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| Titel |
Mechanics of gravity instability in offshore deltas, with reappraisal of fluid overpressures in the Niger Delta |
| VerfasserIn |
Bertrand Maillot, Yves Leroy, Xiaoping Yuan |
| Konferenz |
EGU General Assembly 2017
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 19 (2017) |
| Datensatznummer |
250151028
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| Publikation (Nr.) |
 EGU/EGU2017-15569.pdf |
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| Zusammenfassung |
| Gravity 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 occurrence of gravity instabilities due to the interaction of
these three 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. For any given topographic
profile, the Limit Analysis predicts the normal fault position and dip, marking the
extensional domain, the activation length of the detachment defining the transitional
sliding domain, and the thrust fault position and dip delimiting the compressive
domain.
The frontal thrusting occuring on the downslope compressive region works against
gravity and therefore stabilizes the upslope gravity collapse. As a consequence, the critical
topographic slope at the onset of the instability is found to be several degrees larger than
predicted with the Critical Coulomb Wedge (CCW) theory, which can account for either
the upslope extensional region, or the downslope compressive region, but does
not account for the interaction of the three domains. The difference in predictions
between the two theoretical approaches is important when the downslope sediment
thickness above the detachment is more than a hundredth of the detachment activated
length.
The above length ratio is in the range 1/30 to 1/70 in the offshore Niger Delta. Neglecting
cohesion, we found that, for the gravity instability to occur, 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.
These values are lower than those previously determined (μ′B = 0.5 − 0.9, μ′D = 0.− 0.2)
by applying the CCW theory to the compressive domain only [Bilotti and Shaw, 2005]. These
new values correspond to a pore-fluid pressure in the range of 80 to 90% of the lithostatic
pressure within the bulk material (Hubbert-Rubey fluid-pressure ratio 0.8 − 0.9),
and in the range of 97 to 99% of the lithostatic pressure within the detachment. |
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