Hole 735B is located adjacent to the Atlantis II Transform Fault on the ultra-slow-spreading Southwest Indian Ridge (Fig. F1). It was first described by Engel and Fisher (1975) and was mapped in detail by Dick et al. (1991b; Fig. F2). It is a 199-km, 20-m.y.-old north-south left-lateral ridge offset at 57ºE. Although the spreading rate of the Southwest Indian Ridge has been fairly constant for the last 30 m.y. at 16 mm/yr (Fisher and Sclater, 1983), the transform lengthened at about 4 mm/yr because of asymmetric spreading at both ridge axes for at least the last 20 m.y. (Dick et al., 1991c). The transform valley is deep (up to 6480 m), with flanking transverse ridges shoaling to as little as 694 m (at Atlantis Bank). The walls are extremely steep, typically sloping from 25º to 40º, and are covered with extensive talus and debris. The transform floor has a >500-m-thick sequence of turbidites shed from the walls and is bisected by a 1.5-km-high median tectonic ridge. Extensive dredging shows that more than 80% of the crust exposed in the active transform valley and its walls is serpentinized peridotite, diabase, and gabbro, whereas only 16.3% is pillow basalt corresponding to the shallowest layer of the crust. By contrast, only relatively undisrupted pillow lavas appear to be exposed on crust along the non-transform walls of the inactive fracture zone valleys north and south of the present-day ridge axes (Dick et al., 1991c; e.g., Fig. F3).
Hole 735B is located at 720 meters below sea level (mbsl) on Atlantis Bank, close to the crest of a 5-km-high transverse ridge that forms the eastern wall of the Atlantis II transform valley (Fig. F3A). It is 93 km south of the present-day ridge axis and 18.4 km from the inferred axis of transform faulting (Dick et al., 1991c). The Bank consists of a steep-sided flat-topped platform, roughly 9 km long north-south and 4 km wide. It is the shallowest of a series of uplifted blocks and connecting saddles that form the transverse ridge. The top of the platform has only ~120 m relief over ~25 km2 above the 800-m contour (see "Bathymetry of Atlantis Bank" chapter, which is an oversized figure that accompanies this volume). The boundary between magnetic Anomalies 5r.2n and 5r.2r crosses east-west directly over the platform just north of Hole 735B (Fig. F4).
A 4-hr 200 m × 200 m video survey conducted near the hole during ODP Leg 118 (Fig. F5) shows a surface consisting of flat outcrops of foliated and jointed gabbro with minor relief, featureless sediment of undetermined but likely insignificant thickness, and basement with such a thin coat of sediment that outcrop textures were easily distinguished through it in the video image. The strikes of distinct joints and fractures estimated from the motion vector of the camera frame and the direction of ship motion, (with roughly 5º-10º error) are also indicated on the survey map (Fig. F5).
The generally east-west foliation around Hole 735B is roughly orthogonal to the transform and parallel to the ridge axis. When projected westward across the platform, it intersects a long oblique west-northwest-trending ridge coming up the wall of the fracture zone (Fig. F3A). Ridges produced by landslips and debris flows are normally oriented orthogonal to the fracture zone. Possibly this oblique ridge, and a similar one 2 km to the north, represent the traces of inclined shear zones dipping to the north, exposed on the transform wall. Given the once shallow water depth, the canyon between these ridges may be an erosional remnant trough between resistant foliated gneissic amphibolites (Dick et al., 1991c). Assuming the ridge represents a shear zone, a three-point solution for the dip, based on an east-west strike, gives ~40º--close to the average for the amphibolite mylonites drilled in the upper 100 m of Hole 735B.
