Review of Continental Margin Structural Strength and Stability
Allen Lowrie, Bradley Matthew Battista, 2013. "Review of Continental Margin Structural Strength and Stability", Shelf Margin Deltas and Linked Down Slope Petroleum Systems–Global Significance and Future Exploration Potential, Harry H. Roberts, Norman C. Rosen, Richard H. Fillon, John B. Anderson
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The classic passive continental margin model of previous decades has been one of stratigraphic evolution over a perceived structurally strong feature. That notion of stability is eroding, as the impact of interlocking geologic processes is better understood. The continental margin instead of being stable may itself be dynamic as it slides into the Gulf of Mexico. For this to be true, there must be processes that individually, collectively, and synergistically, are weakening the margin in such a way that it moves minutely, locally, and regionally, each unit possibly moving independently yet forming a single tapestry of deformation. To propose a hierarchy of operating processes of which one(s) may be dominant or subordinate is surely premature. Listing known processes and their interrelationships may be a worthy exercise, leading to fruitful avenues of research’
Major processes that require further exploration include: (a) the critical cohesive Coulomb wedges and their applications; (b) fold-belts at the base of the continental slope; and (c) potential stress fields in the lower continental rise and deeper abyssal plain.
The critical cohesive Coulomb wedge was originally described as a wedge of deformed/deforming material tapering toward the deformation front, a basal surface of detachment or decollement with most of the dynamics above the decollement and much horizontal compression within the wedge.
Originally this interpreted feature was applied to subduction zones. This interpretation may be profitably affixed to dynamic continental margins such as those of the northern Gulf of Mexico and the Gulf of Cadiz, both of which have major gravitational tectonic overprints.
With large gravity-driven downslope mass-wasting, migrating critical wedges that develop can generate compression, yielding fold-belts. Farther downslope, the downdip generated compressive stresses may be more diffuse and ineffective.
How are pressures transmitted laterally at water depths of 3-4 kms? Prime candidates are the entire continental margin itself or the extrusives: salt (Gulf of Mexico), shale (West Africa/Gabon margin), and/or crustal blocks/salt (Gulf of Cadiz). All are operative, yet at different rates, establishing a heterogeneous stress field, varying with time. With pore-pressure approximating fracture pressure, relatively small changes in lateral stresses could create fractures, keep fractures open, or close fractures.
At multi-kilometer depths, the water pressure commences to be sufficient so that natural gases remain in solution and hydrates may or may not be formed. The existence of hydrates changes stresses within sediments; hydrate absence minimizes the possibility of buoyant free gas buildups. The role of hydrocarbons, gas, liquid and/or solid, and of whatever source (bio-genic or abiogenic), given their sheer abundance and ubiquity, needs to be re-examined.