The rifting and breakup of continental lithosphere to form passive or rifted continental margins is one of the most fundamental geological processes on Earth. This process has many physical similarities with the formation of sedimentary basins and relates directly to the onset of seafloor spreading, a process that generates mid-ocean ridges and the crust underlying the ocean basins. However, largely because of the thick to very thick (2-15 km) sediments found on many rifted margins and the consequent inaccessibility of basement rocks to scientific drilling, rifted margins have so far been studied largely by remote geophysical means or by the post-rift and synrift histories recorded in the overlying sediments at a few "sediment-starved" margins. In the northeast Atlantic Ocean, for example, nonvolcanic rifted margins were drilled off Goban Spur (de Graciansky, Poag, et al., 1985), the northern Bay of Biscay (Montadert, Roberts, et al., 1979), Galicia Bank (Boillot, Winterer, Meyer, et al., 1987), Vigo Seamount (Sibuet, Ryan, et al., 1979), and the Mazagan Escarpment (Hinz, Winterer, et al., 1984).
Quantitative models have been developed for the post-rift isostatic and thermal subsidence of margins and basins (McKenzie, 1978) and, more recently, for the critical effect of asthenospheric temperature at the time of the breakup of the lithosphere on the quantity and composition of melt, which may be underplated at the base of the crust and/or erupted as lavas at the surface (White and McKenzie, 1989). Finite-element models have also been developed for modeling the tectonic evolution of the crust and lithosphere under extensional margins (e.g., Harry and Sawyer, 1992). Nevertheless, in spite of the predictive capability of these geophysical models, some uncertainty remains about both the geological processes involved in the crust and mantle and, in particular, the location and nature of features such as the ocean/continent transition.
The North Atlantic Rifted Margins Detailed Planning Group (NARM DPG) was convened by the Planning Committee of the Ocean Drilling Program (ODP) and met in 1991 to plan a program of drilling to study the problems of the formation and evolution of rifted margins. The DPG identified two important classes of rifted margin for study: margins in which magmatism has dominated the rifting process (volcanic margins) and margins in which magmatism seems to have played a minor role in the rifting process (nonvolcanic margins). The DPG recommended that ODP focus on a transect of holes across each class of margin and that each transect include a conjugate pair of margins. The criteria for selecting the locations of the two transects included (1) the existence of high-quality geophysical data about both conjugate margins, (2) the presence of relatively thin sediment cover on the conjugate margins so that drilling into basement would be possible using the JOIDES Resolution, (3) the absence of salt, which might interfere with drilling, and (4) the absence of post-rift volcanism, which may have modified the divergent margin.
Leg 149 represented the first part of the program proposed by the DPG for the study of nonvolcanic margins. The total program, which was expected to require four 2-month legs, was to include drilling ofmultiple sites in both the Iberia Abyssal Plain and the conjugate Newfoundland Basin, and one site on the Galicia Bank margin. Drilling on each of the margins was to include sites that allowed for sampling of significant sections of basement with minimum sediment penetration, and sites having thicker and stratigraphically more complete sequences of synrift and post-rift sediments. During Leg 149, sites of the first type were drilled; a future deep hole in the Iberia Abyssal Plain will be planned to take into account the Leg 149 results. The program also was designed to allow for assessment of the degree of symmetry in the structure and evolution of the conjugate margins. Characterization of crustal type within a wide zone of thin continental or oceanic crust in the Newfoundland Basin and Iberia Abyssal Plain and the position and nature of the ocean/continent transition (OCT) on the two margins also were important scientific objectives. Geo-physical data suggest that seafloor exposures of mantle peridotite on the west side of Galicia Bank, to the north of the Iberia Abyssal Plain transect, may have a counterpart in the Iberia Abyssal Plain. If such exposures were to be found during the proposed drilling, then clearly they would be a feature of more than local significance. Sites designed to sample synrift sequences will constrain the timing of rifting and breakup, the rift environment, and possibly significant anomalous elevation and/or subsidence asymmetries, which were strongly indicated by recently acquired seismic data. The subsidence histories of the conjugate margins were expected to help in determining the relative importance of lithosphere-scale, pure and simple shear mechanisms of extension.
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Reproduced online: 15 October 2004