Evolution of the Central Indian Ridge, Western Indian Ocean

Home , Central Indian Ridge

ROBERT L. FISHER / Scripps Institution of Oceanography, University of California at San Diego, JOHN G. SCLATER \ La Jolla, California 92037 DAN P. McKENZIE Department of Geodesy and Geophysics, University of Cambridge, Cambridge, England

ABSTRACT consists of a series of short ridge segments, cut by fractures subparallel to Vema Trench. Topographic, magnetic, and earthquake Two strike-slip fault plane solutions have epicenter data from the wholly submerged been obtained several degrees north and south Central Indian Ridge were interpreted, using of Vema Trench (Banghar and Sykes, 1969). the Theory of Plate Tectonics. The pole of Such faulting also is inconsistent with Heezen relative motion between the Indian and and Tharp's early (1965) interpretation. Somalian plates, lying at 16.0° N., 48.3° E. Le Pichon (1968) made the first attempt and with opening at 6.2 X 10~7 deg/yr, was to apply the Theory of Plate Tectonics obtained from the strike of fracture zones (McKenzieand Parker, 1967; Morgan, 1968) taken as transform faults and the spreading to the Indian Ocean. He pointed out that rates based on magnetic anomaly patterns. the east-west strike of the "Rodriguez Frac- Since this pole appears to have moved little ture Zone" of Heezen and Tharp (1965) was since the Miocene, the plate positions at that not consistent with the direction of the rela- past time can be obtained by finite rotation tive motion between the Indian and Somalian about the present rotation pole. Such a recon- plates observed on the Carlsberg Ridge and struction shows that the complicated nature in the Gulf of Aden. Although the existence of the present plate margins results from of a north-south Central Indian Ridge is not Miocene to Recent opening along a north- inconsistent with the direction of relative south fracture zone that existed in this area motion obtained by Le Pichon, it would during an interval of rapid spreading in the require the spreading direction to be about late Cretaceous and early Tertiary. 40 to the strike of the ridge over-all. Com- parable differences between the two direc- INTRODUCTION tions have been observed elsewhere, but are The first effort to interpret the Central uncommon. Indian Ridge in terms of ridge segments The present paper employs more extensive offset at large cross-fractures was made as a and detailed topographic and magnetic in- physiographic diagram by Heezen and Tharp formation, principally from Expedition Circe (1965). They presented bathymetric evidence, (R/V Argo, S.I.O., 1968), to establish the augmented by earthquake epicenter data, to complexly faulted and segmented nature of show an active rifted ridge trending north- the Central Indian Ridge. The strike of cross- south, cut by two transverse fractures, " Vema fractures taken to be transform faults and the Trench" and "Rodriguez Fracture Zone" spreading rates obtained from the magnetic (Heezen and Tharp, 1965, Fig. 4). Additional anomalies can then be used to determine the or differently interpreted bathymetric data, angular velocity vector for the motion rather than supporting the existence of a between the Indian plate and the Somalian continuous "rift valley," has shown that the plate, which includes Africa east of the rift most marked, narrow, steep-walled depres- system, Madagascar, and the Mascarene Pla- sions are aligned obliquely to the north-south teau. Since the pole of relative motion between envelope of the Central Indian Ridge (Fisher, these plates has not shifted appreciably in 1966). This led Fisher and others (1967, Fig. the past 15 m.y., the relative position of 5) to doubt that a continuous "rift valley" Chagos-Laccadive Ridge and the Mascarene existed and to propose that the active zone Plateau in Miocene time can be obtained by

Geological Society of America Bulletin, v. 82, p. 553-562, 9 figs., March 1971 553

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Figure 4. Axial magnetic anomalies of the ated normal to a ridge at 1 5 ° S., striking N. 3 5 ° Central Indian Ridge between 14° S. and 18° S. W., with an assumed spreading (half) rate of Observed profiles were projected on a section 1.8 cm/yr. trending N. 55° E. Synthetic profile was gener-

