Detachment tectonics during Atlantic rifting: analysis and interpretation of the S reflection, the west Galicia margin

Beneath the tilted fault blocks of the western Galicia rifted margin an unusually bright reflection, the S reflection, is observed. The waveform, polarity and amplitude of S indicate that it is a reflection from a seismic interface across which the acoustic impedance increases sharply. This result is consistent with its interpretation as a detachment fault juxtaposing a low velocity and density upper plate and a high velocity and density lower plate. The lower plate may represent partially serpentinized mantle material, brought into contact with pre-rift sediments and upper crustal basement by tectonic denudation.

Pre-stack depth migration is applied to determine the true geometry of S, and its relationships with the overlying faults. It is found that S passes continuously beneath the upper crustal faults, which detach onto S. However, S does appear to be truncated westwards by east-dipping reflections associated with the peridotites exposed at the seafloor. We interpret these reflections as the continuation of the top-to-the-east extensional shear zone sampled within the peridotite, and suggest that S is either antithetic to a master mantle detachment, or that S is cut by a later mantle shear zone.