Abstract Interpretation of ION-GXT long-offset 2D depth-imaged and other seismic data suggests that outer continental margins collapse and tilt basinward rapidly as rifting yields to seafloor spreading. The outer continental margin, already thinned by rifting processes, can be viewed as the hanging wall of a mega-half-graben associated with a landward dipping shear zone. The footwall of the shearmega is the serpentinised or serpentinising sub-continental mantle that commonly becomes exhumed as it rises from beneath the thinned and embrittling continental margin. We call this shear zone between the continental crust and the rising subcontinental mantle the “outer marginal detachment.” At magma-poor margins, the outer continental margin collapses to tilt basinward (“outer marginal collapse”) to depths up to 3-3.5 km sub-sea at the continent-ocean transition, so that it normally lies deeper than the adjacent oceanic crust (accreted later by seafloor spreading). We use the term “collapse” because of the rapidity of the deepening, which seems to be <3 Ma, and possibly <1 Ma. Salt deposition or rapid clastic (deltaic) sedimentation at non-magmatic margins, or SDRs and less organised magmatism at magmatic margins, may accompany the collapse, with salt thicknesses rapidly reaching 5 km and volcanic piles reaching 15-20 km, as controlled by isostasy. This mechanism of rapid salt deposition allows some megasalt basins ( e.g ., Gulf of Mexico) to be deposited on synrift sections at global sea level, although other salt basins may form in air-filled depressions below sea level. Outer marginal collapse is “postrift” from the perspective of the continental crust, but of tectonic, and not of thermal, origin, owing to shear on outer marginal detachments. We examine how outer marginal collapse can migrate diachronously along strike, much like the onset of seafloor spreading, as is required in models of ocean creation. We suggest that backstripping estimates of lithospheric thinning (beta) at outer continental margins may be excessive, because they probably attribute outer marginal collapse to thermal subsidence.

Abstract The petroleum geology of the Mississippi Canyon, Atwater Valley, western DeSoto and western Lloyd Ridge protraction areas, offshore northern Gulf of Mexico, is controlled by the interaction of salt tectonics and high sedimentation rate during the Neogene, and has resulted resulting in a complex distribution of reservoirs and traps. Seventy-eight fields/discoveries are evaluated and comprise structures with four-way closures (18), three-way closures (46), and stratigraphic traps (14). Three of these discoveries are in Upper Jurassic eolian reservoirs, the remainder are in Neogene deep-water reservoirs. The tectonic-stratigraphic evolution of the area is analyzed at eleven discrete intervals between 24 Ma and Present. The analyses show how the allochthonous salt systems evolved over time, and their effect on sedimentation patterns and sub-basin evolution. The study area includes some of the largest fields in the northern deep Gulf of Mexico. Thunder Horse produces from an anticlinal (turtle) structure that developed with a basement-controlled allochthonous system. The greater Mars-Ursa sub-basin has nine fields with > 1.5 BBBOE EUR, including Mars, Ursa and Princess, that developed with a counterregional allochthonous salt system. The remaining fields have considerably smaller reserves, which are controlled by the area within closure and number of reservoir intervals. Many of the smaller fields are produced from one well subsea tiebacks. Most of fields in the study area are contained within sheet-like or wedge-shaped stratigraphic intervals and have four-way or three-way trapping configurations. These findings reflect the profound effect that mobile salt has had on the petroleum geology of the region.