Steven Ings

BSc Hons. (Geology), Saint Mary's University, 2002

Ph. D. Thesis

Passive Continental Margin Salt Tectonics: Numerical Modelling, Analytical Stability Analysis, and Applications to the Scotian Margin, Offshore Eastern Canada

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The post-rift structural evolution of many rifted continental margins has been affected by the mobilization and continued flow of weak syn- or early post-rift salt layers. Driven primarily by regional differential sediment loading, salt flow can deform the sedimentary overburden into a range of complex structures, many of which can become effective hydrocarbon traps. In this thesis, controls on salt mobilization and resulting structural styles are investigated using 2D finite element modelling, analytical mechanics, and reflection seismic data from the Scotian Margin.

Lubrication theory combined with limit analysis is used to determine an analytical stability criterion for simple systems where a sediment overburden of laterally varying thickness overlies a linear viscous salt layer. The differential loading produces a pressure gradient and induces salt flow. If the differential loading is large enough, the overburden fails developing a region of landward extension compensated by basinward contraction. For submarine systems, water loading increases solid and fluid pressures in the sediments, reduces the pressure gradient on the salt, and buttresses the overburden.

Slow sediment progradation causes a diachronous structural evolution comprising four main phases: 1) initiation of salt flow and the formation of salt withdrawal mini-basins and diapirs; 2) onset of normal growth faulting and extension of the overburden; 3) large-scale evacuation of the salt, and overburden rafting; 4) formation of an allochthonous salt nappe that overthrusts the depositional limit of the salt. Seaward margin tilt increases the rate of salt flow and overburden deformation, while flexural isostatic adjustment has the opposite effects.

The formation of mini-basins, where the overburden is less dense than salt, requires a dynamical stress to create the necessary dynamic bathymetry. Numerical results indicate that the toe-of-slope contractional domain is one region of a passive margin where this dynamical stress amplifies mini-basins.

Numerical modelling, in conjunction with seismic interpretation, has allowed the development of a new conceptual model for the post-rift structural evolution of the northeastern Scotian Margin. Here, Jurassic deltaic sedimentation squeezed salt basinward producing an open-toed salt nappe. Subsequent sedimentation caused gravity spreading over the nappe and produced regional extensional structures characterized by landward dipping sigmoidal stratigraphy.

Supervisor: Chris Beaumont