ODP drills the West Iberia rifted margin

Abstract

Leg 149 of the Ocean Drilling Program drilled a transect of basement holes across the ocean‐continent transition (OCT) of the rifted continental margin off the west coast of Portugal. The principal objective was to sample a series of basement highs beneath the Iberia Abyssal Plain, which were thought to span the transition from the oldest oceanic crust to a peridotite ridge separating oceanic from extended continental crusts to a broad region of highly‐extended continental crust.Drilling demonstrated that the late rift‐stage exposures of peridotite at the seafloor are more laterally extensive than previously thought; peridotite was found on two highs 19 km apart. From the region expected to be extended continental crust, we cored 57 m of metagabbro basement whose continental or oceanic affinities are not yet clear. This rock may be Hercynian in age, and therefore part of the continental crust that was rifted to form this margin, or it may have been emplaced, uplifted, and exposed during rifting some 125 m.y.a. At our most landward site, Tithonian sediments, deposited about 20 m.y. before the onset of seafloor spreading on this segment of margin, are interpreted to overlie extended continental crust. The results of the leg will shed new light on the mechanisms by which upper mantle and perhaps lower crustal rocks are exposed at the seafloor during the late stages of continental breakup.