Many of the sites drilled during Leg 177 are associated with the Agulhas Basin and are arranged along a latitudinal transect extending from the Agulhas Fracture Zone Ridge in the north, to Meteor Rise in the subantarctic, and to Shona Ridge and Bouvet Island in the south (Figs. 1, 5, 6). The Agulhas Basin lies on the African plate and is bounded by the Agulhas Fracture Zone to the north, the Southwest Indian Ridge to the south, the Meteor Rise on the west, and the Agulhas Plateau to the east. The topographic complexity of the Agulhas Basin bears testimony to its tectonic history (du Plessis, 1977; LaBrecque and Hayes, 1979; Cande et al., 1988; LaBrecque, 1986; Raymond and LaBrecque, 1988, 1991; Henson and Ruppel, in press).

The three northern sites (Sites 1088, 1090, and 1091) are associated with the Agulhas Ridge (Fig. 5), which is an elongate topographic feature that parallels the Agulhas Fracture Zone. The Agulhas Ridge extends from the northern tip of the Meteor Rise and terminates abruptly at 40°S, 15°E, where it intersects the northern end of an abandoned spreading axis in the Agulhas Basin. At ~65 Ma, the spreading axis south of the Agulhas Fracture Zone jumped 825 km to the west, and motion along the transform fault was abandoned (du Plessis, 1977; LaBrecque and Hayes, 1979; Barker, 1979). Before the ridge jump, the very large offset Agulhas transform (1400 km displacement of the spreading axis or maximum age offset of ~45 Ma) was subjected to increasing compressional stress as a result of the changing relative motion of the South American and African plates. This compression across the transform may have resulted in thrusting of the South American plate over the African plate, creating the Agulhas Fracture Zone Ridge (C. Raymond, pers. comm., 1996). Alternatively, the Agulhas Ridge may have formed from extension at the fracture zone resulting in serpentinite diapirism (Bonatti, 1978), or volcanic construction resulting from extension and/or a robust magma source, such as the Shona Hotspot (Menard and Atwater, 1969; Kastens, 1987; Hartnady and le Roex, 1985).

Site 1088 is located at the northeastern end of the Agulhas Ridge, near the intersection of the fossil ridge transform (Fig. 5), where it is a broad feature with 2250 m of relief. Site 1089 is located north of the Agulhas Ridge in the southernmost Cape Basin where the oceanic crust is older than magnetic Anomaly 34 (Upper Cretaceous; Fig. 6). Site 1090 is located on the southwest portion of the Agulhas Ridge, where it narrows considerably and the topography steepens and becomes more intricate.

Sites 1091 and 1092 are associated with the Meteor Rise, which is one of the dominant topographic features in the southeast Atlantic and marks the westward limit of the Agulhas Basin (Fig. 5). This oval-shaped, aseismic plateau rises to water depths of 2000 m and consists of basement highs with intervening depositional basins. The Meteor Rise is the conjugate feature of the Islas Orcadas Rise in the western South Atlantic, and formed either by (1) effusive volcanism at the developing rift zone associated with the ridge jump from the Agulhas Basin to the west at ~62 Ma (Fig. 7; LaBrecque and Hayes, 1979; Mutter et al. 1988) or (2) passage of the Shona Hotspot across the Agulhas Fracture Zone and the subsequent ridge jump to the preweakened hotspot trace (Hartnady and le Roex, 1985). Site 1091 is located on the western flank of Meteor Rise on magnetic anomalies 24-25 (early Eocene-late Paleocene), which represents the oldest oceanic crust between the Meteor Rise and the Mid-Atlantic Ridge (Fig. 6). Site 1092 is located on the north central Meteor Rise (water depth 1976 m) and is probably underlain by volcanic basement of early Eocene age or older.

Site 1093 is associated with the Shona Ridge that was formed by a hotspot (Shona Hotspot) that may presently reside between 50° and 52.5°S near an anomalously shallow segment of the Mid-Atlantic Ridge (Hartnady and le Roex, 1985; Douglass et al., 1995). Acoustic basement is anomalously smooth in the region of Site 1093, and is estimated to be located on late Miocene crust by extrapolating magnetic anomalies of Cande and Kent (1992). Lastly, the southernmost Site 1094 is located in a small sedimentary basin north of Bouvet Island. This site is located close to the Bouvet Fracture Zone, where the basement age is not well defined from the available magnetic data.

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