Detachment Folding in the Alaminos Canyon Area of the Western Gulf of Mexico

John H. Spang
Texas A&M University, College Station, TX

Analysis of published seismic lines reveals a wavetrain of N-S trending detachment folds that are tilted basinward (East). The folds are periodic and change geometry from normal folds in their cores to a box-shaped geometry as you go outward. The flat fold crests of the box-shaped folds are tilted resulting in asymmetric folds. In the folded pre-growth layers, the limb dips increase up section, which is indicative of detachment folding. The age of the oldest growth sediments decreases up dip to the West. Thus, the oldest folds appear to be at the toe or down dip end of the tilted wave train as would be expected in a gravity slide. New geometric models of box-shaped detachment folds are used to model the folding. Natural detachment folds consist of pre-growth layers that can undergo massive thickness changes above a detachment horizon above an undeformed basement. In new geometric models of box folds, inclined axial surfaces with opposed dips intersect both up and down section to form anticlines and synclines with only one axial surface in the core of the folds as observed in the seismic. In most detachment folds the synclines do not actually move down in an absolute sense but are left behind as the anticlines move up. In the natural folds the deformable layer below is thick enough that the synclines may actually move down. The shortening and thickening of the deformable layer forms a wedge-shaped zone that produces an eastward tilted wave train in the models.