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 de tachment faults in these processes, are currently the subject of inten se 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 marg in, where the S reflector was originally defined and is best imaged. I t is shown that faults bounding wedge-shaped units of late synrift sed iments, which hence were active during rifting immediately prior to br eakup, appear to detach at shallow levels onto the S reflector. S itse lf appears as a continuous, locally domal feature and does not general ly appear offset. The waveform of S is compatible with a reflection fr om a single interface such as a sharp tectonic boundary. The depth sec tions show that S was active at 1-3 km below the seafloor during final rifting; S is interpreted as a brittle detachment fault which control led the final breakup of the continent west of Galicia. Furthermore, t he data provide constraints on the sense of shear of S: analogy with d etachment 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 com plex, 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 degrees or less and hence may be considered a genuine low-angle normal fault.