Ocean-continent-transition at magma poor rifted margins, the magnetic signature of a magmatic breakup?

Introduction Magnetic anomalies have been identified from M0 up to anomalies M20-M22 both in the Newfoundland basin and in the Iberian abyssal plain. A large amplitude magnetic anomaly (the J anomaly) marks the beginning of this M sequence of anomalies. These magnetic anomalies have been extensively used to constrain the position of Iberia relative to the European, African and American plates from the late Jurassique to the early Cretaceous times e.g. (Srivastava, 2000). However, the sources for these magnetic anomalies are strongly debated as they were not identified in typical oceanic crust but within the OCT (some have even shown afterwards to be located well within thinned continental crust (Funck, 2003)). They are thus different from standard seafloor spreading magnetic anomalies produced mainly by the cooling of basalts and interpreted as isochrones. The magnetic anomalies within the OCTs in the Iberian abyssal plain and Newfoundland basin are thought to be caused either by the serpentinization of the exposed mantle rocks (Sibuet, 2007) or by magmatic intrusions (Russell, 2003). We investigate the possibility that the magnetic lineation mapped as M0 is in fact not a seafloor spreading lineation. We propose an alternative model based on forward modeling of magnetic anomaly profiles using seismic refraction and drill hole data. We suggest that the main magnetic anomaly (J anomaly) in the OCTs of the Iberian-Newfoundland margins might be explained by underplated magmatic bodies and surface lava flows that post-date mantle exhumation. These magmatic bodies and lavas may result from a late Aptian tectono-magmatic event which marks the transition to seafloor spreading.