Indian Mid-ocean Ridge Research Articles

The tectonics of the Mid-Indian Ocean Ridge and the petrography of the solid rocks of its rift zones

The structure of the Mid-Indian Ridge has been studied by many authors. Especially active investigations relate to the period of activity of the International Indian Ocean Expedition (1960 to 1965), and results of this work are well known from numerous papers cited in bibliographical list edited by Fox (1967), especially those by Matthews (1966), Vine (1966), Cann (1968), Cann & Vine (1966), Laughton (1966), Fisher (1965) and Engel & Fisher (1969). Russian expeditions in the area of the Mid-Indian Ridge (figure 1) paid particular attention to the correlation between different geophysical and geological features of the rift zone. These results are not so well known for language reasons and because there are few publications.

Petrology of ultrabasic rocks from rift zones of the Mid-Indian Ocean Ridge

Ultrabasic rocks from the Mid-Indian Ocean Ridge and peridotites of the alpine type have similar petrographic characteristics. A petrochemical comparison of these rocks shows that oceanic peridotites are enriched in SiO 2 and normative hypersthene. The direction of the development of the composition of ultrabasic rocks after separation of different amounts of basalt from pyrolites of oceanic and alpine type ultrabasic rocks was obtained by calculation. Combination of these data with experimental melting diagrams shows that alpine type ultrabasic rocks melt at higher temperatures than oceanic ultrabasic rocks. On geological and petrochemical grounds we think that the ultrabasic rocks which outcrop in the rift zones of the Mid-Indian Ocean Ridge are a specific formation. This formation consists of slighty differentiated pyrolites of the oceanic type.

Distribution of trace elements in crystalline rocks of rift zones

More than 1000 precise analyses of trace elements in basic and ultrabasic rocks of the Mid-Indian Ocean Ridge have been used for geochemical comparison with similar data for the continents, islands and chondrites. We can draw a preliminary conclusion that the transport of trace elements in the oceanic segment of the Earth was controlled by two main processes: ( a ) degassing of the mantle, and ( b ) primary formation of the oceanic upper mantle and crust. Results of both processes may jointly occur.

Regional gravity anomalies in the Indian Ocean

Free air gravity values averaged over 1°, 5° and 20° squares of latitude and longitude in the Indian Ocean are presented. A free air gravity map based on 20° square averages bears a striking resemblance to a gravity map obtained from satellite-derived spherical harmonic coefficients of the geopotential. However, considerably more detail is present in the map constructed from 5° averages than in the satellite-derived map. A free air gravity map based on the 5° averages shows the important Indian Ocean gravity minimum centered south of India, as well as the minima east of Africa and west of Australia. The East Indies Islanc Arc region has positive anomalies over it. The Mid-Indian Ocean Ridge including the Carlsberg Ridge has positive gravity values with respect to the values over the basins on either side. By comparing gravity values at intersections of ships' tracks as well as the 1° square averages for different cruises, the accuracy of the data is assessed.

Lherzolite, Anorthosite, Gabbro, and Basalt Dredged from the Mid-Indian Ocean Ridge

The Central Indian Ridge is mantled with flows of low-potassium basalt of uniform composition. Gabbro, anorthosite, and garnet-bearing lherzolite are exposed in cross fractures, and lherzolite is the bedrock at the center of the ridge. The Iherzolites are upper-mantle rock exposed by faulting.

Isotopic Dating and Geomagnetic Polarity Studies on Volcanic Rocks from Mauritius, Indian Ocean

Results of a geochronological and paleomagnetic investigation of the volcanic island of Mauritius are reported. Potassium-argon dates indicate that the main shield volcano was built subaerially between about 7.8 and 6.8 m.y. ago in the early Pliocene. Some evidence is presented for caldera formation following the construction of the volcanic shield. Profound erosion subsequently destroyed much of the volcano, leaving only peripheral steep-sided massifs. Lavas of the Younger Volcanic Series were erupted from about 3.5 m.y. ago until less than 0.2 m.y. ago, flooding the denuded stumps of the shield volcano. This activity was intermittent and a hiatus is recognized from about 2 to 0.7 m.y. ago, which we have taken as the break between the eruption of the Early and Late Lavas of the Younger Volcanic Series. The mean directions of magnetization of the three groups of lavas are not significantly different from one another and are close to an axial dipole direction. The combined paleomagnetic and dating studies on the lavas also yield further information on the geomagnetic polarity time scale; 16 results on rocks with ages lying between 3.5 and 0.17 m.y. are consistent with the established scale for this period of geological time. Of particular significance are results that confirm the Mammoth reversed event at 3.0 m.y. ago, and the age of the boundary between the Gauss normal and Gilbert reversed polarity epochs at 3.35 m.y. Nineteen results were obtained on rocks whose ages are between 4.6 and 7.9 m.y., and these data generally are consistent with the polarity time scale based upon extrapolations from the sea-floor spreading hypothesis. The evolution of the Mascarene Islands, comprising Mauritius, Reunion and Rodriguez is briefly reviewed and it is concluded that each island had developed independently, and that no correlation of age with respect to distance from the Mid-Indian Ocean Ridge is observed. The volcanism that has built Mauritius and Reunion may be a continuation of the development of the Mascarene Plateau with migration of the activity generally southward with time.

