Localisation of steady-state ultra-slow oceanic spreading along magma-poor rifted margins: Case example offshore Galicia Bank (West Iberia).

The kinematic of the southern North Atlantic is still debated and new kinematic markers are needed to improve our knowledge of the earliest movements. In this frame, we focus on the location of the first evidence of steady-state oceanic spreading offshore Galicia Bank. Such marker is a spatial criterion that can be used to propose refined new kinematic models. Galicia Bank is part of the magma-poor rifted margins of the southern North Atlantic. The margin is located west of Iberia and is conjugated to the southeastern margin of Flemish Cap. These plate corners are key for understanding the kinematics of the Iberia plate, as they are suspected to act as microplates with complex movements during the Late Jurassic – Early Cretaceous. Studies already proposed domains of exhumed continental and oceanic mantle along a seismic reflection and wide-angle profile offshore Galicia Bank (Dean et al., 2015; Davy et al., 2016) but this boundary is poorly defined on a large scale along the margin. As rift phases occurred during the ‘Cretaceous Quiet Zone’ (118–83 Ma), it is not possible to identify the first oceanic crust using Earth's magnetic field reversal. We propose to interpret several E/W to NW/SE oriented seismic reflection profiles from the BREOGHAM-2005 cruise (P.I. Luis Somoza) to better constrain these areas of exhumed mantle. We based our interpretation method on previous studies of the eastern part of the Southwest Indian Ridge (SWIR) that described a domain of exhumed mantle with successive detachment faults on either side of the ridge axis occurring over the last 11 million years (e.g. Sauter et al., 2013; Reston et al., 2018). In addition, recent seismic reflection data allowed the definition of new criteria for characterising ultra-slow nearly amagmatic spreading ridges. We therefore map these criteria in order to locate this domain along the West Iberia margin. We provide new spatial observations of landward-dipping reflectors and exhumed mantle ridges. They are interpreted as seismic indicators of the presence of flipping detachments. A new boundary is thus proposed along the West Iberia margin separating continental mantle exhumation from steady-state ultra-slow oceanic spreading, which could serve as a constraint in kinematic constructions. The indicators of early steady-state oceanic spreading may be applied to other magma-poor rifted margins. This study may indeed be supported by the presence of the same flip-flop structures in symmetry offshore the Flemish Cap southeast margin.