RECOGNITION AND MECHANICAL SIGNIFICANCE OF BUCKLE-FOLD VS. CLASSIC DETACHMENT-FOLD STYLES

In the classic Jamison detachment-fold model for compressional structures, synclines maintain constant thickness, whereas the anticlines thicken. This passive-syncline model also applies to fault-propagation fold models. In buckle-style detachment folds, the synclines are active and subside as detachment-zone material flows from beneath the synclines into the anticlines. The difference appears to be caused by the mechanical properties of the detachment-zone material and leads to significant differences in layer-parallel strain (LPS) magnitudes in the folded beds. Even though the resulting folds can look very similar, they can be separated using excess area vs. depth diagrams. For passive-syncline folds, the area-depth line goes to zero area at the depth of the lower detachment, whereas for the buckle style, the area-depth line indicates a much deeper, and incorrect, lower detachment. The distinction is clear if the detachment position is known or can be inferred from the mechanical stratigraphy. The displacements and LPS calculated from the area-depth lines are accurate for both fold styles. The differences are illustrated with kinematic models and seismic profiles across two anticlines from deep-water fold belts on the west coast of Africa. The Alpha/Bobo field of Nigeria is a passive-syncline style fold and is developed over a shale detachment zone. The Angola fold is an active-syncline style and is developed over a salt detachment zone. Above the detachment zone, the passive-syncline Alpha/Bobo fold has calculated LPS magnitudes in the range -6 to -7 % in pregrowth strata, whereas the active-syncline Angola fold has LPS of approximately zero. Questions remaining include whether shale detachments always favor passive synclines and salt detachments always favor active synclines, and if the LPS consistently correlates with the syncline style. An important caveat is that this application of the area-depth method requires well-constrained cross sections. A section constructed by long-distance constant-bed-thickness extrapolation will probably give an area-depth detachment that is too deep, looking like a buckle fold, regardless of the true style.