INSIGHT INTO THE NATURE OF THE OCEAN-CONTINENT TRANSITION OFF WEST IBERIA FROM A DEEP MULTICHANNEL SEISMIC-REFLECTION PROFILE

A high-quality multichannel seismic reflection profile across the West Iberia margin confirms the existence of a 130-km-wide transition zone between tilted continental fault blocks and unequivocal oceanic crust . The zone is characterized by a 1.0-2.5-km-thick seismically unreflec tive upper basement layer lying over reflective basement that contains major landward-dipping reflectors that mostly truncate against the ba se of the upper layer and penetrate to depths of 10-19 km below the ba sement surface, and minor seaward-dipping reflectors. Moho reflections are absent in this zone. We suggest that the unreflective layer repre sents tectonically exposed upper mantle that has undergone extensive s erpentinization enhanced by vigorous seawater circulation. About 40 km farther north the transition zone narrows to 35 km and the unreflecti ve layer is less continuous and widespread.