An appraisal using magnetic data of the continent-to-ocean transition structure west of Iberia

About half of the rifted margins purportedly formed by extension with minor magmatism. The conceptual models of those magma-poor systems are greatly influenced by the continent-to-ocean transition structure of the archetypal magma-poor West Iberia Margin. In the past, interpretation of magnetic data of West Iberia has been used to constrain conceptual rifting models, including the structure of the transition from the exhumed mantle domain to the oceanic crust formed at a spreading centre. However, uncertainties on geophysical data were generally not considered leading to overdetailed interpretations. We use synthetic magnetic modelling to show that magnetic data acquired at sea level cannot resolve subhorizontal lithological layering in deep-water continental margins. We then present new magnetic modelling guided by a refined velocity model of the wide-angle seismic IAM-9 profile that shows that the magnetic J-anomaly correlates with oceanic crust that abuts exhumed mantle across a vertical boundary. This well-constrained observation supports that seafloor spreading initiated abruptly, terminating mantle exhumation. Conventional wisdom dictates that the sudden efficient melt extraction relates to a mechanical threshold during lithospheric thinning and concomitant asthenospheric upwelling under which melt can migrate toward the surface. However, our results support that mantle melting creating oceanic crust was probably not driven by gradual lithospheric thinning and asthenospheric upwelling, but by seafloor spreading centre propagation that cut across the lithosphere, creating the abrupt structure.

Automated summary

About half of the rifted margins are believed to have formed through extension accompanied by minor magmatism. A primary influence on the conceptual models of magma-poor rift systems is the structure of the continent-to-ocean transition exemplified by the West Iberia Margin. However, previous interpretations of magnetic data from West Iberia have not adequately considered uncertainties, leading to overly detailed interpretations. Our study uses synthetic magnetic modelling and a refined velocity model to show that magnetic data at sea level cannot resolve deep-water lithological layering. Our results support an abrupt seafloor spreading initiation that terminated mantle exhumation, rather than gradual lithospheric thinning and asthenospheric upwelling driving mantle melting.