Leg 149 of the Ocean Drilling Program (ODP) explored the ocean-continent transition (OCT) on the Iberia Abyssal Plain and its role in the opening of the Atlantic Ocean approximately 130 Ma ago. Leg 149 was the first of several legs in the Atlantic Ocean to study rifted-margin formation and evolution. Transects were chosen across both volcanic and nonvolcanic conjugate rifted margins with basement being the major drilling target to better define the nature of such margins and their role in the opening of the Atlantic Ocean. The Iberia Abyssal Plain was chosen as the best defined nonvolcanic rifted margin on the eastern edge of the Atlantic Ocean (Fig. 1), the geologically conjugate Newfoundland margin was to be drilled on a later leg. The Iberia continental margin consists of several topographically distinct regions. In the northern part of the margin continental crust extends seaward under shallow water as the 200 km by 150 km Galicia Bank. The Galicia Bank contains a series of isolated seamounts along its southern edge and is separated from Iberia by a broad submarine valley. The Iberia Abyssal Plain lies south of the Galicia Bank (Fig. 2). Here the continental margin has a straight narrow shelf and a steep continental slope. South of 40°N the slope is cut by numerous deep canyons and at 39°N the east-west Estremadura Spur separates the Iberia Abyssal Plain from the Tagus Abyssal Plain that forms the southern part of the Iberia continental margin. South of the Tagus Abyssal Plain the east-northeast Gorringe Bank marks the seismically active plate boundary between Eurasia and Africa.
Three main episodes of Mesozoic rifting marked the separation of the Iberia Margin from the Newfoundland Grand Banks. The first episode was a Triassic to Early Jurassic rifting phase that produced graben and half-graben structures in which evaporites were deposited (Wilson et al, 1989; Murillas et al., 1990). A second rifting event consisted of extension in Late Jurassic time. A third episode of Early Cretaceous extension marked the south-to-north breakup of Iberia from the Grand Banks (Boillot, Winterer, Meyer, et al, 1987; Whitmarsh et al., 1990; Pinheiro et al., 1992). These studies found that the OCT crust in this region is only about 4 km thick and is underlain everywhere by a continuous layer with a velocity near 7.6 km/s that is probably serpentinized mantle peridotite. Plate reconstructions of Iberia with Europe and North America are difficult because Iberia was alternately attached to Europe and Africa (Srivastava et al., 1990).
Leg 149 was designed, on the basis of geophysical data, to drill across the entire OCT from the oceanic to the continental edge of the Iberia rifted margin but instead found an ocean-continent gap at least 130 km wide. Geophysical studies by Whitmarsh et al. (1990), Beslier et al. (1993), and Whitmarsh et al. (1993) have defined the crustal characteristics used to target drill sites on the Iberia Abyssal Plain. The major drill targets on the Iberia Abyssal Plain were a series of basement highs (Fig. 3) beneath several hundred meters of sediment cover. At Hole 900A a series of fine-grained recrystallized and foliated metamorphic rocks of mafic composition were drilled. Primary textures were completely destroyed and the only chance to determine the genesis of these rocks rests in determining and interpreting their compositional characteristics. This study deciphers the primary igneous petrogenesis of the ODP Hole 900A metamorphosed mafic rocks rather than the nature of their subsequent metamorphism. Consequently, our samples were chosen from parts of the core with the cleanest recrystallized plagioclase and clinopyroxene assemblage, avoiding regions that were largely retrograded to amphibole and other hydrous minerals.