John H. Spang1
(1) Texas A&M University, College Station, TX
Abstract: New area-constant detachment fold models for the origin of shale/salt-cored anticlines with withdrawal basins in the Gulf of Mexico
In shale/salt cored anticlines the two new geometric models aid significantly in interpreting areas of poor quality seismic and in estimating the amount of material that was mobile. In existing models for detachment folds in thin-skinned fold and thrust belts, an anticline grows above a detachment, but there is no downward motion in the adjacent synclines. In the anticline, layer-parallel strains are required, and layer parallel shortening decreases upward. At higher limb dips, the outer arc of the anticline begins to elongate. The amplitude of the anticline and related limb dips increase upward. The new models incorporate a unit that can flow from beneath the syncline into the core of the anticline. In model 1, the withdrawal basin only moves down vertically, while in model 2, the withdrawal basin moves both down and laterally. Both models result in an anticline, however the anticline in the second model grows at a significantly higher rate. Both new models result in “true” withdrawal basins that grow at a significantly lower rate than the anticlines. In model 1, limb dips in the pre-growth sediments are the same at all levels, while in model 2, the limb dips increase upward similar to the original model for detachment folding. For overfill of growth sediments, limb dips in the growth sediments decrease upward ultimately to zero at the top of the growth sediments. Elongating growth sediments would facilitate the development of normal faults that could propagate downward into elongating pre-growth sediments and die out without becoming layer-parallel.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana