Leg 45 of the Deep Sea Drilling Project was the inaugural cruise of the International Phase of Ocean Drilling. Its primary scientific objective was to develop an understanding of the composition, structure, and origin of oceanic crust by drilling as deeply into it as technology would permit.The feasibility of drilling deep into the oceanic crust was demonstrated during Leg 37 when one hole was drilled 586 meters into basaltic volcanic rocks near the crest of the Mid-Atlantic Ridge at about 37 °N. Drilling of this hole, 332B, required repeated retrieval of the drill string to replace worn bits. This was the first successful multiple re-entry hole in the history of DSDP. On the basis of the Leg 37 experience, the re-entry system was strengthened and improved, in the hope that a hole as deep or deeper would be drilled on Legs 45 and 46.Legs 45 and 46 were conceived as a combined attempt either to drill a single deep hole into North Atlantic oceanic crust, using, if possible, all the time of both cruises, or, in the event of early loss of a deep hole, to drill holes in several surveyed areas in a roughly eastwest transect, out to crust about 110 million years old. The areas of principal interest for Leg 45 were survey areas AT-5 (see Hussong et al., this volume), situated near 23 °N, 46 °W (Figures 1 through 3), and AT-6 (see Purdy et al., this volume), near 23 °N, 43°W (Figures 4 and 5). The two survey areas are on opposite sides of the Mid-Atlantic Ridge, south of the Kane fracture zone in the vicinity of magnetic anomalies 4,4', and 5 (see Plate 1 in pocket). Both regions were generated along approximately the same segment of the Mid-Atlantic Ridge between 6 and 13 million years ago, and have since been separated by sea-floor spreading.The drilling on Legs 45 and 46 was only the initial stage of a longer term program aimed at determining the petrologic, geochemical, magnetic, physical, and structural properties of the crust beneath the ocean floor, to obtain an understanding of the processes which produce these properties, and to outline the evolution of oceanic crust through time. At this writing, the North Atlantic portion of this program has been completed and crustal
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