Since a large percentage of the mid-ocean ridge system remains uncharted and unsampled,
studying the diverse processes involved in the generation of new oceanic crust is an ambitious
goal that requires international cooperation. The Ridge InterDisciplinary Global Experiment
(RIDGE) initiative is doing just that in cooperation with InterRIDGE, an international
organization of countries active in mid-ocean ridge research. The two programs are working to
characterize the morphology, structure, composition, biological communities, and energy fluxes
of the mid-ocean ridge system on a global scale. Over the next few years RIDGE will explore
the virtually unstudied spreading centers of the Indian Ocean, and InterRIDGE will target the
slow-spreading Southwest Indian Ridge.
Fig. 1. Satellite-derived gravity field over the Indian Ocean [from Sandwell and Smith, Eos, vol. 73, p. 133, 1992] showing the three major spreading centers: the Southwest Indian Ridge (SWIR), the Central Indian Ridge (CIR), and the Southeast Indian Ridge (SEIR).
In 1996 a RIDGE field program will explore an unmapped section of the SWIR between 15° E and 35° E. The objectives of this geophysical study are to characterize crustal accretion and segmentation in two contrasting domains: one consisting of linear ridge segments and one dominated by transform faults.
Several important scientific problems can be addressed by studying the SWIR. Most geophysical and geochemical studies to date have focused on the slow-spreading Mid-Atlantic Ridge, intermediate-spreading ridges such as the Juan de Fuca Ridge, or the fast-spreading East Pacific Rise. Little is known about the crustal or lithospheric structure, the chemical systematics of magmatism, or the nature of hydrothermal activity at the slowest end of the spreading spectrum.
Theoretical work suggests that the style of crustal accretion and magmatism may differ significantly at very slow spreading rates, although there are few data to test these models. From a geochemical standpoint, because crust forming along some portions of the SWIR may be produced by very small degrees of melting, the rocks recovered may preserve a clearer record of the variability in upper mantle source composition. In addition, because the crust along portions of the SWIR is likely to be thinner than normal, the large topographic relief of the rift valley and transform faults along the SWIR should offer an excellent opportunity to sample the lower crust and upper mantle and compare their compositions to spatially related basalts. Contrasting styles of lithospheric accretion and deformation at triple junctions can also be studied at the SWIR.
The SWIR is also a region of high priority for InterRIDGE. Beginning in September - October, the French "Dorsales" field program will study the segmentation characteristics of the SWIR near the Galliani Fracture Zone (~52° E). British scientists recently completed a study of the SWIR near the Atlantis II Transform and west of the Rodriguez Triple Junction and another British group plans to conduct a geophysical study of the Bouvet Triple Junction this year. Finally, a return to ODP Hole 735B near the Atlantis II Fracture Zone, where ~450 m of gabbro were recovered during drilling in 1987, remains a high priority of the Lithosphere Panel of the Ocean Drilling Program.
The SEIR is divided into two supersegments. The first extends from the Tasman Zone to about 90° E and includes the Australian-Antarctic Discordance (AAD). The second supersegment extends from 90° E to the Rodriguez Triple Junction and is influenced along part of its length by the Amsterdam/St. Paul and Kerguelen hotspots. The relative proximity of the AAD, which is thought to overlie an anomalously cold portion of the mantle, and the Amsterdam/St. Paul and Kerguelen hotspots offers the opportunity to study the effects of varying mantle temperature on crustal accretion and magmatism at almost constant spreading rate. In addition, the presence of the Amsterdam/St. Paul and Kerguelen hotspots allows further investigation of the interaction between mid-ocean ridges and mantle plumes.
A number of geochemical and tectonic factors make the SEIR particularly exciting for further study. It is well known that Indian Ocean mid-ocean ridge basalt (MORB) has isotope systematics that are distinct from those of Atlantic and Pacific MORB. However, because no samples have been recovered over ~3000 km of the SEIR, the distribution of this distinctive isotope signature remains unclear. Sampling suggests that the AAD represents the eastern boundary between the Indian Ocean and Pacific isotope provinces but this, too, must be tested by sampling the vast, sparsely sampled SEIR west of the AAD.
