Sofie Gradmann

Salt tectonics is a key player in the evolution of many worldwide sedimentary basins on rifted continental margins. For more than a century, the evolving structures have been studied; but focus remained primarily on the onshore and shallow-water regions. The evolution of the poorly studied deep-water salt-tectonic structures is the focus of this thesis. Investigations are performed using 2D numerical models that comprise a viscous salt layer overlain by a frictional-plastic passive margin sedimentary sequence from shelf to deep water. This thesis addresses multiple salt-tectonic processes (gravity spreading, evolution of fold belts and salt canopies, diapirism) in a general context but with special focus on the structural evolution of the northwestern Gulf of Mexico (GoM). Here, multiple phases of gravity-spreading induced salt mobilization and thin-skinned deformation occurred throughout the Cenozoic. During the latest, late Oligocene-Miocene phase, the Perdido Fold Belt (PFB) formed from a 4.5km thick pre-kinematic section as a prominent salt-cored deep-water structure above the pinch-out of the autochthonous salt. It is here demonstrated with analytical as well as numerical calculations that the folding of the PFB can have formed by gravity spreading alone without basement tectonics. A requirement for this deformation is very high pore-fluid pressure in the sediments, which effectively reduces the sediments' mechanical strength. These values are refined using numerical models that couple compaction-induced fluid pressure to mechanical deformation. It is shown that very high fluid pressure is only necessary at the landward base of the deforming system; fluid pressure in other regions may remain moderate. This study shows, for the first time, the regional and dynamic evolution of pore-fluid pressure in a continental margin sedimentary system above salt. Additionally, the contribution of `lateral compaction' during fold-belt evolution is addressed. Landward of the PFB, a large-scale canopy developed during the Eocene. Its evolution is studied by investigating three different concepts of canopy evolution that have been proposed in the scientific literature. A canopy evolving via the mechanism of squeezed diapirs is most similar to the Eocene canopy of the northwestern GoM. A canopy evolving via the mechanism of breached anticlines is similar to that observed above the landward end of the PFB. Dynamic diapir growth is addressed in a neutral stress regime under uneven sedimentation employing a new mechanism of diapir initiation and evolution.

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Pages: 355
Supervisor: Chris Beaumont