Fisher, 1969). Such trends, inferred from the Magnet Profile 516 (Fig. 5) just east of topography, are confirmed by correlation of Rodriguez Island, while K-Ar dating magnetic anomalies (Fig. 2), especially of (McDougall and others, 1965) gives ages of the large axial anomaly. 1.3 to 1.5 m.y. for that island's volcanic Rodriguez Ridge is the only east-west- rocks. Apparently Rodriguez Ridge is a later tending feature on the sea floor between the feature not pertinent to the tectonics of the Mascarene Plateau and the Chagos-Laccadive Central Indian Ridge. Similar large aseismic Ridge. Heezen and Tharp (1965) and Fisher volcanic ridges occur in the Pacific and and others (1967) postulated that Rodriguez Atlantic and are not related in any simple way Ridge is part of a major fracture zone off- to the tectonics of the surrounding sea floor. setting the Central Indian Ridge. Bathymetric They remain as the major unexplained topo- and magnetic interpretations (Figs. 1 and 2) graphic features of the ocean floor. now do not appear to support that view. North of 10° S. (Fig. 2), there is little Furthermore, magnetic anomaly "5" (about definitive magnetic information. The trans- 10 m.y.) can be identified clearly on Project form faults, most especially E-E' ("Vityaz

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3 5 III H HUB II

Figure 3. Axial magnetic anomalies of the ated normal to a ridge at 15 ° S., striking N. 3 5 ° Central Indian Ridge between 10° S. and 14° S. W., with an assumed spreading (half) rate of Observed profiles were projected onto a section 1.8 cm/yr. trending N. 55° E. Synthetic profile was gener-

rotation about this pole. The resulting re- on information available through late 1968 construction suggests a simple explanation and the interpretation is weighted toward for the present complexity of the Central recent soundings taken with precision depth Indian Ridge. recorders. These data support the over-all north-south trend of the ridge. This trend is TOPOGRAPHIC, MAGNETIC, AND not, however, that of either the transform SEISMIC OBSERVATIONS faults, which trend east-northeast to north- Bathymetric contours for the Central east or of the actively spreading short ridge Indian Ridge presented in Figure 1 are based segments which trend northwest (Engel and

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fracture zone" of Udintsev, 1965, p. 159) and I-I' ("Argo fracture zone," this paper) is 90 the more spectacular G-G' ("Vema Trench," km, as determined from the offset of the Heezen and Nafe, 1964) appear as transverse axial magnetic anomaly, although the flank- deeps which are seismically active, and the ing anomalies are not easy to identify (Fig. ridge segments between them are all very 4). The existence of J-J' is suggested by short. Between 10° S. and 15° S. (Fig. 3), correlation of magnetic anomalies, but neither the magnetic anomalies have been used to its strike nor its offset are well established. identify the ridge axis; however, in this The large-lipped K-K' ("Marie Celeste frac- region, correlations of other flanking anom- ture zone," this paper) is clearly defined by alies are less convincing. Within the ex- topography (Fig. 1), seismicity, and mag- pected precision of location, almost all of netics (Figs. 4 and 5) and has an offset of 220 the epicenters lie either on the ridge axis or km (Engel and Fisher, 1969). Fracture zones on transform faults. South of 15° S., L-L' and M-M' (Figs. 5 and 6) differ from there are fewer transform faults but with all others on the Central Indian Ridge in larger displacements, and longer ridge seg- marking left- rather than right-handed dis- ments. The displacement of the ridge axis at placement of the ridge crest. The offset and

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Figure 5. Axial magnetic anomalies of the ated normal to a ridge at 20° S., striking N. 30° Central Indian Ridge between 18° S. and 21° S. W., with an assumed spreading (half) rate of Observed profiles were projected on a section 1.8 cm/yr. trending N. 60° E. Synthetic profile was gener-

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strike of each is better determined from the 70° E., where the seismically active zone crestal anomalies and the seismicity, respec- bifurcates (Fig. 2 and Fig. 7). The eastern tively, than by the topography in Figure 1. branch of the ridge system is the Southeast The most southerly fracture zone, N-N', on Indian Ridge, the western branch is the the Central Indian Ridge marks right-handed Southwest Indian Ridge. The intersection is displacement of the axis. Like the previous discussed in detail by McKenzie and Sclater two, its offset and strike were inferred from (in prep.), who show that the southeast magnetic anomalies (Fig. 7) and epicenters branch is spreading at a half-rate of 3 cm/yr. rather than from topography. This value, if taken with the half-rate of 2.4 The ridge axis can be followed farther cm/yr. obtained from magnetic anomaly pro- south to the triple junction at about 25.5° S., files Nl to N7 (Fig. 7), indicates that the

