The Ipod Programme on Passive Continental Margins

During the past 200 Ma (1 Ma = 10⁶ years) the arrangement of continents and ocean basins has been reorganized from a pattern of one supercontinent, with mainly plate edge, subduction, or active continental margins bordering one essentially contiguous ocean basin, to the present configuration of dispersed continents and several oceans. Most of the world's present continental margins which were formed during that 200 Ma period are `passive' margins lying within the interiors of lithospheric plates. Several models of rifting and evolution of these passive margins have been proposed. The objectives of Ipod include testing of these models by learning as much as we can about the history of rifting of passive continental margins, their internal structure, distribution of facies, subsidence history, and the nature of the transition and modification of the crust at the margin. These objectives cannot be attained by drilling alone, and geophysical surveying and analysis of samples from the drilling are essential parts of the overall programme.Legs 41, 47a and 50 have been devoted to the study of the eastern Atlantic and its margin by drilling on the slope, on the rise and in the adjacent basins off North Africa. South of the Canary Islands the Mesozoic history of the basins is extremely similar to that of the northwest Atlantic since at least the early Late Jurassic. Carbonates have been deposited in moderately deep basins on newly formed oceanic crust until both subsidence and partial isolation of the basins during the early Cretaceous favoured accumulation of carbonaceous shales. During the same times carbonate platforms then carbonaceous mudstones are found higher on the adjacent margins. North of the Canary Islands the Moroccan basin received massive terrigenous deposits during the latest Jurassic and early Cretaceous, possibly as a consequence of the tectonic evolution of the western Atlas region. The Cainozoic is characterized both by major changes is palaeo-oceanographic conditions that influence greatly the amounts and nature of the biogenic pelagic sediments, and by renewed tectonic activity in the basins as evidenced by the creation of volcanic edifices in the Cape Verde and Canary Islands areas and the uplift of the Cape Verde Rise.