Large, coherent, submarine landslide associated with Pan-African foreland flexure

The Ombonde detachment is a primary low-angle normal fault that developed i n an undeformed Neoproterozoic carbonate shelf succession as it entered a w est-dipping Pan-African subduction zone. The fault is mappable from the top of the shelf succession to the granitic basement surface at a paleodepth o f 1.5 km, and the hanging wall has not been significantly deformed. The pri mary fault geometry is well constrained by stratigraphic cutoff relationshi ps, irrespective of secondary rotations. The dip direction of the fault was similar to 270 degrees, and its horizontal separation was 15-18 km. The fa ult plane is composed of two ramps separated by a long flat segment at a pa leodepth of 0.55 km. The ramps are inclined 8 degrees-14 degrees relative t o the carbonate strata, which underwent little or no compaction, and the me an cutoff angle overall is 1.3 degrees. Given constraints on the contempora neous tectonic setting, the primary fault dips must equal the stratigraphic cutoff angles augmented by a taper angle for lithospheric flexure of not m ore than 4 degrees. Primary mean dips of <5 degrees are mechanically implau sible for a shallow tectonic fault related to extension of non-thickened cr ust because of the high normal- to shear-stress ratio. However, large gravi ty slides have moved on detachments dipping 1 degrees-5 degrees on modern c ontinental margins. A gravitational origin is therefore favored, although n o toe thrust is observed on account of younger cover. Stratigraphic and sed imentologic observations indicate a relative sea-level drop of >200 m, whic h would have significantly reduced the water load and thereby the normal st ress on a subhorizontal plane, possibly leading to excess pore-fluid pressu res. This scenario is consistent with the virtual absence of macroscopic sh ear deformation adjacent to the fault plane.