Continental margin shale tectonics: preliminary results from coupled fluid-mechanical models of large-scale delta instability

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Abstract: We investigate the evolution of rifted continental margin shale tectonics using 2D finite-element models that couple sediment deformation to compaction-driven Darcy fluid flow via the effective stress. Fluid overpressures are generated in the models by a combination of mechanical compaction (grain rearrangement) and viscous compaction (grain dissolution and local reprecipitation), and lead to failure and flow of visco-plastic Bingham shale. Model results indicate that pore fluid pressures must be 90–95% of the lithostatic pressure to cause shale failure and delta destabilization. Mechanical compaction alone is insufficient to generate fluid overpressures required for failure in the models and viscous compaction is the primary source. The numerical models include delta progradation, lateral lithology variation, and flexural isostatic compensation. Seaward shale flow and associated overburden deformation results in the formation of landward regional and counter-regional fault-bounded extensional basins, a transitional domain of thickened and folded shale beneath the continental slope, and a seaward fold and thrust belt at the delta toe. The structural styles generated by the preliminary numerical models are compatible with features observed in unstable Cenozoic deltas (e.g. the Niger Delta) and provide additional insight into the fundamental relationships between deltaic sedimentation, fluid pressure generation, and margin-scale gravity spreading.