An appraisal using magnetic data of the Continent to Ocean Transition Structure West of Iberia

The conceptual models of magma-poor rifted margins are greatly influenced by the continent-to-ocean transition structure of the archetypal magma-poor West Iberia Margin. Some previous works with West Iberia magnetic data have been used to constrain the structure and interpret the transition from the exhumed mantle domain to the oceanic crust formed at a spreading center. However, it is found that the resolution uncertainty of the geophysical data was generally overlooked, leading to over-detailed interpretations. In this work we use synthetic magnetic modelling to show that magnetic data acquired at sea-level cannot resolve sub-horizontal lithological layering in deep-water continental margins. Then, we present a new magnetic model guided by a refined velocity model of the wide-angle seismic IAM-9 profile in the Iberia Abyssal Plain. This new model supports that the J-anomaly is caused by a ~6 km thick oceanic crustal structure with locally increased magnetization compared to regular oceanic crust. This J-anomaly crust abuts the exhumed mantle across a nearly vertical boundary, and is the oldest accreted oceanic crust. These results support that mantle exhumation was abruptly terminated by the accretion of oceanic crust. Mantle melting creating oceanic crust was probably not driven by gradual lithospheric thinning and asthenospheric upwelling, but may be the result of seafloor spreading center propagation cutting across the lithosphere and creating the abrupt structure.

This work was funded by the Portuguese Fundação para a Ciência e a Tecnologia (FCT) I.P./MCTES through national funds (PIDDAC) – UIDB/50019/2020- IDL. Support from FCT (PTDC/CTA-GEF/1666/2020), Spanish Ministry of Science and Innovation (CTM2015-71766-R, PID2019-109559RB-I00) and Spanish Research Agency (CEX2019-000928-S) is also acknowledged.