Supplementary Objectives (if deepening Hole 735B is not achieved)
• Begin a transect of 500-m-deep offset holes, which will have the eventual goal of
determining how the stratigraphy of the lower crust at a very slow-spreading ridge
varies in time and space.
• Establish a natural laboratory for future hole-to-hole seismic, magnetotelluric, and
permeability experiments for in situ direct determination of the physical properties
of oceanic seismic layer 3 at geologically relevant intervals.
• Recover the shallow basaltic carapace originally emplaced above Site 735 by
drilling a single-bit hole into the pillow basalt sequence at the conjugate drill site
north of the SWIR on crust of the same age.
The principal goal of Leg 176 will be to deepen Hole 735B to a depth sufficient to determine the nature of the magmatic, metamorphic, and tectonic processes in the lower oceanic crust and seismic layer 3. At the present time several different models fit the existing Hole 735B stratigraphy. This is because the 500-m sequence drilled to date may represent only the uppermost portion of the gabbroic crust, which could be anywhere from 1.5 to 5 km thick at this location. As this zone effectively represents a major thermal boundary layer during accretion of the crust, these rocks may not be representative of all of the remaining section. For example, three different models for the stratigraphy of the lower ocean crust are shown in Figure 6, any one of which can be reconciled with the cores already recovered from Hole 735B.
Model "A" proposes that the crust is made of relatively small gabbroic bodies discordantly cutting each other and partially serpentinized upper mantle. The proportion of gabbroic bodies to mantle lithologies diminishes with depth (see Swift and Stephen, 1992; Cannat, 1993; Sleep and Barth, 1994). This model suggests that the primitive troctolites recovered from the bottom of Hole 735B represent deep level gabbroic rocks, and continued drilling should rapidly intersect upper mantle lithologies. A second interpretation (Fig. 6B) is that the lower crust is made up of an assembly of small discordant gabbroic bodies that transit abruptly into the upper mantle following the concept proposed by Cann (1970) and Nisbet and Fowler (1978) and recently amplified by Smith and Cann (1993). Finally, a third model, based on ophiolite studies (Fig. 6C) suggests that the discordant gabbroic bodies occur only in the upper crust, and then grade downward into large layered gabbro intrusions, which have a sharp magmatic contact with upper mantle peridotite (Pallister and Hopson, 1981; Smewing, 1981).
An additional goal for this leg is to determine if seismic layer 3 can contain substantial quantities of partially serpentinized peridotite, and whether the MOHO could be an alteration front in the mantle. Inversion of the major and trace element composition of the basalts from crust that is the same age and in a similar position as Hole 735B on the conjugate lithospheric flow line north of the present day spreading center (Fig. 3) indicates an original igneous crustal thickness of about 3±1 km (Muller et al., 1997). At the same time the presence of coarse troctolites at the base of Hole 735B and serpentinized peridotite dredged downslope suggest that the igneous crust/mantle transition may occur within a kilometer or so of the bottom of Hole 735B (Dick et al., 1992). Wide-angle seismic refraction profiles over and around the Atlantis II Bank, however, image a distinct MOHO 4-5 km below the seafloor (Muller et al., 1997; Fig. 7), despite the fact that the original basaltic carapace of pillow lavas and dikes has been tectonically removed. The seismic boundary could represent a magmatic transition from gabbroic rocks to peridotite; however, it is much deeper than anticipated for gabbroic crust at the SWIR. The MOHO reflector can also be traced eastward into "normal" ocean crust away from the transform, where it shoals to about 4 km below basement. At the same time, crust with layer 2 velocities, which is absent at Site 735, also appears about 3 km to the east of Hole 735B, and gradually increases in thickness to about 1.5 km. Thus, the seismic crustal section becomes thinner away from Site 735B, and seismic layer 3 thins to about 2.5 km. The increase of seismic layer 3 from 2.5 km thick well away from the Atlantis II Transform to 5 km near the transform seems inexplicable in the light of current ocean crust models, unless layer 3 can, at least locally, contain significant amounts of partially serpentinized peridotite and the MOHO beneath Site 735 is an alteration front in the mantle (Dick, 1996; Muller et al., 1997).
Thus, Leg 176 will return to Site 735 on the Southwest Indian Ridge with the objective of deepening Hole 735B to at least 1.5 km or deeper, if conditions permit. Such a section should be sufficient to adequately characterize the lower ocean crust at a slow-spreading ridge and may be sufficient to penetrate the igneous crust/mantle boundary, if this indeed does not coincide with the Mohorovicic discontinuity. If the latter occurs, scientists will have their first opportunity to sample the fundamental transition between rocks produced by crystallization of magma extracted from the Earth's mantle and the residues from which these liquids were derived. Our goals include documenting the depths to which fluids penetrate the lower ocean crust and perhaps the sub oceanic mantle, and defining gradients in metamorphic facies, if they exist. From a tectonic perspective, deepening Hole 735B will establish the spacing and morphology of the major ductile shear zones recognized in the earlier cores, and hence the role of tectonic extension as opposed to simple magmatic accretion in the formation of the lower ocean crust. Layered cumulate gabbros, as might be expected according to the ophiolite model of ocean crust stratigraphy, were not included in the nearly complete section recovered during Leg 118, and continued drilling will establish whether they are absent in this environment, thereby discounting the hypothesis of a long-lived magma chamber.
In the event that drilling conditions preclude deepening Hole 735B, we will start a new hole, based on a short local-scale survey with the video camera. This would be drilled to the maximum depth possible in the remaining time to fulfill our original objectives to the extent possible. The position of this hole would be in the proposed offset transect of 500-m-deep holes centered around Hole 735B. This transect would be situated either along a lithospheric flow line or on an isochron to the east depending on the local geology and site availability. A nominal spacing of 800 m was originally proposed for these holes, as a suite of five holes would represent a distance of 3.2 km, which is roughly equal to the typical half-width of the inner rift valley floor of the SWIR, and thus a logical length scale to first test the variability of crustal accretion. This spacing equates to 80,000 yr age increments along the lithospheric flow line. The precise location of the first offset hole, however, will be based on geologic information from the further deepening of Hole 735B and on site local surveys with the Resolution's video system, and it is possible that the co-chiefs, with the advice of the shipboard party, may find that a site offset along an isochron to the east may produce a greater scientific return.
A final back-up site is located at SWIR 6 (Fig. 2). This site will be drilled in the event that further drilling at Site 735 is precluded, or the time remaining does not warrant setting a new guide base. This site is situated on crust of the same age as Site 735 on the counter-lithospheric flow line to the north of the present day SWIR axis. Based on seafloor morphology, dredging, and single channel seismics the volcanic carapace corresponding to the Hole 735B gabbros is preserved intact beneath a sediment cover. This would be a single-bit hole, drilled to destruction or until time allotted for operations expires, and spudded in a sediment pond designed to recover a 100-m section of the volcanics.