Well-defined magnetic anomalies cross over Atlantis Bank, and Site 735 is the only location in the ocean basins where the age of the magnetic anomaly (Anomaly 5r.2n, 11.75 Ma; Figs. F2, F3, F4) has been confirmed, within error, by a zircon U-Pb isotopic age date of 11.3 Ma from a trondhjemite sampled in situ (Stakes et al., 1991). Given the position of the site, the relatively constant spreading direction over the last 11 m.y., and the ridge-parallel strike of the local foliation, the Atlantis Bank gabbros must have accreted beneath the ridge axis 15 to 19 km from the ridge-transform intersection around 11.5 Ma. The smoothly domed, very nearly flat surface of the platform suggests that it formed by wave erosion of a small island created when the gabbros were unroofed and uplifted at the inside-corner high of the ancestral Southwest Indian Ridge at 11 Ma. The platform then subsided to its present depth by means of normal lithospheric cooling (Dick et al., 1991c). A similar wave-cut platform, from which rounded cobbles of peridotite have been dredged, exists on the Southwest Indian Ridge at the southern ridge-transform intersection of the DuToit Fracture Zone (Fisher et al., 1986).
The exposure and emplacement of the Hole 735B section likely occurred by unroofing at a long-lived detachment fault on the rift valley wall followed by block uplift into the inside-corner high at the ancestral ridge-transform intersection of the Atlantis II Fracture Zone (Fig. F6; Dick et al., 1991c). Similar exposures of plutonic rock have been found at several present-day ridge-transform intersections, most notably at the eastern inside-corner high of the Kane Fracture Zone in the Atlantic. There, as at the present-day northern ridge-transform intersection of the Atlantis II Fracture Zone, an intact volcanic carapace is found in the rift mountains on only one side of the rift valley, spreading in the direction away from the active transform. On the other side of the rift valley, deep crustal rocks and mantle peridotite are exposed on the rift valley wall at the inside-corner high. This remarkable asymmetry is well illustrated by comparing the seafloor topography around Hole 735B to that for crust of the same age situated on the opposite northern lithospheric flow line (Fig. F3B). This asymmetry is inferred to result from the periodic formation of a crustal weld between new ocean crust and the old cold lithospheric plate at the ridge-transform intersection. Because of this weld, the more rigid shallow levels of the newly formed ocean crust spread with the older plate in the direction away from the active transform. At depth, beneath the brittle-ductile transition, the plutonic section spreads symmetrically in both directions (Dick et al., 1981; Karson and Dick, 1983). At the Kane Fracture Zone, the surface of the detachment fault has been directly observed by submersible (Dick et al., 1981; Karson et al., 1987, Mével et al., 1991). Such detachment faults are suggested to form periodically by fault capture during amagmatic periods at slow-spreading ridges (Harper, 1985; Karson, 1990, 1991).
The ultra-slow-spreading rate of the Southwest Indian Ridge has important implications for crustal structure at Site 735. Although crustal thickness is roughly constant with spreading rate above a half rate of 10 mm/yr, it appears to drop off rapidly at slower rates (Reid and Jackson, 1981; Jackson et al., 1982). Seismic refraction measurements at both the Arctic and Southwest Indian Ridges indicate typical crustal thicknesses of ~4 km, including normal ocean crust immediately east of Hole 735B (Jackson et al., 1982; Bown and White, 1994; Minshull and White, 1996).
Muller et al. (1997) found that the seismic thickness of the crust north and south of Atlantis Bank is 4 km, with Layer 2 having a normal thickness of ~2 km (Fig. F7). Beneath Atlantis Bank, however, the depth to the Moho is 5 ± 1 km below the seafloor (Muller et al., 1997), despite the absence of either pillow lavas or sheeted dikes corresponding to seismic Layer 2. It is unlikely, in this environment, that this can all be igneous crust. In addition, mantle peridotites were dredged on the walls of the bank not far from Hole 735B at 4382 m, 1.3 km above the Moho (Dick et al., 1991c). Inversion of rare earth element concentrations in basalts dredged from the conjugate site to the north, exposing crust of the same age as Atlantis Bank, suggest a crustal thickness of 3 ± 1 km (Muller et al., 1997). Thus, Muller et al. (1997) agree with the geologic evidence (Dick et al., 1991c) that the crust is thin beneath Atlantis Bank and that the Moho there represents an alteration front in the mantle.