Hole 735B is located 18 km east of the Atlantis II Transform Fault on the Southwest Indian Ridge (SWIR) at a unique outcrop of lower oceanic crust exposed on a 15-km-long wave-cut terrace at 700 m water depth. The hole represents the deepest penetration into plutonic basement in the world's oceans. Leg 176 will deepen Hole 735B to a nominal depth of 1.5 km, although it will attempt to go deeper if conditions permit. The proposed drilling has two principle objectives: (1) to test whether seismic layer 3 is principally constituted of gabbroic rock produced by igneous intrusion or whether it may contain substantial portions of partially serpentinized peridotite; and (2) to obtain a representative section of seismic layer 3 from which the magmatic, tectonic, and hydrothermal processes responsible for the accretion of the lower crust at slow-spreading ridges can be accurately inferred. The drilling results will provide a direct test of the accuracy of common assumptions made as to the composition of the lower crust and its thickness when computing global geochemical fluxes between the earth's interior, its crust, the oceans, and the atmospheres.

Extensive logging will be conducted, including Formation MicroScanner and a downhole Vertical Seismic Profile with a packer set for stability during the experiment. Downhole temperatures will be obtained along with the caliper log upon first re-occupying the hole.

If drilling conditions prevent deepening of Hole 735B, an alternate strategy, originally envisioned as the second of a two-leg drilling plan, will be adopted. This alternate strategy will test the lateral and temporal variability in the deep ocean crust by drilling a series of 500-m-deep holes offset by 100,000-yr increments along a lithospheric flow line. In the event that further drilling on the bare rock platform becomes impossible, or if the time remaining in the leg is short, the drill ship will move to backup Site SWIR 6 to drill a single-bit hole as deep as possible through a sediment pond and into basaltic basement. This site is located on crust of the same age as Hole 735B, but is situated north of the SW Indian Ridge where a volcanic carapace like the one that once covered Site 735 is preserved.

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