Reappraisal of gravity instability conditions for offshore wedges: consequences for fluid overpressures in the Niger Delta

Abstract

S U M M A R YGravity instabilities in offshore deltas often involve three structural domains in interaction by the weak detachment plane: an upslope extensional region, a transitional domain sliding seaward, and a downslope compressive region. We provide the fluid pressure conditions for the gravity instabilities due to the interaction of these three structural domains. For that purpose, we apply the kinematic approach of Limit Analysis which relies on the mechanical equilibrium and on the assumption that the onset of the instability is indeed triggered by the motion of the three domains if the Coulomb criterion is met on all slipping faults. The Limit Analysis predictions include the detachment activation length and the normal and thrust fault dips for any given topographic profile. The approach is validated by showing that our predictions match the experimental results on normal faulting triggered by air overpressure in sand analogues. For offshore wedges, the stabilizing effect of the frontal thrusting and of the transitional zone sliding requires large overpressures to reduce friction within the detachment and upslope sediment deposition to trigger the instability. As a consequence, the topographic slope is found to be several degrees larger than predicted with the Critical Coulomb Wedge (CCW) theory which does not account for the interaction of the three domains. The difference in predictions between the two theoretical approaches is important for length ratio less than 100, defined by the ratio of the detachment activated length to the downslope sediment thickness. Fitting our prototype to the offshore Niger Delta and estimating the above length ratio to be in the range of 30-70, it is found that, for cohesionless materials, the effective friction coefficient μ B is less than 0.27 within the bulk material and μ D is less than 0.017 in the detachment for the gravity instability to occur. These values are lower than those previously determined (μ B = 0.5−0.9, μ D = 0. − 0.2) by considering only the compressive domain and applying the CCW theory. These new values correspond to a pore-fluid pressure in the range of 80-90 per cent of the lithostatic pressure within the bulk material (Hubbert-Rubey fluid-pressure ratio 0.8-0.9), and in the range of 97-99 per cent of the lithostatic pressure within the detachment.Key words: Geomechanics; Sedimentary basin processes; Dynamics and mechanics of faulting; Dynamics: gravity and tectonics; Mechanics, theory, and modeling; Africa. I N T RO D U C T I O NRegional seismic studies across the Amazon Fan (Silva et al. 1998;Cobbold et al. 2004), the offshore Niger Delta (Weber & Daukoru 1975;Hooper et al. 2002;Maloney et al. 2010), and the offshore Brunei wedge (Tingay et al. 2009;King et al. 2010) show normal faulting in the thick, coastal part on the shelf and simultaneous thrusting in the thin, deep part, with all faults rooted on a common detachment level. These structures are thus typically characterized by three successive domains above the weak detachment: the upslope e...

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