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Figure 6. Axial magnetic anomalies of the ated normal to a ridge at 20° S., striking N. 30° Central Indian Ridge between 20° S. and 21° S. W., with an assumed spreading (half) rate of Observed profiles were projected onto a section 2.3 cm/yr. trending N. 60° E. Synthetic profile was gener-

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northeast segment of the Southwest Indian and Fisher, 1969, their Fig. 1), the absence Ridge should be spreading approximately of large amplitude magnetic anomalies just north-south at a half-rate of about 1 cm/yr. southwest of the triple junction, and the This direction agrees with the fault plane paucity of epicenters (Fig. 2) may be con- solution for the only earthquake in this area sequences of the slow spreading rate. so examined by Banghar and Sykes (1969). Spreading rates and azimuths of fracture The relatively minor width of topographic zones (Table 1) have been used to determine expression at 4000 m depth (compare Engel the pole of relative motion between the

518

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PROFILES N,

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Figure 7. Axial magnetic anomalies of the erated normal to a ridge at 25° S., striking N. Central Indian Ridge between 23° S. and 26° S. 25° W., with an assumed spreading (half) rate Observed profiles were projected onto a section of 2.4 cm/yr; the Magnet synthetic is based on trending N. 65° E. Synthetic profiles were gen- an assumed altitude of 2000 m.

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TABLE 1. OBSERVED AND CALCULATED AZIMUTHS AND SLIP RATES FOR A POLE OF RELATIVE MOTION BETWEEN THE SOMALIAN AND INDIAN PLATES COMPUTED TO BE AT 16.0° N., 48.3° E. WITH A ROTATION RATE OF 6.2 X 10~7 DEG/YR

Observed Calculated Fracture zone Ridge Lat.° N. Long." E. Az Rate in Az Rate in cm/yr cm/yr Owen 11.0 57.5 30° 30° 0.6 Carlsberg 8.0 59.0 1.5 38° 0.8 Carlsberg 4.0 63.0 1.6 41° 1.1 G-G' ("Vema") -9.0 67.0 52° 53° 1.8 Central Indian — 12.0 67.0 1.8 56° 1.9 U' ("Argo") — 14.0 66.0 55° 59° 1.9 Central Indian — 16.0 66.5 1.8 60° 2.0 J-J' — 17.0 67.0 60° 60° 2.1 K-K' ("Marie Celeste") — 17.5 66.0 63° 62° 2.1 L-L' —20.0 67.0 60° 62° 2.2 M-M' —20.0 68.0 63° 60° 2.2 Central Indian —21.0 68.5 2.3 60° 2.3 N-N' —22.5 69.0 65° 60° 2.4 Central Indian -24.5 69.0 2.4 62° 2.4 Indian and the Somalian plates. They are plates on either side of the ridge changed. consistent with a pole at 16.0° N., 48.3° E., Although it is possible for ridges to spread with an angular rotation rate of 6.2 X 10~7 obliquely to the direction of relative motion deg/ yr. Excellent agreement between the ob- between two plates, they more commonly served slip rates and directions (Table 1 and break up into a series of short ridge seg- Fig. 8A) gives us confidence in this pole ments joined by transform faults. This de- position. velopment must involve either asymmetric spreading, which has not yet been convinc- EVOLUTION OF THE CENTRAL ingly demonstrated for any region, or jump- INDIAN RIDGE ing of the ridge axis. One or the other process Examination of Figure 1 shows that frac- must be involved since a symmetrically ture zones F-F' to K-K', indicated as trans- spreading ridge cannot change its shape by form faults in Figure 2, extend in subdued evolution. form well beyond the ridge zone; they end Not all transform faults result from changes on the northeast against the Chagos-Lacca- in spreading direction. Those in the Central dive Ridge and on the southwest against the Atlantic, the Gulf of Aden (Laughton and southern portion of the Mascarene Plateau. others, 1971), and the Southeast Indian Ocean Current seismicity is almost entirely on active (McKenzie and Sclater, in prep.) were formed ridge crests or transform faults, and lies in a by the breakup of continental plates, and the north-south band approximately median shape of the initial crack can still be dis- between the shoal regions (Fig. 8A). If the cerned in the presently active zone. Mascarene Plateau and the Chagos-Laccadive The complicated nature of the Central Ridge are each rotated through 10° toward Indian Ridge does not appear to have been each other about the Indian-Somalian pole, governed by continental fragmentation but then Chagos Bank nestles into the neck by an earlier ridge offset along a transform between Saya de Malha and Nazareth Bank fault. McKenzie and Sclater (in prep.) have (Fig. 8B), and the 2000-m contour extends shown that magnetic anomalies 23 and older nearly continuously from Mauritius through in the northern part of the Arabian Sea and the Maldive Islands. The present band of in the south-central Indian Ocean lie ap- epicenters therefore marks the initial break proximately east-west. These authors have when each plate is rotated back through half suggested that at the time of formation of the angular distance between them. anomalies 30 through 21 (Cretaceous through Studies of magnetic anomalies in the early Eocene, according to the time scale of Northeast Pacific (Atwater and Menard, Heirtzler and others, 1968, p. 2123) the active 1970) have shown that some of the large ridge crest trended east-west and a very long fracture zones in that region were formed transform fault, the "Chagos fracture zone," when the direction of motion between the linked an ancient "Carlsberg Ridge" to its

