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Abstract: Sequential Geometric Models of Detachment Folds: Alaminos Canyon Area, Gulf of Mexico
Based on published seismic lines, the Alaminos Canyon in the Western Gulf of Mexico contains a N-S trending wave train of detachment folds that are tilted basinward (eastward). The folds are periodic and change geometry from normal folds in their cores to a box-shaped geometry outward. 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 updip to the west, with the oldest folds at the toe or downdip end (to the east) as would be predicted for internal shortening within a gravity slide. New sequential 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. The geometric models produce box folds that have inclined axial surfaces with opposed dips that intersect both upsection and downsection to form anticlines and synclines with only one axial surface in the core of the folds (Figs. 1 K-O) as observed on the seismic data. In these new geometric models, box-shaped folds grow more slowly in amplitude than those with a single anticlinal axis. After initial layer-parallel shortening, all of the models undergo a line length elongation (stretching) which occurs first at the highest levels in the anticline and migrates downward with increasing shortening. This initial shortening followed by elongation is shown by the requisite strains in Figure 1. In most natural 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. Figure 2 shows geometric models for a wave train in detachment folds for the case of vertical axial surfaces. In the natural detachment folds the deformable layer below is thick enough that the synclines may actually move down. The sequential geometric models reveal that shortening and thickening of the deformable layer forms a wedge-shaped zone that produces a tilted wave train similar to the natural detachment folds (not shown in Figure 2).
Epard, J.-L., and R. H. Groshong, Jr., 1995, Kinematic model of detachment folding including limb rotation, fixed hinges and layer-parallel strain: Tectonophysics, v. 247, p. 85-103.
Groshong, R. H., Jr., and J.-L. Epard, 1994, The role of strain in area-constant detachment folding: Journal of Structural Geology, v. 16, p. 613-618.
Acknowledgments and Associated Footnotes
1 Department of Geology & Geophysics, Texas A&M University, College Station, TX 77843-3115
Copyright © 2007 by The Gulf Coast Association of Geological Societies