The S reflector west of Galicia (Spain): Evidence from prestack depth migration for detachment faulting during continental breakup

The mechanism of continental extension and breakup, and the role of detachment faults in these processes, are currently the subject of intense debate. One possible detachment fault is the S reflector, imaged as an undulating (in time), locally discontinuous reflection on existing seismic profiles west of Iberia. Here we present new images in depth of the four margin-normal profiles across the west Galicia rifted margin, where the S reflector was originally defined and is best imaged. It is shown that faults bounding wedge-shaped units of late synrift sediments, which hence were active during rifting immediately prior to breakup, appear to detach at shallow levels onto the S reflector. S itself appears as a continuous, locally domal feature and does not generally appear offset. The waveform of S is compatible with a reflection from a single intertace such as a sharp tectonic boundary. The depth sections show that S was active at 1-3 km below the seafloor during final rifting; S is interpreted as a brittle detachment fault which controlled the final breakup of the continent west of Galicia. Furthermore the data provide constraints on the sense of shear of S: analogy with detachment terranes, the present, synrift and structural dips of S, and the identification of a breakaway to S imply that S accommodated top-to-the-west shear. Toward the east of the profiles, S becomes more complex, possibly because of different phases of detachment faulting and the development of both incisement and excisement structures. From the geometry of wedge-shaped sedimentary units deposited during faulting above S it also appears that S was active at an angle of 20° or less and hence may be considered a genuine low-angle normal fault.