Influence of stratigraphic setting and simple shear on layer-bound compaction faults offshore Mauritania

We have used three-dimensional seismic data to examine the geometry of layer-bound compaction faults on the Mauritanian continental margin. The faults deform a fine-grained Neogene unit which mantles buried canyons. Above the canyons, most layer-bound faults strike perpendicular to the canyon axes, whereas adjacent to the canyons the faults are omni-directional and intersect bedding planes in polygonal patterns. On the flanks of a large structural high, faults also strike perpendicular to the slope direction, displaying a concentric pattern. Faults that show preferential alignment are located in areas with increased bedding dip, and most aligned faults are antithetic to the bedding dip. Where the bedding dip is greater than ∼1°, the synthetic faults dip more steeply than the antithetic faults. This discovery was unexpected because the stress tensor due to the effect of gravity on a dipping margin would cause the antithetic faults to form with steeper dips, which we show theoretically. We hypothesise that the layer hosting the faults has been subjected to simple shear of ∼20° or more, with the top of the layer displaced downdip by ∼100 m relative to its base. A potential application of these, and similar, observations is to assess slope stability on continental margins.