Indian Ocean
Indian Ocean
The field program carried out a high resolution bathymetric, sidescan, dredging, glass coring, gravity, and magnetics study of a 930 km-long portion of the Southeast Indian Ridge (SEIR) in the vicinity of the Amsterdam and St. Paul hotspot. About 700 km of additional ridge axis adjacent to the main study area was surveyed and sampled with coarser spacing. With the exception of one reconnaissance survey in the mid-1980's, most of this portion of the intermediate-rate SEIR was virtually unsampled and unexplored by modern, high resolution swath mapping and geophysical methods. This section of the ridge is closest to the large Kerguelen hotspot and Plateau, which may have played an important role in the development of globally distinctive Indian Ocean mantle.
The major goals of the program were: (1) to map and characterize the distribution of mantle components that define the unique isotopic signature of the Indian Ocean MORB mantle; (2) to study the style, extent, and length scale of the geochemical effects of the interaction between the SEIR and the two hotspots in the region, the large Kerguelen hotspot and the smaller ASP hotspot; (3) to characterize the structural nature of the interaction between the ASP hotspot and the SEIR; and (4) to characterize the morphology, segmentation style, degree of melting, and tectonic history (out to 1 Ma) of a long section of this poorly explored spreading center.
To achieve these goals, we carried out 47 dredges and 51 wax cores in the study area. These samples are being analyzed for major, minor and trace elements, and radiogenic isotope compositions, and the compositions will characterize the mantle melting domains, reflect melting processes, and trace hotspot/SEIR interactions. Underway geophysical lines were spaced at 10-20 km producing nearly complete SeaBeam2000 bathymetry and sidescan sonar coverage in addition to high resolution gravity and magnetics from the axis out to crust of age 1 Ma on either side. These measurements will characterize the properties of the crust, the lithosphere, sub-lithospheric flow, and the history of segmentation over this time interval. The data will refine existing hypotheses and develop geophysical and geochemical models of plume-ridge interaction, mantle contamination by plumes, mantle melting, fracture zone damming of along-axis flow, transition from axial valley to no axial valley, along-axis variations in mantle Bouguer anomalies, and relationships between seafloor roughness, segmentation style, and spreading rate. The program we have completed, combined with the results of previous SEIR programs to the east, now provides complete coverage of the SEIR ridge axis from 77°E to 120°E (the Australian-Antarctic Discordance), a distance of well over 4000 km.
