The spreading rate along the Southwest Indian Ridge has been relatively constant for the last 34 Ma, near 0.8 cm/yr. This is at the ultra-slow end of the spreading-rate spectrum (Fisher and Sclater, 1983). This has important implications for crustal structure at Site 735, because although crustal thickness is relatively 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 about 4 km, including normal ocean crust immediately west of Hole 735B (Jackson et al., 1982; Bown and White, 1994; Minshull and White, 1996; Muller et al., 1997). All of this indicates that, with the absence of basalt extrusives and gabbros, the depth to the crust-mantle transition should be less than 2.5 km at Hole 735B.
All the characteristic features of slow-spreading ridges, including rough topography, deep rift valleys, and abundant exposures of gabbroic and ultramafic 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, when compared to dredge collections of similar size from the North Atlantic Ocean, indicates an unusually thin crustal section near Southwest Indian Ridge transforms. Moreover, the paucity of dredged gabbro near these transforms suggests that magma chambers were small or absent there as well.
Thin crust adjacent to fracture zones is thought to reflect segmented magmatism, which produces rapid along-strike changes in the structure and stratigraphy of the lower ocean crust (Whitehead et al., 1984). Thus, the Southwest Indian Ridge is seen as consisting of a series of regularly spaced 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 is therefore situated near the projection of the midpoint of the magmatic center beneath the Southwest Indian Ridge at 11.5 Ma (Dick et al., 1991b).
Geology of the Atlantis II Fracture Zone
The Atlantis II Fracture Zone (Fig. 2) was first described by Engel and Fisher (1975) and was mapped in detail by Dick et al. (1991b). It is a 199-km, 20-Ma northsouth left-lateral offset of the Southwest Indian Ridge at 57°E longitude. The transform is marked by a 6480-m-deep transform valley, with high flanking transverse ridges shoaling to as little as 700 m. The valley walls are extremely steep for large distances, typically sloping from 25° to 40°, and are covered with extensive talus and debris. The floor of the transform has a >500-m sequence of turbidites shed from the walls of the transform and is bisected by a 1.5-km-high median tectonic ridge. The ridge-transform intersections are marked by deep nodal basins lying on the transform side of the neovolcanic zones that define the present-day spreading axes, and which intersect clearly defined rift valleys with a relief of more than 2200 m and widths of 22 to 38 km. Extensive dredging showed that more than two thirds of the crust exposed in the transform valley and its walls is plutonic rocks, principally gabbros and partially serpentinized peridotites. By contrast, only relatively undisrupted pillow lavas appear to be exposed on crust of the same age and position relative to the transform fault on the conjugate lithospheric flow line to the north (Muller et al., 1997; e.g., Fig. 3).
Well-defined magnetic anomalies were mapped over the flanking transverse ridges and transform valley, even over large areas where extensive dredging shows that basalts are absent, including Site 735. This was the first direct demonstration that the gabbros and peridotites can constitute a magnetic source layer (Dick et al., 1991b), a possibility originally raised by the laboratory work of Fox and Opdyke (1973) and Kent et al. (1978). These results were confirmed in Hole 735B by downhole logging (Pariso et al., 1991) and measurements on cores (Kikawa and Pariso, 1991; Kikawa and Ozawa, 1992; Pariso and Johnson, 1993). In fact, the gabbro massif at Site 735 is the only location in the ocean basins where the age of the magnetic anomaly (Anomaly 5r.2n, 11.75 Ma; Figs. 2 4) has been confirmed, within error, by a zircon U-Pb isotopic age date of 11.3 Ma from a trondhjemite sampled in situ in basement (Stakes et al., 1991).
Hole 735B is located on a shallow bank, informally named Atlantis Bank, on the crest of a 5-km high mountain range, termed a transverse ridge, which constitutes the eastern wall of the Atlantis II transform valley (Fig. 3A). The hole is situated some 93 km south of the present-day Southwest Indian Ridge axis, and is 18.4 km from the inferred axis of transform faulting on the floor of the Atlantis II Fracture Zone (Dick et al., 1991b). The bank consists of a platform, roughly 9 km long in a northsouth direction and 4 km wide, which is the shallowest of a series of uplifted blocks and connecting saddles that form a long, linear ridge parallel to the transform. The top of the platform is flat, with only about 100 m relief over 20 km2. The boundary between magnetic Anomalies 5r.2n and 5r.2r crosses eastwest directly over the platform just north of Hole 735B (Fig. 4).
