the material with and 25øS, only the scarpson the west side of oceaniccrustal velocity, layer 3, lies about I km the ridge at 7øS and on the east side at 25øS shallowerbeneath the ridge than in the adjacent have enoughrelief to accountfor the elevation basin and that the compressionalwave velocity of the acoustic basement at the crest of the of the material underlyingthis layer appearsto ridge, and thus theseprofiles also do not offer increasefrom ridge crest to basin. They postu- adequate support for the conclusionthat the late that perhaps,after the ridge was elevated, elevationof the ridgeis principallydue to block the discontinuity migrated downward as a faulting. result of phasechanges and left a high crustal Other evidence of faulting on the eastern velocity (7.07 km/•c) as a 'fossil' boundary. flank in the area between 0 ø and 5øS is seen The seismic refraction profile reported by in the small block faulting structures of the Neprochnov et al. [1964] (shownby Laughton easternmostedge (profiles 1, 3, and 4; Figure et al., 1970, Figure 11) indicates that normal 3). These structures, however, occur along a mantle velocities (8.0 km/sec) occur beneath nearly north-southline subparalld to the over- the Indian Ocean basin on the west side of the all north-south trend of the ridge (Figure 2) ridgeas well as in the Whartonbasin. and at a large angle to the trend (about N45 ø- If the Ninety East ridge were simply an up- 80øE) of the blocklike structures of the ridge lifted blockof oceaniccrust as in a simplehorst- in plan. They thereforesuggest the existenceof type feature,a marked positivefree air anomaly a throughgoingnorth-south fracture zone on (more than q-160 mgals) over the crest would the east side. Irregular subbottomtopography be expected.For example,if the body in Figure shown on the east edge of profile 2 is in line 7 with a densityof 3.05 g/cm-• had a densityof with the postulated fracture zone, although, as 3.40 g/cm% the same as the mantle, the maxi- a result of the en echelonstructure, the ridge mum free air anomaly value over the crest proper lies some distance to the west. This would be q-180 mgals. In the area between0 ø fracture zone may mark the principal boundary and 5øS there is a positive free air gravity betweenthe central Indian Oceanplate and the anomaly, but the anomaly differencebetween Wharton basin plate, and hence I suggestthat the crest and the flanks is not great, between B0wIN: ORIGIN OF NINETY EAST RIDGE 6035 30 and 75 mgals, or about 47 mgals on the air anomaly high is associatedwith this ridge, average (see inset, Figure 4). The gravity pro- but it is generally lower than that over the files (Figure 3) show that, for the most part, Ninety East ridge on the other side of the the free air anomaly parallels the topography. free air minimum [Talwani and Kahle, 1973]. The variations with topographyare presumably It seemsprobable that the ridge to the east of due to the indirect effect of the positivemass of the topographicdepression will have a structure the topography, which is closer to the site of and an origin similar to those of the Ninety the measurementthan the compensatingnega- East ridge. It may be a complementarystruc- tive mass at depth. In the chart of free air ture that developed within the margin of the anomaly (Figure 4) the western edge of the Wharton plate in a manner similar to that of ridge is better defined than the eastern, where the Ninety East ridge in the Indian Ocean the contour pattern is more irregular. Because plate. the free air anomaliesassociated with the ridge FORMATION OF THE NINETY EAST RIDGE are small, its mass must in some way be compensatedat depth. Similarly, only small free air Several types of possiblestructures for the anomaly values (up to somewhat over 325 Ninety East ridge are diagrammedin Figure 6. mgals) occur elsewhere over the crest of the The origin proposed by Francis and Raitt ridge [Talwani and Kahle, 1973], so that these [1967] has been discussedabove. The initial conclusionsmay also apply to other parts of structure of their proposedmodel would have the ridge. been, presumably, like that in Figure 6f. Le The simple Bouguer anomalies (Figure 5) Pichon and Heirtzler [1968] concludedthat the show steeper gradients over the western margin blocky profile of the ridge at 17øS and the than over the eastern margin, where there is steep east-facingscarp there suggestedoverrid- generally a very gradual gradient. The mini- ing of the eastern crustal plate by the western mum is located near the crest. The steeper crustal plate, presumably as shown in Figure western-flank Bouguer anomaly gradient sug- 6a. They accounted for the overthrusting by gests that the boundary of the compensating assuming differential movement on either side mass has a steeper slope here than at the of the Ninety East ridge as the result of con- eastern flank. verging strikes of the central and southern The fracture zone occurringto the east of the Indian Ocean ridges on either side of the Am- ridge appears to have an associatedfree air sterdam fracture zone. This could yield an anomaly minimum (Figures 3 and 4). A pro- isostatically compensated structure, but it nouncedminimum (with values more negative would not be concordant with the seismic than --75 mgal) is shown south of 8øS on the refraction results of Francis and Raitt [1967]. compiledfree air anomaly chart of the Indian Laughton et al. [1970] adopted a horst-type Ocean prepared by Talwani and Kahle [1973]. origin for the ridge (as in Figure 6e), depicted The free air anomaly minimum continues to in their Figure 11, a composite crustal section the south to at least 21øS and is coincident with across the central Indian Ocean. a topographic depressionshown on the unpub- Simple formation of the Ninety East ridge lished compiled contour map (southern bound- by horst-type uplift (Figure 6e or 6f) or by a ary of the map is at 16øS) prepared by R. L. fold or upthrust as a result of convergenceof Fisher (1968). The topographic depression two plates (Figure 6g or 6h), would lead to an and free air anomaly minimum appear to lie elevation of excess mass that would result in along a southern extensionof the fracture zone marked positive free air gravity anomalies. found to the east of the Ninety East ridge be- Earlier in this paper it was pointed out that tween the equator and 5øS. It is inferred that large free air anomalies do not occur over the the topographic depressionand free air anom- ridge; thus the mass of the ridge is in some aly minimum south of 8øS do indicate the way compensatedat depth (as in Figures 6a, location of the transform fault that separated 6b, 6c, and 6d). The ridge is too broad (nearly the Indian Oceanand Wharton basinplates. 200 km wide) to be a horst in isostatic equi- Southof 10øSa ridgeoccurs, trending parallel librium (Figure 6b), and, as was mentioned to, and on the east of, the depression]A free previously, the seismic refraction data do not 6036 BowIN' ORIGIN Or NINETY EAST RIDGE

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o: i ! o o o • • • o' o• BOWIN: ORIGIN OF NINETY EAST RIDGE 6037 indicate a thickened crust of volcanic material tion of overlying normal oceaniccrustal layers. (Figure 6c), although, clearly, new seismic I infer that this material is a mixture of gabbro refraction data are needed to substantiate the and serpentinized(hydrated) peridotite. Since results of Francis and Raitt [1967]. The north- these rock types are less dense than normal ward age gradient on the ridge crest docu- mantle at equivalent depths, they provide the mented by the DSDP resultsrule out an origin requisite compensationand are capable of hav- of the ridge as either a 'leaky' fracture zone ing producedthe uplift of the ridge. or an axis of sea floor spreading. A simplified structure model across the Francis and Raitt's [1967] hypothesized Ninety East ridge that is consistentwith bathy- uplift of the ridge, followed by a downward metric, seismic, and gravity observations is migration of the discontinuity due to phase shown in Figure 7. The general magnitude of changes,would have resulted in initially large the root of lower-density material is also indi- positive free air anomalies that would have cated. The root may have a stratified density decreasedwith time as compensationresulted variation, as