In terms of ridge morphology, the AAD and the SEIR to the west exhibit transitions in axial topography from a pronounced rift valley to an axial high at nearly constant spreading rate. These transitions are observed in satellite-derived gravity data as well as in the limited amount of bathymetric data available. Such transitions are thought to reflect variations in the thermal structure of the plate boundary. These transitions also appear to coincide with changes in the geochemical variability of axial lavas that mimic those associated with fast- and slow-spreading ridges.
The SEIR offers the opportunity to study the parameters that control the axial morphology of spreading centers, the nature of mantle upwelling within spreading segments, and the thermo-mechanical structure of mid-ocean ridges. Another subject for study is the influence the Amsterdam/St. Paul and Kerguelen hotspots have on crustal accretion along the SEIR. For example, although the geophysical signature of the Amsterdam/St. Paul hotspot is more marked along the SEIR southeast of the Amsterdam and St. Paul Islands, the geochemical signature is most pronounced northwest of the islands.
RIDGE activities along the Southeast Indian Ridge started in December 1994 with the first of two back-to-back field programs designed to study the transitions in axial topography observed along the plate boundary, the geochemical and geophysical effects of mantle temperature on crustal accretion, and the origin of the distinctive isotopic character of the Indian Ocean at the spreading center between 90° E and 120° E. The first leg focused on the geophysical characterization of the plate boundary, while the geochemical leg is concentrating on geochemical sampling.
In the austral summer of 1995 - 1996, a geochemical and geophysical study of the SEIR between 32° S and 42° S will be conducted. This section of the SEIR is centered on the Amsterdam/St. Paul hotspot. The objectives of this field program are to characterize the dispersal of the Kerguelen and Amsterdam/St. Paul plume signature as well as other components of the Indian Ocean mantle, to study the geophysical and geochemical segmentation of the plate boundary along this hotspot-influenced spreading center, and to study the effects of plume-ridge interaction on mantle flow. As part of the Japanese InterRIDGE program, a multidisciplinary study of the Rodriguez Triple Junction and the adjacent SEIR were recently carried out.
Like the SWIR and the SEIR, the CIR is a good site for studying the interaction between mantle plumes and spreading centers. The Reunion-Rodriguez hotspot is located on the west flank of the CIR and its influence on the spreading center is manifested near 19° S. The CIR exhibits significant variations in axial topography, nontransform offset morphology, and segmentation characteristics between 19° S and the Rodriguez Triple Junction. These changes may be related to the proximity of the Rodriguez/Reunion hotspot or to the triple junction. The CIR also offers opportunities to study crustal accretion at intermediate spreading rates and to address problems such as the relationship between axial topography, crustal thickness, and basalt chemistry.
Sampling of Indian Ocean MORB reveals a diversity of isotopic compositions, the spatial distribution of which can only be constrained by further sampling along each ridge including the CIR and Carlsberg Ridge. At present, no RIDGE field programs are planned for the CIR, but RIDGE and InterRIDGE efforts along the CIR will increase over the next few years. Because the few hydrothermal measurements available from the Indian Ocean were made on the CIR, opportunities for hydrothermal investigations are more advanced along the CIR than along any other Indian Ocean ridge. Although no active vent sites have been located, evidence for hydrothermal activity has been found along at least two segments of this spreading center.
At present we do not even have first-order information on the distribution of hydrothermal activity or biological diversity associated with Indian Ocean spreading centers. Thus, the first steps in the hydrothermal and biological characterization of the Indian Ocean must be largely exploratory. Once active hydrothermal vents sites have been located, clear plans can be made for in-depth studies of important problems such as hydrothermal fluxes, larva dispersal, and biogeographical distribution of vent organisms. Several U.S. groups are actively developing programs to characterize the Indian Ocean spreading centers from a hydrothermal and biological perspective.
Return to Science and
Society
Return to Starting
Point
