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

¹ Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts 02138, USA

The Ombonde detachment is a primary low-angle normal fault that developed in an undeformed Neoproterozoic carbonate shelf succession as it entered a west-dipping Pan-African subduction zone. The fault is mappable from the top of the shelf succession to the granitic basement surface at a paleodepth of 1.5 km, and the hanging wall has not been significantly deformed. The primary fault geometry is well constrained by stratigraphic cutoff relationships, irrespective of secondary rotations. The dip direction of the fault was 270°, and its horizontal separation was 15–18 km. The fault plane is composed of two ramps separated by a long flat segment at a paleodepth of 0.55 km. The ramps are inclined 8°–14° relative to the carbonate strata, which underwent little or no compaction, and the mean cutoff angle overall is 1.3°. Given constraints on the contemporaneous tectonic setting, the primary fault dips must equal the stratigraphic cutoff angles augmented by a taper angle for lithospheric flexure of not more than 4°. Primary mean dips of <5° are mechanically implausible for a shallow tectonic fault related to extension of nonthickened crust because of the high normal- to shear-stress ratio. However, large gravity slides have moved on detachments dipping 1°–5° on modern continental margins. A gravitational origin is therefore favored, although no toe thrust is observed on account of younger cover. Stratigraphic and sedimentologic observations indicate a relative sea-level drop of >200 m, which would have significantly reduced the water load and thereby the normal stress on a subhorizontal plane, possibly leading to excess pore-fluid pressures. This scenario is consistent with the virtual absence of macroscopic shear deformation adjacent to the fault plane.