LEG 149

Iberian Abyssal Plain

Leg 149 was the first of a series of ODP legs designed to study the sedimentation history and tectonic evolution of the Iberia-Newfoundland conjugate rifted margins. The west Iberia margin comprises three segments which have experienced progressive breakup from south to north in Early Cretaceous times. The Ocean-Continent Transition (OCT) in the central segment has been located by seismic reflection and refraction profiles and by magnetic and gravity modeling which indicated that the oceanic crust adjacent to the OCT is thin, of the order of 4 km. Geophysical data suggested that seafloor exposures of mantle peridotite on the western margin of Galicia Bank extend southward beneath the sediments of the Iberia Abyssal Plain.

Drilling was conducted at five sites (Site 897 to Site 901) along a transect of the OCT, the main sites located over basement highs. The location of a peridotite ridge marking the OCT beneath the sediments of the Iberia Abyssal Plain was correctly predicted, but the discovery an outcrop of peridotite, possibly 19 km wide, was completely unexpected. Apparently, the peridotites were brought up to the seabed by the final stretching and break-up of continental crust as Newfoundland separated from Iberia about 130 Ma. One basement high was capped by a set of three unusual breccia flow units composed almost entirely of serpentinized peridotite and underlain by a mass- flow deposit. These breccia units have the characteristics of rapidly deposited flows with shear deformation under low normal stress typical of high fluid pressures. The unusual character of these flows, and their rare occurrence in the oceans, makes it difficult to establish the mechanism of their deposition. The mass-flow deposit was cored close to the top of the highest known basement high in the area and, since mass flows are driven by gravitational potential, it is hard to see how clasts from tens of miles away reached their present position unless there was a different arrangement of basement relief in the Early Cretaceous and the site, which is now a high, underwent significant uplift after deposition of the mass flow. From the region predicted to be extended continental crust, basement consists of mettagabbro which may be part of the continental crust that was rifted to form the margin or may have been emplaced, uplifted, and exposed during rifting at 125 Ma.

At the most landward site, sediments of Tithonian (Late Jurassic) age were recovered. These sediments, deposited about 20 m.y. before the onset of sea-floor spreading on this segment of the margin, are syn- or pre-rift in origin and suggest that this site overlies thinned continental crust.