New Excess-Area VS. Depth to Detachment and Geometric Models with Applications to Deepwater, Western Gulf of Mexico, USA

A new excess-area vs. depth to detachment calculation correctly interprets geometric models for detachment folds that have a deformable lower layer (salt). The weak lower layer can both accommodate massive thickness changes and flow laterally allowing anticlines to move absolutely up and synclines absolutely down displacing the weak (autochthonous) layer until it thins to zero. Anticlines grow rapidly in response to compression coupled with addition of material from beneath subsiding synclines. Previous models consider anticlines to move up but constrain synclines to form in-situ. Our work expands on these by including the effect of compressionally formed downward moving synclines with upward moving anticlines above a mobile (salt) substrate. The resulting plots of excess-area vs. depth to detachment identify (1) the amount of material either added (to the core of the anticlines) or lost (from under the synclines), (2) the onset of folding and deformation, (3) the termination of folding.

Geometric analyses of seismic data from several natural folds in the Perdido fold belt; (deep-water western Gulf of Mexico, USA) indicate flow of material from beneath compressionally formed synclines. The base of the autochthonous Louann salt serves as the detachment horizon for this NE-SW trending fold belt. Plots of excess-area versus an arbitrary depth to detachment for independently interpreted anticlines and synclines provide both the timing of folding and the area either lost or added during deformation. The results of the seismic interpretations and geometric models show that the oldest and largest amplitude folds (most northwestern) were formed as early as the early Paleocene, with potential growth to near present day. As the fold belt progresses to the southeast, the structural formations become younger with a majority of the folds originating at the end Oligocene/earliest Miocene, along with their being smaller in amplitude due to a limited salt budget.