Sediment failure types, preconditions and triggering factors in the Gulf of Cadiz

A series of morphological structures, such as scars and escarpments related to seafloor instabilities, were observed in the Gulf of Cadiz using multibeam bathymetry and acoustic imagery. According to the geometry of the slide scars, the slope angle, the surrounding seafloor morphology and the mechanical parameters of the sediment, we suggest the likely mechanisms initiating the failures for the different types of observed structures. Most of the small-scale sediment failures (≤2 km 2 ) seem directly related to dome-like structures (where slopes are steep) or are located in the vicinity of such structures (fluid flows). It appears that progressive deformation or fluid flow related to the growing of dome-like structures may have weakened the sediments sufficiently to bring 7°-steep slopes to metastable conditions (with a factor of safety close to 1.0). The other instability types are likely related to high-magnitude (Ms > 6) earthquakes, which are prone to occur in this area (located in the neighbourhood of the 1755 Lisbon earthquake area). Some particular large-scale structures were observed among these seafloor features, for example on the Guadalquivir Bank. On this bank, a series of successive large scars (at least 4 km long), composed of multiple and very regular arcuate segments (1 km in diameter), were observed at different bathymetric levels (every 40 m). These structures might be related to a deep-rooted detachment zone (e.g. successive listric faults) and triggered by high-magnitude earthquakes or by accumulated displacement along a tectonic discontinuity. This would explain such a large-scale deformation, providing a regular escarpment of 40 m high without any sediment flow downslope, thereby suggesting an ongoing (or unfinished) deformation.