Rocks from the Mid-Oceanic Ridge in the Indian Ocean

Previous study of rocks from the Mid-Indian Oceanic Ridge has been limited to specimens from dredge hauls taken from a few restricted areas. Those described in this report were recovered exclusively from piston cores taken from thousands of miles apart between 06°50′N, 60° 12′E; and 31° 38′S, 83° 48·5′E and at depths between 2400 and 4300 m. They have an average diameter of about 4 cm and were buried under 20–856 cm of unconsolidated sediment of Pleistocene and younger age.Twenty-four of the thirty-two samples examined are basalts and two are spilites derived from basaltic lava on the Cralsberg Ridge near 02° 50′N; 67° 31′E. Four specimens of pyroclastic glassy sand and vesiculated basalt are a product of aerial volcanism between the islands of Réunion and Mauritius. Two serpentinized periodotites were collected close to the Rodriguez Fracture Zone between 20°S; 61°E and 21°S; 71°E. The basalts and serpentinites of the Mid-Indian Oceanic Ridge are similar to those from the Mid-Atlantic Ridge.Chemical analyses of the Indian Ocean rocks indicates that different petrologic provinces exist along the Mid-Indian Oceanic Ridge. Differentiation processes have concentrated more mafic elements on the Carlsberg Ridge than any other portion of the Ridge investigated, with the exception of high volcanoes which are built up of alkali-basalts.

Recent heat flow measurements in the Indian Ocean

Forty-one new heat flow measurements were made in the Indian Ocean by Lamont Geological Observatory ships. Three measurements were made in the Red Sea and indicated a higher than average heat flow. Six heat flow values in the axis of the mid-Indian Ocean ridge at 21°30′S indicate a local region of very high heat flow. On the basis of recent geophysical observations in the Indian Ocean, the trend of the axis of the mid-Indian Ocean ridge is reinterpreted. The zone of rough topography between the equator and 35°S and longitude 64° and 68°E is interpreted as a fracture zone that displaces the Carlsberg ridge from the southeast branch of the mid-Indian Ocean ridge. At two stations in the southern Indian Ocean, the temperature gradients measured at each station changed greatly with depth in the sediment. These changes are thought to result from recent changes in the bottom water temperature. The average of all measurements to date in the Indian Ocean is 1.46 μcal/cm² sec. The average of 135 measurements in the deep basins with thick sediment is 1.32 μcal/cm² sec and may better represent the average heat flow through the floor of the Indian Ocean.

Mascarene Plateau, Western Indian Ocean

The submarine Mascarene Plateau, lying east and northeast of Madagascar in the west central Indian Ocean, extends as a faulted composite arc for 2300 km, from the Precambrian granitic “micro-continent” of Seychelles Bank southward through the coral–reef–capped volcanic structures (?) of Saya de Malha, Nazareth, and Cargados Carajos banks to the faulted Tertiary–Quaternary oceanic volcanic island Mauritius. Locally the aseismic plateau is buttressed by spurs, is steep-sided and angular in plan, or is extended, disrupted, or offset by cross–faulting. Other major structural elements in a sector centered on the Mascarene Plateau are: (1) the seismically active Mid-Indian Ocean Ridge and rift zone (including that portion known as the Carlsberg Ridge) between the equator and its bifurcation near 23°S.; (2) the angular and faulted Amirante Trench lying between Seychelles Bank and the Madagascar-Farquhar complex; (3) the east–west late Tertiary–Pleistocene Rodriguez Ridge and its possible extension eastward as a fracture zone offsetting the Mid-Indian Ocean Ridge; (4) the Mauritius Trench, a 1400-km border deep-trending southwest from east of Mauritius to 28°S., 51°E. Irregular topography, attributed to volcanism and block faulting, is strikingly developed southeast of the intersection of Rodriguez Ridge with the Mascarene Plateau. Here linear, slightly sedimented deeps and narrow ridges lie subparallel to Mauritius Trench and to the southwest branch of the Mid-Indian Ocean Ridge. Except in the northwest sector where northwest and north trends lie within the Agalega–Seychelles–Amirante–Owen Fracture Zone complex, the dominant topographic lineations are northeast to east–northeast. These are displayed by large features such as fractures offsetting the mid-oceanic ridge (for example, Vema Trench), the Mauritius Trench and the linear topography southeast of Mauritius, the apparent offsets of the southern half of the Mascarene Plateau, spurs on the west flank of the mid-oceanic ridge, and the smaller ridges protruding; from the sedimented rise and abyssal plain between Tromelin and the Mascarene Plateau. Offsets of the Mascarene Plateau along northeast-trending faults-occurred in Tertiary time as adjustments accompanying formation of the Mascarene Basin. Activity on several of these faults has continued where they intersect the crestal portions of the mid-oceanic ridge system, but growth of the exceedingly rough, ridge in this region is primarily by addition of igneous material along its axis.