is suggestedby the changein the from the phase changesoccurring during the deepestdetermined seismic • velocities from 7.94 downward migration. Following the downward km/sec at the eastern edge to 7.07 km/sec at migration of the discontinuity, the structure the ridge crest [Francis and Raitt, 1967], but would be like that in Figure 6d and could there are insufficientdata tq resolve its distri- result in a compensatedridge. However, Francis bution. and Raitt [1967] did not state the specificsof When did the emplacement of the material the proposedphase change, nor did they explain with intermediate compressionalvelocity, and why the phase changewould have occurredat a hence the uplift of the Ninety East ridge, shallower depth below sea level beneath the occur? McKenzie and Sclater's [1971] argu- crest of the ridge and not also further below ment, prior to the DSDP drilling, was that up- sea level beneath the crust on the flanks of the lift of the ridge probably occurred after some ridge. Furthermore, I claim that the Ninety time in the Eocene. Their evidence was the East ridge has always had free air anomalies lack of an elevated fracture zone south of the near zero and thus has remained in near iso- southeast Indian ridge, the occurrenceof cal- static equilibrium throughout its development, careous microfossils of Paleocene and Eocene a theory that also requiresanother explanation age in dredge hauls from various parts of for the origin of the compensatingroot for the the ridge, and the recoveryof calcareousmicro- ridge. fossilsof Tertiary age from one haul indicating Although fault scarps do occur locally on that the elevation of the ridge must have been the margins of the ridge, they are not every- above the carbonate compensationdepth by where apparent.From this evidence,I postulate the Eocene at the latest. They thus concluded that the basicstructure of the ridge is anticlinal that ridge uplift occurred at the end of the with local secondaryfaulting. The ridge origi- period of rapid spreadingin the Late Cretace- nated from the emplacementof 7.07- to 7.94- ous to early Tertiary. km/sec material concomitantlywith the forma- The leg 22 DSDP resultsnow provide evidence for a different interpretation. It seems clear that the oldest sediments on the crest Fig. 4. (Opposite) Free air gravity anomaly chart of a part of the Ninety East ridge. Contour of the ridge were depositedat, or near, sealevel interval is 25 mgals. Bathymetry corrected for and that in the adjacentbasin the oldestsedi- variation of sound velocity in sea water (Mat- ments of about the same age were depositedin thews tables) is shown by contours (at intervals deeper water. The underlyingbasalt also pro- of 500 meters) from R. Fisher (unpublished data, 1968) and modified in accordance with data ob- vides some evidencethat the flows on the ridge tained during cruise 100 of the RV Chain. Inset occurred in shallower water than the flows in shows a plot of the free air anomaly as a func- the borderingbasins. The vesicularand amyg- tion of water depth. Ship track control is shown daloidal basalt obtained from holes 214 and by the thin lines. Data are from cruise 100 of the RV Chain and from NOAA ships Oceanog- 216 on the ridge is in contrast to the pillow rapher and Pioneer. Dotted line indicates loca- lava basalt without notable vesicles obtained tion of crustalstructure sectionshown in Figure 6. from the deep-seafloor (holes211,212,213, and 6038 BOWIN'ORIGIN Or NINETY EAST RIISGE

l" BOWIN: ORIGIN OF EAST RIVG•. 6039 215). A lower confiningpressure on the ridge COMPENSATED UNCOMPENSATED crest during solidificationof the basalt is likely E ß to be the reason for the vesicular basalt there. This evidences.uggests that the ridge was uplifted, at least in large part, before the upper- most basaltic crust there was formed and thus is approximatelycontemporaneous with the sea floor spreadingof the Indian Oceanplate. Subse- quent subsidenceof the ridge crest (from near c G sea level) (Figure 8) is most probably in responseto the general Sdbsiden.ceof accreted ocean floor with time [von der Botch et al., 1972]. If, as it is inferred, the elevation of the ridge