BACKGROUND (continued)

Geology of the Atlantis II Fracture Zone
The Atlantis II fracture zone (FZ) (Fig. 3) was first described by Engel and Fisher (1975) and mapped in detail by Dick et al. (1991a, 1991b). It is a 199-km, 20-Ma left lateral offset of the SW Indian Ridge running almost due north-south at about 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, sloping from 25° to 40°, and are covered with extensive talus and debris. The floor of the transform has a thick, >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 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 are 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 (Fig. 3).

Well-defined magnetic anomalies were mapped out to 11 Ma over the flanking transverse ridges and transform valley, even over large areas where dredging during the site survey for Leg 118 showed 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., 1991a), a possibility raised by the early laboratory work of Kent et al. (1978), and subsequently confirmed by down hole logging (Pariso et al., 1991) and measurements on recovered cores from Hole 735B (Kikawa and Ozawa, 1992; Kikawa and Pariso, 1991; Pariso and Johnson, 1993). In fact, the gabbro massif at Site 735 is the only location in the ocean where the age pick of the magnetic anomaly (anomaly 5A'', 11.5 Ma; Dick et al., 1991a) 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. It 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., 1991a). The bank consists of a platform, roughly 9 km long in a north-south 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. A video survey of a 200- x 200-m area in the vicinity of the hole showed a smooth, flat wave-cut platform exposing foliated and massive jointed gabbro locally covered by sediment drift (Robinson, Von Herzen, et al., 1989). The platform was probably formed by erosion of an island similar to St. Paul's Rocks in the central Atlantic, and then subsided to its present depth from normal lithospheric cooling (Dick et al., 1991a). A similar wave-cut platform occurs on the ridge flanking the DuToit Fracture Zone (Fisher et al., 1986).

The boundary between magnetic anomaly 5 and 5A crosses east-west directly over the platform (Fig. 4). The boundary trends southward down the wall of the transform at a sufficiently shallow angle that, based on a simple projection into the massif, a successful penetration down to 2 km below seafloor at Hole 735B would likely penetrate the magnetic transition between them.

The foliation apparent in the images from the Leg 118 video survey and at the top of the drill core strikes east-west, parallel to the ridge axis and orthogonal to the fracture zone. The orientation of similarly foliated peridotites exposed on St. Paul's Rocks has been measured and is also parallel to the Mid-Atlantic Ridge and orthogonal to St. Paul's Fracture Zone (Melson and Thompson, 1970). This foliation, projected along strike across the Atlantic Bank platform, intersects a long ridge coming up the wall of the fracture zone, which is oriented obliquely west-northwest to the transform. Ridges produced by land-slips and debris flows normally are oriented orthogonal to the fracture zone. Our suspicion then is that this oblique ridge, and a similar one 2 km to the north, represent the trace of the thick zone of foliated gabbros down the wall of the transform. Given the once shallow water depth, the canyon between the two ridges may be erosional and the foliated gneissic amphibolites may be resistant remnants. A three-point solution for the dip of the shear zone, based on the trend of this ridge and an east-west strike, gives a dip of approximately 40°, which is close to that observed in the drilled amphibolites.

This shear zone represents a ductile fault and, thus, does not represent a simple stratigraphic discontinuity. The rocks at the top of the shear zone are 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 an unspecified amount. Any offset drill sites to the north would, therefore, start higher in the stratigraphic section. 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 crystallized and been ductily deformed (~11.5 Ma) beneath the median valley of the Southwest Indian Ridge 15 to 19 km from the ridge/transform intersection.

The gabbros were subsequently uplifted in a large horst from beneath the rift valley 5 to 6 km up into the transverse ridge (Dick et al., 1991a; Magde et al., 1995). The single uniform magnetic inclination throughout the section demonstrates that there has been no late tectonic disruption of the section, although the relatively steep inclination suggests block rotation of up to 18° (Pariso et al., 1991). Thus, unlike dredge samples from the transform walls, those drilled in Hole 735B formed beneath the rift-valley floor away from the transform fault and faults formed during formation of the valley. Petrologically, these rocks likely represent a typical igneous section of Southwest Indian Ridge ocean crust with an intact metamorphic and tectonic stratigraphy recording brittle-ductile deformation and alteration at high temperatures beneath the rift valley, as well as subsequent unroofing and emplacement on ridge-parallel faults.

The unroofing and exposure of the Hole 735B section relates to the present-day asymmetric distribution of plutonic and volcanic rocks north and south of the ridge axis near the fracture zone, as well as to the striking physiographic contrast between crust spreading in opposite directions at the ridge/transform intersection ( Dick et al., 1991a; Fig. 3). These features suggest that a crustal weld periodically formed between the shallow levels of the ocean crust and the old cold lithospheric plate at the ridge/transform intersection. This weld caused the shallow levels of the newly formed ocean crust to spread with the older plate away from the active transform, thereby causing the creation of long-lived detachment faults. Beneath the faults, the deep-ocean crust that was spreading parallel to the transform was unroofed and emplaced up into the rift mountains to form a transverse ridge. A similar model was proposed (Dick et al., 1981; Karson and Dick, 1983) to explain the asymmetric physiography and distribution of plutonic and volcanic rocks at the Kane Fracture Zone in the North Atlantic. There, the surface of the detachment fault has actually been observed by submersible (Dick et al., 1981; Mével and Cannat, 1991). It has been suggested that detachment faults similar to the one proposed to explain unroofing of the lower crust at the Atlantic Fracture Zone occur periodically within rift valleys by fault capture during amagmatic periods (Dick et al., 1981; Harper, 1985; Karson, 1991). Thus, the structures and fabrics seen in core from Hole 735B are likely to be representative of the kinds of fabrics generally found in lower crustal sections formed at slow-spreading ridges (Fig. 5). It is true, however, that because of the proximity to the transform the extent of the ductile shear may be greater than elsewhere beneath the rift valley.

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