GEOMETRIC MODELS OF BOX-SHAPED DETACHMENT FOLDS IN THE ALAMINOS CANYON AREA OF THE WESTERN GULF OF MEXICO

Published seismic lines in the Alaminos Canyon Area in the Western Gulf of Mexico reveal a wave train of N-S trending detachment folds that is 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. In the folded pre-growth layers, the limb dips increase up section, which is indicative of detachment folding. The limb dips decrease up section in the growth sediments, and 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. Natural detachment folds consist of layers of pre-growth sediments that can undergo massive thickness changes above a detachment horizon on top of an undeformed basement. New geometric models of box-shaped detachment folds are used to model the box-shaped folds. In the models, pairs of parallel, inclined axial surfaces with opposed dips intersect both up and down section to form box-shaped anticlines and synclines with only one axial surface in the core of the folds as observed in the seismic. The box-shaped portions of the folds in the new geometric models grow more slowly in amplitude than the existing geometric models with only vertical axial surfaces (Groshong and Epard, 1994) or with inclined synclinal and vertical anticlinal axial surfaces (Epard and Groshong, 1995). After initial layer-parallel shortening, all of the models undergo a line length elongation (stretching), which occurs first in the outermost part of the folds and migrates inward with increasing shortening. Alternating the dip direction of successive pairs of parallel axial surfaces forms a wave train of folds. 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 an absolute sense. In the natural folds the deformable layer below is thick enough that the synclines may actually move down into the deformable layer below. If the wave train in the geometric models is pinned at one end (e.g. at the end of the detachment horizon or pinch out of the salt), then the positive downward movement of the synclines and the shortening and thickening of the deformable layer results in a tilting of the wave train as observed in the Alaminos Canyon Area.