is primarily due to the emplacementof gabbro and serpentinizedperidotitc at a localizedspot ...... •:•::•:•::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::: ======on an axis of sea floor spreading, the crustal Fig. 6, Possible cross section structures for the layers, or at least the upper basalt, appear to Ninety East ridge. Those on'the left are isostatic- have formed on top of an uplifted part of •he ally compensated, and those on the fight are not. cres• of the ancient Indian Ocean sea floor Only the structure of diagram D is compatible with both seismic refraction and gravity data. spreading rise. Formation of the crust (layers 2 and 3) under theseelevated conditions may account for the fact that their combined thick- composition different from that of normal ocean nessis less over the ridge than in the basin •o basaltsis to be expected[Morgan, 1971, 1972; the east [Francis and Raitt, 1967]. Presumably, Schilling, 1973]. local faulting occurred concurrently with the However, it may be that the sourcedepth of uplift of the ridge and in responseto later ad- the Ninety East mantleplume was not as deep justments of stress. Possibly, other aseismic as th&• beneaththe Hawaiian-Emperorchain ridgeshave a structure similar to that proposed or Iceland. Perhaps it arose from a shallower here for the Ninety East ridge. depth where horizontal motions very similar Localizedemplacement of gabbro and serpen- to thos.e at the surfaceoccur. This might explain tinized peridotitc on the axis of a spreading the apparent concordanceof the trend of the ridge may be the result of a mantle plume or Ninety East ridge with that of the inferred deep-seated'hot spot' [Wilson, 1965; Morgan, transform fault on the east. Otherwise,it would 1971, 1972] that perhaps did not have the be necessary to postulate that the absolute conditions requisite for the formation of a motion direction (relative to a deep source of large volcanic structure (Figure 6c) such as the mantle plume) of the old Indian Ocean Iceland, the Azores, or Hawaii. If so, the trend plate was the same as the relative motion direc- of the Ninety East ridge shows,as is proposed tion betweenit and the Wharton basin plate in by Morgan [1972], the direction that the old orderto explainwhy the trend of the ridgeand Indian Ocean plate moved as it was formed. that of the fracture zoneon the eastare parallel. It is conceivablethat the mantle plume may A shallowermantle source in some way may have risen beneath a location just slightly off have resultedin the emplacementof gabbroand the axis of spreadingand thus might explain serpentinized peridotire beneath the ridge the apparent lack of a feature similar to the rather than in the fo•rmation of a large volcanic Ninety East ridge southwestof the Kerguelan structure. ridge. Dependingon the offset of the plume If the inferencesof the previous paragraph from the spreading axis, the basaltic crust are correct, the Ninety East ridge may offer an shouldbe somewhatyounger over the Ninety opportunity to estimate the differential velocity East ridge crest than in the basin to the west. between the top of the lithosphere and the Also, if the basalt of the crest were formed depth in the mantle where the plume originated, from deep mantle plume material, a chemical perhaps within the asthenosphere.Estimates 6040 BOWIN: ORIGIN OF NINETY EAST RIDGE of the depth of the plume sourcemight be further DSDP drilling. The Ninety East ridge possiblefrom chemicalcomposition studies. may thus offer a unique opportunity to de- Differential velocitiesmight be obtained by termine a velocity gradientbetween the mantle determiningthe magnitudeand directionof an and the lithosphere. increasingage differencealong the length of The inferred trend of the fracture zone on the ridge,between the crust of the ridgeand the east side of the Ninety East ridge (Fig- the crust in the basin to the west, through ure 1) has a small changein directionbetween

