Ocean Drilling Program, United States

The Galicia Bank and Iberia Abyssal Plain segments of the west Iberia margin were drilled during Ocean Drilling Program Legs 103 and 149 and have been extensively studied geophysically. Leg 149 determined landward and oceanward limits to the ocean-continent transition (OCT) of the crust off western Iberia by drilling an east-west transect of holes. However, only one of these holes penetrated basement between these limits. This site, Site 900, cored 56 m of fine- to coarse-grained gabbro that had experienced synrift dynamic re-crystallization under granulite facies conditions at 136.4+ or -0.3 Ma, according to (super 40) Ar/ (super 39) Ar dating. Geophysical data clearly show that the OCT has magnetic and seismic velocity properties that are in some sense transitional between continental and oceanic crust. Multichannel seismic reflection profiles, one of which has been recently reprocessed, strongly indicate that, although the eastern (landward) part of the OCT is dissected by deeply penetrating normal faults and low-angle detachments, these die out westward (oceanward) into a region of smoother basement that lacks significant intrabasement reflectors and is of uncertain origin. Leg 173 will drill a small number of holes to basement, on basement highs mainly within the OCT, to complete the Leg 149 transect. These holes will characterize the OCT, test models of lithospheric (crustal) extension, determine the extent of synrift magmatism, and examine the nature of the oldest oceanic crust.