Ocean Drilling Program, United States

This paper presents a general overview of the geology and geophysics of western Iberia, and in particular of the western Portuguese Margin. The links between the onshore and offshore geology and geophysics are especially emphasized. The west Iberia Margin is an example of a nonvolcanic rifted margin. The Variscan basement exposed on land in Iberia exhibits strike-slip faults and other structural trends, which had an important effect on the development, in time and space, of subsequent rifting of the continental margin and even perhaps influences the present-day offshore seismicity. The margin has had a long tectonic and magmatic history from the Late Triassic until the present day. Rifting first began in the Late Triassic; after about 70 Ma, continental separation began in the Tagus Abyssal Plain. Continental breakup then appears to have progressively migrated northwards, eventually reaching the Galicia Bank segment of the margin about 112 Ma. Although there is onshore evidence of magmatism throughout the period from the Late Triassic until 130 Ma and even later, this was sporadic and of insignificant volume. Important onshore rift basins were formed during this period. Offshore, the record is complex and fragmentary. An ocean/continent transition, over 150 km wide, lies beyond the shelf edge and is marked on its western side by a peridotite ridge and thin oceanic crust characterized by seafloor spreading anomalies. Rifted fault blocks are recognized within the ocean/continent transition along the whole margin. Mostly, they merge westwards into a transitional zone where the basement often has low relief of unknown origin, and linear magnetic anomalies parallel the seafloor spreading anomalies. There is indirect geophysical evidence that this zone is underlain by intrusions in the lower crust. The most plausible, but not the only, explanation seems to be that this part of the ocean/continent transition consists of fragments of magmatically disrupted and intruded thinned continental crust. The margin also underwent important postrift compression in Eocene and Miocene time, as demonstrated by folding and nondeposition or erosion of abyssal plain sediments. The Eocene deformation is clearly visible off Galicia but the Miocene deformation is dominant in the rest of the margin, where it may have overprinted the former compressional episode. The margin is still seismically active in its southern part at the present day, mainly because of its proximity to the Azores-Gibraltar plate boundary.