SUMMARY We discuss the magmatic development of the west Iberia non‐volcanic rifted continental margin in the North Atlantic Ocean. So‐called ‘non‐volcanic’ rifted continental margins are characterized by a lack of syn‐rift magmatism and are considered to be largely amagmatic. However, this is clearly an oversimplification since seafloor spreading itself is a magmatic process and it is implausible that seafloor spreading begins instantaneously. We concentrate our attention on the recently described zone of exhumed continental mantle (ZECM) to investigate what magmatic processes accompanied the breakup of the continental lithosphere and the subsequent formation of the ZECM leading to the onset of seafloor spreading. We use magnetic anomalies supplemented by the interpretations of multichannel seismic reflection profiles and wide‐angle seismic experiments presented elsewhere. Forward and inverse modelling of a sea‐surface magnetic anomaly chart and of surface and deep‐towed magnetometer profiles shows that anomalies within the ZECM differ in trend, amplitude and source type from those in the adjacent oceanic crust and thinned continental crust. The ZECM anomalies appear to be caused by elongated source bodies within 8 km of the top of the acoustic basement aligned parallel to the margin. We interpret such bodies as syn‐extensional intrusions that increase in volume oceanward. They eventually merge in the vicinity of a margin‐parallel, basement peridotite ridge to give rise to a continuous crust that records reversals in the Earth's magnetic field from the time of anomaly M4(N)–M5(R), i.e. to mark the onset of seafloor spreading. We find no evidence for anomalies formed by seafloor spreading, at either slow or ultraslow rates, before M5(R) (128 Ma).

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The presence of a well-defined ocean-continent transition (OCT) and the absence of large volumes of extrusive or intrusive rocks on the West Iberia margin make it a good place to investigate how the largely amagmatic rifting and break-up of continental lithosphere evolves into oceanic crust produced by magmatic sea-floor spreading. In the southern Iberia Abyssal Plain there is a broad OCT with a characteristic seismic and magnetic character, distinct from both thinned continental crust and normal oceanic crust, which supports the notion that it consists predominantly of exhumed and serpentinized mantle. Interpretations of magnetic and seismic data indicate that on average only small amounts of syn-rift melt exist within the OCT. Isolated, probably margin-parallel, intrusive melt bodies are scattered within the eastem part of the OCT well beneath the top of acoustic basement. Within the western part of the OCT, closer to unambiguous sea-floor spreading magnetic anomalies, such bodies were later(?) emplaced at higher levels and more closely together in the basement until eventually sea-floor spreading began. The evidence does not support the hypothesis that ultraslow sea-floor spreading can explain the magnetic anomalies observed within the wider parts of the West lberia OCT, where the OCT evolution is best resolved.

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