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 Evolution of the Central Indian Ridge, Western Indian Ocean 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 TOPOGRAPHIC, MAGNETIC, AND SEISMIC OBSERVATIONS 555 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 554 FISHER AND OTHERS-EVOLUTION OF THE CENTRAL INDIAN RIDGE 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 556 FISHER AND OTHERS-EVOLUTION OF THE CENTRAL INDIAN RIDGE 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 i •mi H H i in i 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- Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/82/3/553/3432609/i0016-7606-82-3-553.pdf by guest on 25 September 2021 TOPOGRAPHIC, MAGNETIC, AND SEISMIC OBSERVATIONS 557 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.