Movement along a low-angle normal fault: The S reflector west of Spain

[1] The existence of normal faults that moved at low angles ( less than 20 degrees) has long been debated. One possible low-angle fault is the S detachment at the west Galicia ( Spain) margin and thought to occur at the top of serpentinized mantle. It is unlikely that S was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts, it appears to have rooted beneath the conjugate Flemish Cap margin, and it is similar to structures elsewhere that also appear to be rooted detachments. Here we analyze depth images to identify synrift sediment packages above S and use the geometry of these synrift packages to constrain the angle at which S both formed and remained active. We find that S must have remained active at angles below 15 degrees, too low to be explained simply by the low friction coefficient of partially serpentinized peridotites. Instead, we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed, enabling moderately high fluid pressures to develop.