THIN CRUST AT THE WESTERN IBERIA OCEAN-CONTINENT TRANSITION AND OPHIOLITES

Western Iberia is bounded by a nonvolcanic rifted continental margin made up of three apparently independent segments. The age of breakup decreases from south to north. Seismic refraction and reflection profiles, and magnetic and gravity data from each segment, show a consistent pattern of geophysical observations across the ocean-continent transition (OCT) zone, which is a few tens of kilometers wide. We emphasize here the discovery of thin (2-4 km) oceanic crust underlain by 7.6 km s-1 material within the OCT. The available evidence favors the suggestion that the 7.6 km s-1 layer is serpentinized periodotite and that the thin oceanic crust is primarily the result of a poor magma supply for a few million years immediately after continental breakup. This thin crust may be the source of some ophiolites which exhibit thin crustal sections and continental margin affinities.