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The west Galicia passive margin exposes extensional structures (north-south normal faults and tilted crustal blocks; northeast-southwest trans-verse faults) formed during a Valanginian-latest Aptian rifting episode. The structural setting was controlled by older Variscan lineaments and by Early Cretaceous plate kinematics. Prior to the Hauterivian, crustal stretching was apparently northeast-directed, and subsequently northwest- directed. The amount of crustal extension is clearly less than the amount of crustal thinning, which is explained better by detachment faulting than by uniform stretching of the lithosphere.

To the north, basaltic basement borders Galicia Bank. To the west, the margin is bounded by a ser- pentinized peridotite ridge. Petrological and struc-tural analysis and 39Ar-40Ar dating demonstrate uplift and exposure of mantle rocks onto the sea floor during rifting. We conclude that stretching of the lithosphere beneath a continental rift system can result in total removal of the continental crust and tectonic unroofing of the upper mantle. A com-posite model is suggested, combining astheno- spheric diapirism and normal simple shear of the thinned lithosphere.

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