The Petrology of Rodriguez Island, Indian Ocean

Rodriguez Island is offset from the center of an elliptical submarine platform on the south side of the Rodriguez Fracture Zone, about 400 km west of the crest of the Mid-Indian Ocean Ridge. It consists of rather uniform alkali basalts (nepheline-normative), with a local development of phonolitic segregation veins and occasional pyroclastic deposits. Gabbroic xenoliths and xenocrysts, which are widely distributed and locally abundant, are interpreted as fragments of a layered intrusion (cryptic variation at least An 64→38 ) formed by gravitative differentiation in a relatively high-level rnagma chamber at an earlier period of the volcano9s history, and brought to the surface during a rejuvenation phase of volcanic activity. This phase, which has produced the present island, seems to have occurred after marine planation of the main volcanic infrastructure. A fringe of strongly false-bedded calcarenites on the windward side of the island is interpreted as an aeolian deposit formed by accumulation of debris derived from the lagoonal reef-flat during a period of glacially lowered sea level in Pleistocene times.

A discussion concerning the floor of the northwest Indian Ocean - Heat flow and magnetic profiles on the Mid-Indian Ocean Ridge

Sixty heat flow values were measured along nine profiles across the Mid-Indian Ocean Ridge. The results were roughly of the same character as the ones previously reported for the South Atlantic Ridge. The correlation of high heat flow with the centre of the ridge was less pronounced. The scatter of heat flow values when plotted as a function of distance from the ridge was even greater. The average of all values is 1.35 /tcal cm -2 s -1 , indicating that over the surveyed area the heat flow is normal. The cause for the low values on the flanks of the ridge remains unknown. A right lateral displacement of about 200 km across the Vema Trench was measured from the offset of the magnetic anomaly on the ridge crest.

A discussion concerning the floor of the northwest Indian Ocean - Physciography of the Indian Ocean

As a result of the International Indian Ocean Expedition, the bottom of the Indian Ocean is now one of the best known areas of the ocean floor. The Mid-Indian Ocean Ridge, a rugged mountain range, lies in the centre of the Indian Ocean. North-northeast trending fractures offset the axis of the ridge. In the Arabian Sea these fractures are right lateral; in the southwest Indian Ocean they are left lateral. Displacements range from a few miles* to over 200 miles. The northeast Arabian Sea and the Bay of Bengal are occupied by huge abyssal cones built by sediments discharged from the Indo-Gangetic plain. Extensive abyssal plains lie seaward of the abyssal cones. In low latitudes smooth topography is characteristic of the continental rise, the abyssal cones, and the oceanic rises. However, near the polar front smooth c swale9 topography laps over the normally rugged Mid-Oceanic Ridge. This c swale5 smoothing appears the result of the higher organic productivity of the Antarctic seas. Microcontinents, mostly linear meridional ridges, are unique features of the Indian Ocean. These massive but smooth-surfaced blocks contrast markedly with the broad rugged Mid-Oceanic Ridge.

Igneous Rocks of the Indian Ocean Floor

Four dredge hauls from near the crest and from the eastern flank of the seismically active Mid-Indian Ocean Ridge at 23 degrees to 24 degrees S, at depths of 3700 to 4300 meters, produced only low-potassium tholeiitic basalt similar in chemical and mineralogic composition to basalts characteristic of ridges and rises in the Atlantic and Pacific oceans. A fifth haul, from a depth of 4000 meters on the lower flank of a seamount on the ocean side of the Indonesian Trench, recovered tholeiitic basalt with higher concentrations of K and Ti and slightly lower amounts of Si and Ca than the typical-oceanic tholeiite of the ridge. The last sample is vesicular, suggesting depression of the area since the basalt was emplaced. Many of the rocks dredged are variously decomposed and hydrated, but there is no evidence of important chemical modification toward conversion of the lava flows to spilite during extrusion or solidification.