ABSTRACT

In shale/salt cored anticlines two new geometric (kinematic) models aid significantly in interpreting areas of poor quality seismic data (e.g. areas of high limb dip) and in estimating the amount of material which was mobile. In the original model 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, such that homogeneous layer parallel shortening decreases upward, and 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 lost-area method is used to determine depth to the detachment, fault displacement and original, undeformed line length of the layers. The new models incorporate a unit, which can flow (e.g. over pressured shale or salt) from beneath the syncline into the core of the anticline. In the first new model, the withdrawal basin only moves down vertically, while in the second model, 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 of the new models result in "true" withdrawal basins (synclines), which grow at a significantly lower rate than the anticlines. In the first model, limb dips in the pre-growth sediments are the same at all levels, while in the second model, the limb dips increase upward similar to the original model for detachment folding. In the case of 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 (crestal?) normal faults that could propagate downward into elongating pre-growth sediments. More complex fold shapes can be modeled by adding more fold hinges, which also changes the magnitude of the layer parallel strains (shortening and/or elongation) necessary to produce the final geometry. The models also serve as a tool to calculate the area of material (or volume in three-dimensions) which has moved into or out of a given structure. For example, the mobile layer can also be modeled as a salt tongue, which can be placed in different positions on the structure. In this example, knowledge of the area of the salt and the geometry of the fold from the model can lead to more accurate interpretation of the salt geometry.