THE S REFLECTOR WEST OF GALICIA - THE SEISMIC SIGNATURE OF A DETACHMENT FAULT

The so-called S reflector is a group of bright, continuous reflections underlying the landward-tilted fault blocks of the west Galicia rifte d margin. S has been interpreted as the brittle-ductile transition, th e top of an intrusion, a detachment fault, and the crust-mantle bounda ry. To constrain the internal structure of the reflector, we have carr ied out detailed analyses of these reflections. We compare the wavefor ms of the seafloor reflection and its first multiple, both to determin e the amplitude of the seafloor reflection and to show that the seaflo or is in effect a spike in the reflectivity series so that the seafloo r reflection can be used as the far-field wavelet, including both sour ce and receiver ghosts. We compare the waveform of the seafloor and S and show that, within the resolution of our data, S is a reflection fr om a step increase in acoustic impedance. This result is confirmed thr ough complex trace analysis, and in particular the determination of th e apparent polarity of S, and the examination of the instantaneous fre quency function: S is consistently positive polarity, and shows no sig nificant frequency anomaly, Simple modelling shows that S is very unli kely to be a reflection from a thin layer. We thus conclude that S is probably a single steplike interface. From the varying frequency conte nt of the data, we determine a value for the effective Q between S and the seafloor, and use this to assess the amplitude loss due to attenu ation and scattering. We use a comparison between the seafloor and the S reflection to constrain the amplitude of S, and estimate a reflecti on coefficient for S of at least 0.2 in places, decreasing landwards. By analogy with structures developed in the highly extended regions of the western United States, we consider that the most likely interpret ation of S is as a sharp west-dipping detachment fault separating a 'g ranitic' upper plate from a higher-velocity lower plate, locally proba bly serpentinized mantle.