The ocean-continent transition (OCT) on nonvolcanic rifted continental margins is important in that it contains evidence concerning the breakup of the continents and the onset of seafloor spreading. The nature of the OCT off western Iberia has recently been attracting attention following seismic and other geophysical studies there and drilling of acoustic basement by Leg 149 of the Ocean Drilling Program. Here we concentrate on the interpretation of a new digital magnetic anomaly chart for the Iberia Abyssal Plain between 39.5¿ and 42.2¿N. The most striking anomaly is a trough which exists immediately east of, and locally parallel to, anomaly J and is closely coincident with a basement peridotite ridge. We conclude from modeling surface magnetic anomalies and a deep-two magnetometer profile that seafloor spreading began about 129.9 m.y. ago (Barremian) at a rate of 10.0 mm/yr, consistent with drilling and seismostratigraphic results from the southern Iberia Abyssal Plain. A second feature is the existence of linear low-amplitude isochron-parallel magnetic anomalies east of the peridotite-ridge trough and a clear change in the trend of these anomalies east of about 11¿15'W. It is possible to model these anomalies by a series of 7- to 30-km-wide weakly magnetized crustal blocks of about the same thickness as the oceanic crust. The west-to-east anticlockwise change in trend at 11¿15'W suggests either a major change in stress regime and/or in the type of magnetic anomaly source.

We speculate that the region between the peridotite ridge and 11¿15'W represents a transitional crust whose formation was dominated by the same stress regime which accompanied eventual seafloor spreading. We explain the change in trend at 11¿15'W by a model featuring a rift which propagated through continental crust from south to north. We consider three scenarios for the formation of the transitional zone crust; first, continental crust is surrounded and impregnated by intrusive and extrusive material, formed from passive upwelling of magma created by the limited adiabatic decompression partial melting which occurs as the two continental lithospheric plates first begin to separate, second, such crust is surrounded by unroofed upper mantle material, or third, the zone is the product of ultraslow seafloor spreading. Present evidence tends to support the first scenario. In this scenario, the top of the asthenosphere eventually becomes shallow enough to enable magma to penetrate the thinnest continental crust, fill the space caused by additional extension, and form the transitional crust, already described, over a broad zone at least several tens of kilometers wide. At breakup, upper mantle rocks become exposed on the seafloor and undergo extensive serpentinization. Ina final stage we envisage focusing of the igneous activity leading to the onset of seafloor spreading, initially producing thin oceanic crust because of a restricted melt supply. The width of continuous tectonically extended continental crust in the southern Iberia Abyssal Plain is less than was previously supposed. ¿ American Geophysical Union 1995