Rifted continental margins are one of the primary topographic features on the Earth's surface. They provide evidence of one of the most significant geological processes, namely the rifting and breakup of continental lithosphere, which ultimately leads to the creation of new oceanic crust by seafloor spreading and the eventual accretion of the underlying mantle of the oceanic lithosphere. The rifting and breakup of continental lithosphere involves both tectonism on a crustal and lithospheric scale and magmatism that has its roots in adiabatic decompression melting within the asthenosphere. Rifted margins at which magmatism dominates are often described as volcanic, and those where it plays an apparently insignificant role are called nonvolcanic, even though a spectrum of magmatic intensity probably exists between these two extremes.
Although the initial stages of rifting, and the sedimentation that accompanies them, can be examined on land or even offshore on continental shelves, as has been done in the many basins drilled during hydrocarbon exploration, the final stages of the process invariably take place under oceanic water depths. Therefore, their study involves marine geological and geophysical investigations and, ideally, deep drilling. Nevertheless, the thick sediments commonly found on rifted margins hinder investigation, by deep seismic reflection profiling, of the pre- and synrift sediments and the acoustic basement, and presently prevent the use of deep scientific drilling to sample these rocks. Studies have therefore been restricted to those margins that, as a result of particular geographical or geological circumstances, have been relatively starved of sediment. Even so, the whole gamut of geological and geophysical techniques has not often been applied to any one such margin. Further, by its nature, continental rifting and breakup is a bilateral process involving a pair of conjugate margins so that to obtain the most complete picture, such margins should be studied in pairs. Given the logistical and funding problems of studying pairs of margins, now located on different continents, such conjugate pairs of margins have been studied even more rarely. Between 1988 and 1991, a dozen proposals for scientific drilling on rifted margins in the North Atlantic Ocean were put forward. The JOIDES Planning Committee (PCOM), recognizing the importance of studying pairs of conjugate margins and the rather piecemeal approach of the past, set up a North Atlantic Rifted Margins Detailed Planning Group (DPG) to recommend the way forward. The DPG identified two drilling transects, one designed to investigate a nonvolcanic margin and the other a volcanic margin. Consequently, Ocean Drilling Program (ODP) Leg 149 was programmed in the Iberia Abyssal Plain as the first of a series of legs to address a transect across the Iberia/Newfoundland pair of conjugate nonvolcanic margins.