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 o o 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 ,i i I i r J / 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 sedi- ments (54-58 m.y.) in DSDP hole 213 at. that site suggeststhat the observedmagnetic anoma- lies 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.