A 4-hour video survey of a 200 by 200 m area near the hole using the shipboard Vibration-Isolated Television (VIT) system during ODP Leg 118 showed a smooth, flat, evidently wave-cut platform exposing roughly eastwest foliated and jointed massive gabbro locally covered by sediment drift (Robinson, Von Herzen, et al., 1989). Using the time stamp on the video image and voice-over data on the tapes, an outcrop map was constructed of the survey area (Fig. 5). The surface consists 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 resolved through it in the video image. All of these indicate a monotonous, nearly flat hard-rock pavement. Relative depths measured by a length of weighted cable tied to the base of the VIT frame, though probably in error by a few meters, clearly showed that the southwest half of the survey area slopes gently toward the wall of the transform. Also indicated on the survey map are the estimated strike of distinct joints and fractures visible in the video image. While camera rotation and lack of orientation precluded exact and continuous measurements, the weight dragging across the seafloor, combined with the motion vector of the VIT frame deduced from the direction of ship motion, allowed orientation (within probably a 5° 10° error) of many of these features. These are plotted as lines along the side of the swath path on the map adjacent to the time stamp location where the observation was made, oriented according to the estimated attitude of the joint/fracture.
The generally eastwest foliation, which is orthogonal to the transform and parallel to the ridge axis, is similar in both respects to the orientation of foliated peridotites exposed on St. Paul's Rocks. These are also orthogonal to the adjacent St. Paul's Fracture Zone and parallel to the Mid-Atlantic Ridge (Melson and Thompson, 1971). The foliation on Atlantis Bank, projected from the position of Hole 735B westward along strike across the Atlantic Bank platform, intersects a long oblique west northwest trending ridge coming up the wall of the fracture zone (Fig. 3A). Ridges produced by land-slips 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 toward the axis of the paleo-Southwest Indian Ridge exposed on the wall of the transform. The shear zone crosses over the top of the platform in the vicinity of Hole 735B. 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., 1991b). A three-point solution for the dip of the shear zone, based on an east west strike gives a dip of approximately 40°, which is close to that observed in the amphibolites drilled in the upper 100 m of Hole 735B. The shear zone represents a ductile-fault, and thus is not a simple stratigraphic discontinuity. The rocks at the top of the shear zone at Hole 735B are largely gabbronorites that pass gradually into a zone of olivine gabbro toward its base. The shear sense determined from drill cores is normal and the rocks north of the drill site are down-thrown by an unspecified amount.
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 and been deformed beneath the median valley of the Southwest Indian Ridge, 15 to 19 km from the ridge transform intersection, around 11.5 Ma. The smooth flat surface of the platform suggests that it formed by wave erosion of a small island created when the gabbros were uplifted at the inside-corner high of the ancestral Southwest Indian Ridge at 11 Ma as a large horst block, similar to St. Paul's Rocks in the central Atlantic Ocean; the block then subsided to its present depth by means of normal lithospheric cooling (Dick et al., 1991b; Magde et al., 1995). A similar wave-cut platform, from which rounded cobbles of peridotite have been dredged, occurs on the Southwest Indian Ridge at the southern ridge-transform intersection of the DuToit Fracture Zone (Fisher et al., 1985).
The single uniform magnetic inclination measured in Hole 735B gabbros demonstrates that there is little significant late tectonic disruption of the section, although the relatively steep inclination suggests block rotation of up to 20° (Pariso et al., 1991). Thus the Hole 735B gabbros, which were drilled from the center of an uplifted horst block originally emplaced at an inside-corner high, formed beneath the rift-valley away from the major faults in the transform or on the rift valley walls. The section likely preserves a typical record of accretion, hydrothermal circulation, and brittle-ductile deformation beneath an active rift. However, the rocks also contain a later history, reflecting static block uplift to shallow depths and rapid cooling, but this is not the full record of thermal equilibration and alteration of mature ocean crust cooled at depth beneath an intact section of ocean floor during seafloor spreading.
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. 6; Dick et al., 1991b). Similar exposure of plutonic rock has 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 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. 3B). 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).
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