Tectonic Setting
Site 735 is located in the rift mountains of the Southwest Indian Ridge (SWIR), 18 km east of the present-day axis of the Atlantis II Transform Fault (Fig. 2). The Southwest Indian Ridge has existed since the initial breakup of Gondwanaland in the Mesozoic (Norton and Sclater, 1979). Shortly before 80 Ma, plate readjustment in the Indian Ocean connected the newly formed Central Indian Ridge to the Southwest Indian Ridge and the Southeast Indian Ridge to form the Indian Ocean Triple Junction (Fisher and Sclater, 1983; Sclater et al., 1981; Tapscott et al., 1980). Steady migration of the triple junction to the northeast has created a succession of new ridge segments and fracture zones including the Atlantis II. Thus, the Atlantis II Fracture Zone and the adjacent ocean crust is entirely oceanic in origin, free from complications due to continental breakup as postulated for some equatorial fracture zones along the Mid-Atlantic Ridge (Bonatti and Honnorez, 1976).

Over the last 34 m.y., the spreading rate along the Southwest Indian Ridge has been relatively constant, near 0.8 cm/yr, at the very slow end of the spreading-rate spectrum (Fisher and Sclater, 1983). All the characteristic features of slow-spreading ridges, including rough topography, deep rift valleys, and abundant exposures of plutonic and mantle rocks are present on the Southwest Indian Ridge (Dick, 1989). Significantly, two thirds of the rocks dredged from the walls of the active transform valleys are altered mantle peridotites, whereas most of the remainder are weathered pillow basalts. This exceptional abundance of peridotite, compared to dredge collections of similar size from the North Atlantic Ocean, indicates an unusually thin crustal section in the vicinity of Southwest Indian Ridge transforms. Moreover, the paucity of dredged gabbro along the Southwest Indian Ridge suggests that magma chambers were small or absent near fracture zones.

The thin crust adjacent to fracture zones is thought to reflect segmented magmatism along the Southwest Indian Ridge, which produces rapid along-strike changes in the structure and stratigraphy of the lower ocean crust (Whitehead et al., 1984). This model views the Southwest Indian Ridge as a series of regularly spaced, long-lived shield volcanoes and underlying magmatic centers, which undergo continuous extension to form the ocean crust (Dick, 1989). Site 735 is located some 18 km from the Atlantis II Transform Fault, and was accordingly situated near the mid-point of a hypothetical magmatic center beneath the Southwest Indian Ridge 11.5 Ma (Dick et al., 1991a).

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