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MASS PEt? UNIT AI•EAIN A COLUMN •2•.oo-• • 40 /rmDEEP •2,o0o• - • L ( •'g/crn z) 12•00 -• Fig. ?. Crustalstructure model acrossNinety East ridge at 4øN (locationshown by the dotted line in Figure 4). Zero horizontaldistance in the model is at $gø(YE.The projected locationsof the seismicrefraction pro•es of Franc• and R½• [ 1967] are shownby arrows. The elevationsof the crustal]ayers for LSD-52 were raisedso that the sea floor elevations would coincide at the location of the structure model. The densities for the model are in gramspe• cubiccentimeter and are inferred from the •afe-Drake relationof densityto compressionalwave velocity give• by •• e• •. [1959].The gravitationalattraction of the modelwas computed by thepolygon method of •a• a• •. [1959].The dottedand crossed circles indicate the inferred motion toward and away from the reader acrossthe transform fault alongthe easternside of the ridge,respectively. The uppermostcurve is the complete Bougueranomaly corrected for the two-dimensionalshape of the watermass. The two curves beneath are the measured(thin line) and calculated(line with dots) free air anomalies.The rms differencebetween the two curvesis 5.8 mgals.The massper unit area is determined from the densitiesof the model. The material with density3.05 g/cm• is inferredto be gabbroand serpentinizedperidotite. I•OWIN' ORIGIN OFNINETY EASTRIDGE 6041

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o o 6042 BOWIN: ORIGIN OF NINETY EAST RIDGE the straight ridge part and the segmented Heezen, B.C., and M. Tharp, Physiographic blocklike part. Further, it appears probable diagram of the Indian Ocean, the Red Sea, the South China Sea, the Sulu Sea and the Celebes that there is another changeof direction at the Sea, Geol. Soc. of Amer., Boulder, Colo., 1964. south end of the dashedline in Figure I to a Heezen, B.C., and M. Tharp, Physiography of more nearly north-south trend along the south- the Indian Ocean, Phil. Tran•. Roy. $oc. Lon- ern section of the Ninety East ridge. From the don, Set. A, 259, 137-149, 1966. limited bathymetric data available, it appears Heirtzler, J. R., G. O. Dickson, E. M. Herron, W. C. Pitman III, and X. Le Pichon, Marine that this sectionmay have a more blocky and magnetic anomalies,geomagnetic field reversals, en echelon topography than the adjacent sec- and motions of the ocean floor and continents, tion to the north. This, in turn, suggeststhat a J. Geophys. Res., 73, 2119-2136, 1968. relation may exist between the nature of ridge IAGA Commission 2, Working Group 4, Interna- tional geomagneticreference field 1965.0,J. Geo. topography and relative plate motions. phys. Res., 74, 4407-4408, 1969. The fracture zone near which the Ninety Knott, S. T., and E. T. Bunce, Recent improve- East ridge developedappears to be a transform ment in technique of continuous seismic profil- fault of exceptionally great offset. It lies on a ing, Deep Sea Res., 15, 633-636, 1968. great circle and hence presumably was 90ø Laughton, A. S., D. H. Matthews, and R. L. Fisher, The structure of the Indian Ocean, in from a pole of rotation and does not seem •o The Sea, vol. 4, edited by A. E. Maxwell, pp. have any spreadingsegments. The fracture zone 543-586, Interscience, New York, 1970. may have offset a central Indian ridge from a Le Pichon, X., and J. R. Heirtzler, Magnetic center of sea floor spreadingat the north edge anomalies in the Indian Ocean and sea floor of the Wharton plate that may have subse- spreading, J. Geophys. Res., 73, 2102-2117, 1968. Luyendyk, B. P., T. A. Davies, K. S. Rodolfo, quently disappearedas a result of underthrust- D. R. C. Kempe, B.C. McKelvey, R,. D. Leidy, ing beneaththe Indonesianisland arc. G. J. Horvath, R. D. Hyndman, H. R. Their- stien, E. Boltovskoy, and P. Doyle, Leg 26, Acknowledgments. Captain Palmeri, the sci- Deep Sea Drilling Project: Across the southern entific party, and the officers and crew of the Indian Ocean aboard Glomar Challenger, Geo- Chain all contributed to successful observations times, 18, 16-19, 1973. at sea. S. J. Abbot, J. W. Mahoney, Jr., and N. McKenzie, D., and J. G. Sclater, The evolution Serotkin assisted in the digital data reduction. of the Indian Ocean since the Late Cretaceous, Z. Ben-Avraham helped prepare the line draw- Geophys. J. Roy. 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