Steven J. Ings

A new conceptual model for the structural evolution of a regional salt detachment on the northeast Scotian margin, offshore eastern Canada

In this study, we examine, using seismic data in conjunction with numerical modeling, a regional-scale salt detachment and associated synkinematic sediments from the Scotian margin, offshore eastern Canada. This part of the Scotian margin is characterized by an up to 4.5-km (2.8-mi)-thick and approximately 175-km (108-mi)-long synkinematic wedge of Jurassic sediments with internal sigmoidal, landward-dipping reflectors. The synkinematic wedge is laterally extensive, encompassing an area of approximately 30,000 km2 (11,600 mi2), and soles into an interpreted salt detachment. The Jurassic synkinematic wedge, which is interpreted to have formed as an open-ended allochthonous salt nappe, was loaded by prograding sediments during the Jurassic. This loading squeezed the salt seaward and caused the overlying sediments to undergo extension and gravity spreading and gliding, detaching on the salt sheet. The open-ended nappe model provides a mechanism for producing a large amount of extension with very little compensating contraction. Numerical model results indicate that high rates of extension, detaching on even a thin salt layer, can result in similar sigmoidal, landward-dipping strata. Based on the numerical modeling and seismic interpretation results, we propose a new conceptual model for the Jurassic–Paleogene structural evolution of the study area; this model may also have implications for other passive-margin salt basins with regional salt detachments.

Shortening viscous pressure ridges, a solution to the enigma of initiating salt 'withdrawal' minibasins

Salt ‘withdrawal’ sedimentary minibasins, common features of salt tectonic provinces, are typically subcircular or polygonal, 10–30 km in diameter, and contain as much as 10 km of sediment above evaporite that has been expelled into surrounding diapiric structures. The early development of minibasins typically is not buoyancy driven (Rayleigh-Taylor instability) because thin sediment overburden is usually less dense than salt (strictly, halite) or other evaporites. We analyze an alternative mechanism that applies in compressional regimes involving sedimentation onto salt, lateral flow and shortening of sediment and salt to form viscous pressure ridges (VPRs) (the key process). Loading by sediment ponded between VPRs provides a positive feedback that grows the pressure ridges. We show using lubrication theory and numerical models that this mechanism quantitatively solves the enigma and grows minibasins until the compacting sediment is sufficiently thick and dense for Rayleigh-Taylor instabilities to take over.

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

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.

Geodynamic Modeling of Sedimentation-induced Overpressure, Gravitational Spreading, and Deformation of Passive Margin Mobile Shale Basins

We investigate the differential loading and pore-fluid pressure required for failure and subsequent prolonged gravitational spreading of passive margin shale basins using two-dimensional analytical limit analysis and plane-strain finite element modeling. The limit analysis, supported by the models, indicates that narrow margins (slope regions that are approximately 50 to 100 km [31 to 62 mi] wide) require pore-fluid pressures that are 80–94% of the overburden weight for failure to occur, whereas for wider margins (400 km [250 mi] wide) like the Niger Delta, the corresponding values are 95–99%; these ranges depend on the intrinsic strength of sediments. In the large deformation models, gravitational spreading in response to sedimentation-induced overpressure caused by delta progradation is investigated. Shale is modeled as a viscoplastic Bingham fluid that is frictional-plastic below yield and has a yield criterion that depends on the effective pressure (mean stress minus pore-fluid pressure). The velocity of the postyield flow of the shale is limited by the viscosity of the Bingham fluid, chosen for this study to be 1018 Pas. Pore-fluid pressure is predicted parametrically to be proportional to the local sedimentation rate during progradation, where the proportionality constant, kc, depends inversely on the hydraulic conductivity. Varying the sediment progradation rate, the depth of onset of excess pore pressure, and kc produces model deformation patterns consistent with seaward-directed squeeze-type flow of overpressured shale (Poiseuille flow) or wholesale seaward motion of the shale and overburden (Couette flow), depending on the overall mobility of the model.