Through the last three decades, one of the main scientific goals of both the Deep Sea Drilling Project (DSDP) and the more recent Ocean Drilling Program (ODP) has been to drill deep into the Earth to study the composition and structure of the lower oceanic crust and upper mantle. The return to Hole 735B during ODP Leg 176 resulted in the deepening of the previous depth of 500 meters below seafloor (mbsf), achieved during ODP Leg 118, to the deepest penetration of a lower oceanic crust segment ever accomplished, at 1508 mbsf. The 86.3% core recovery of gabbroic rocks achieved during both legs (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999) provided an excellent opportunity to study lower crustal rocks analogous to ophiolitic sequences and assess the seismic nature of oceanic Layer 3. These combined drilling results make Hole 735B a significant accomplishment in understanding the nature of the oceanic crust because we are now able to describe the physical properties and the magmatic, structural, and metamorphic history of a block of the lower ocean crust that formed at a very slow-spreading ridge.
Site 735 is located at 32°43.40´S, 57°16.00´E (Fig. F1) on the rift mountains along the eastern transverse ridge of the Atlantis II Fracture Zone, which is a 210-km-long left-lateral offset of the Southwest Indian Ridge (SWIR). At ~80 Ma, the SWIR became the western segment of the Indian Ocean triple junction as a direct result of the initial breakup of Gondwanaland (Norton and Sclater, 1979; Fisher and Sclater, 1983; Sclater et al., 1981; Tapscott et al., 1980). A subsequent northeastern migration of the triple junction created a series of transform systems that include the Atlantis II Fracture Zone. Since ~34 Ma, the spreading rate along the SWIR has been an ultra-slow half-rate of 0.8 cm/yr (Fisher and Sclater, 1983; Dick et al., 1991). This slow-spreading environment has caused segmented magmatic events that are responsible for along-strike changes in the structure and stratigraphy of the thin crustal sections in this area and provides excellent targets to drill deep crustal sections.
Hole 735B was drilled on a shoal platform, known as the Atlantis Bank, in ~700 m of water. It is believed that this platform, which is 5 km long in a north-south direction and 2 km wide, is part of a series of uplifted blocks that lie on a ridge parallel to the transform valley. These irregularly shaped blocks are defined by steep slopes attributed to normal faulting and may represent simple horsts uplifted relative to both the rift valley floor at which this crust originated and the adjacent oceanic crust to the east. Similar topographic features have been previously found in fracture zones and are commonly underlain by upper mantle and lower crustal rocks (Engel and Fisher, 1969, 1975; van Andel et al., 1971; Bonatti, 1978; Bonatti and Hamlyn, 1978; Bonatti and Crane, 1982; Karson and Dick, 1983). The age of the crust at this site, based on magnetic anomalies (Dick et al., 1991) and a zircon U-Pb isotopic age date (Stakes et al., 1991), is ~11.5 Ma.
The lithostratigraphic units recovered from Hole 735B (Fig. F2) are mainly metagabbros, olivine gabbros, gabbros, Fe-Ti oxide gabbros, gabbronorites, and troctolitic gabbros (Robinson, Von Herzen, et al., 1989; Dick, Natland, Miller, et al., 1999). Several prominent shear zones are present, with the uppermost 40 m of the hole (Unit I) showing the most intense deformation, whereas the bottommost 900 m of the hole has significantly less deformation and a more uniform composition. The unusually high core recovery for the entire 1500-m section represents more igneous rock recovery than that on any previous DSDP or ODP leg. Therefore, this hole provides an unprecedented opportunity to determine the physical properties of the lower crust, calibrate results with compositional and structural variations, and refine previous models of the seismic stratigraphy of oceanic Layer 3.
Here we present results from laboratory measurements from the entire section and downhole logging data from the upper 600 m of Hole 735B in an attempt to understand the factors controlling the seismic signature of the lower oceanic crust. Logging data are available only from the top 600 m because 1400 m of drilling pipe was lost during Leg 176 as a result of severe weather conditions, and only the upper 600 m of the hole was cleared of the obstruction prior to abandoning the site.