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counterpart in the Southeast Indian Ocean (Fig. 9A, this paper). Between anomalies 21 and 6, spreading must have been very slow, since anomalies of this interval cannot be recognized anywhere in the Indian Ocean west of the Ninetyeast Ridge. Perhaps during this interval, the volcanic foundations of Chagos-Laccadive Ridge and the southern portion of Mascarene Plateau (Saya de Malha to Mauritius) were extruded with very slow widening of the "Chagos fracture zone" (Fig. 9B). Radioisotope dating of rocks from Mauritius suggests an age of little more than 20°S 10 m.y. for initiation of that volcanic pile, and volcanism moving north to south along the plateau (McDougall and Chamalaun, 1969, p. 1439). When the spreading rate again became appreciable, probably between 30"S anomalies 6 and 5 in Miocene time, the direction of relative motion between the Indian and Somalian plates was not the same as in late Cretaceous and early Tertiary time, but was northeast-southwest. The Chagos fracture zone no longer was parallel to the direction of relative motion and therefore broke up into a series of ridge segments joined by transform faults. These faults must show left- handed displacements on what were the ancient ridge crests and right-handed displacements along the old Chagos fracture zone- (Fig. 9C). Transform fault N-N' is the only exception to this over-all pattern; it may owe its existence to a small north-south transform fault on the earlier ridge. The steep scarps on the eastern side of the Mascarene Plateau and the western side of the Chagos Bank probably resulted from their being torn apart in the Miocene. Finally, the present Carlsberg and South- east Indian Ridges, less segmented by transform faults, reflect only the northeast-southwest spreading since mid-Miocene time. CONCLUSIONS Examinations of topography, magnetic anomalies, and present-day seismicity have been employed to demonstrate that these Figure 8. A. Tectonic elements, earthquake data, taken with assumptions from the Theory epicenters, and identified magnetic anomaly lineations plotted on a transverse Mercator pro- of Plate Tectonics, can provide a rather simple jection (McKenzie and Parker, 1967) about a evolutionary model for an extremely com- pole at 16°N., 48° E. B. Same, with Chagos- plex portion of the mid-ocean ridge system Laccadive Ridge and Mascarene Plateau rota- in the Indian Ocean. Perhaps more important, ted 10° toward one another about a pole at the present Central Indian Ridge, rather than 16° N., 48° E. Present tectonic elements, Re- being anomalous, has behaved in accor- union, and Rodriguez Ridge have been omitted. dance with two observations made in other Present-day earthquake epicenters mark the ridge areas: (l) spreading is symmetrical on plate boundary.

