Fig. 5.8. The southeastern Pacific Ocean showing magnetic lineations (after Cande et al., 1982) with anomaly numbers after Heirtzler et al. (1968). Spreading ridges shown by heavy solid lines. Fracture zones are marked by dotted lines. Inactive plate boundaries by cross lines. 2,000 m bathymetry contours marked by dashed line (Mercator projection).

Aluk Ridge has been subducted under West Antarctica (Fig. 5.8). When this spreading centre, between the Tula Fracture Zone and Peter I Island, reached the trench axis, sometime after anomaly 22 time (55 Ma B.P.), the corresponding part of the Aluk (Phoenix) plate had been completely subducted and the plate boundary became inactive. Likewise, between the Tula and Hero Fracture Zones to the northeast, the margin stabilized at about anomaly 6 time (30 Ma B.P.) (Cande et al., 1982). Subduction of the Aluk flank of the Antarctic-Aluk spreading system is continuing along the adajcent continental margin, between the Shackleton and Hero Fracture Zones, under the South Shetland Islands, and a spreading centre was active in Drake Passage, east of the Hero Fracture Zone from early Miocene time until 5 Ma B.P.

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