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CARLSBERG RIDGE

SEYCHELLES

SOUTHEAST

INDIAN RIDGE

30 Figure 9. A. Schematic diagram of the Cen- tral Indian Ocean in Eocene time (anomaly 21, about 50 m.y. B.P.). B. Schematic diagram of the Central Indian Ocean in Miocene time (anomaly 6, about 20 m.y. B.P.). C. Schematic diagram of structural trends of the Central Indian Ridge at the present time.

each side of the ridge axis and (2) the strike Doherty Geological Observatory, provided of ridge axes is generally close to being at Vema and Conrad magnetic data in digitized right angles to the direction of motion form. We would like to thank David Crouch between the plates on either side. for his many significant contributions to the preparation of the paper. Field and laboratory ACKNOWLEDGMENTS work was supported by the Office of Naval Officers and crew of R/V Argo carried out Research and NSF Grants G-22255, GP-5469, arduous and meticulous maneuvering opera- and GA-11350. tions during our study of the Central Indian Ridge on Expedition Circe. Lynn Abbott and the S.I.O. shipboard computer group and REFERENCES CITED Misses Linda Meinke and Uta Ritter assisted Atwater, T.; and Menard, H. W. Magnetic in reducing the magnetic information. Ellen lineations in the northeast Pacific: Earth Herron and Walter Pitman, III, of Lament- Planet. Sci. Lett., Vol. 7, p. 445-450, 1970.

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Banghar, A. R.; and Sykes, L. R. Focal mech- Jones, M. T. The evolution of the Gulf anisms of earthquakes in the Indian Ocean of Aden: Roy. Soc. London, Proc., 1971 and adjacent regions: J. Geophys. Res., (in press). Vol. 74, p. 632-649, 1969. Le Pichon, X. Sea-floor spreading and con- Engel, C. G.; and Fishet, R. L. Lherzolite, tinental drift: J. Geophys. Res., Vol. 73, anorthosite, gabbro, and basalt dredged p. 3661-3697, 1968. from the M id- Indian Ocean Ridge: Science, McDougall, I.; and Chamalaun, F. H. Iso- Vol. 166, p. 1136-1141, 1969. topic dating and geomagnetic polarity Fisher, R. L. The median ridge in the south studies on volcanic rocks from Mauritius, central Indian Ocean; in The world rift Indian Ocean: Geol. Soc. Amer., Bull., system;Rept, of UMCSymposium,Ottawa, Vol. 80, p. 1419-1442, 1969. Sept., 1965: (T. N. Irvine, ed.)p. 135-147, McDougall, I.; Upton, B. J. G.; and Wads- Canada Geol. Surv., Paper 66-14, 469 p., worth, W. J. A geological reconnaissance 1966. of Rodriguez Island, Indian Ocean: Nature, Fisher, R. L.; Johnson, G. L.; and Heezen, Vol. 206, p. 26-27, 1965. B. C. Mascarene Plateau, western Indian McKenzie, D. P.; and Parker, R. L. The Ocean: Geol. Soc. Amer., Bull., Vol. 78, North Pacific; an example of tectonics on p. 1247-1266, 1967. a sphere: Nature, Vol. 216, p. 1276-1280, Heezen, B. C.; and Nafe, J. E. Vema Trench, 1967. western Indian Ocean: Deep-Sea Res., Vol. Morgan, W. J. Rises, trenches, great faults, and 11, p. 79-84, 1964. crustal blocks: J. Geophys. Res., Vol. 73, Heezen, B. C.; and Tharp, M. Physiographic p. 1959-1982, 1968. diagram of the Indian Ocean (with de- Udintsev, G. B. Results of Upper Mantle scriptive sheet): Geol. Soc. Amer., New Project studies in the Indian Ocean by the York, 1965. research vessel Vityaz, p. 148-172 in The Heirtzler, J. R.; Dickson, D. O.; Herron, World Rift System; Rept, of UMC Sym- E. M.; Pitman, W. C, III; and Le Pichon, posium, Ottawa, Sept., 1965 (T. N. Irvine, X. Marine magnetic anomalies, geomag- ed.): Canada Geol. Surv., Paper 66-14, netic field reversal, and motions of the 469 p., 1966. ocean floor and continents: J. Geophys. Res., Vol. 73, p. 2119-2136, 1968. MANUSCRIPT RECEIVED BY THE SOCIETY SEP- Laughton, A. S.; Whitmarsh, R. B.; and TEMBER 14, 1970

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