New Mexico Geological Society Annual Spring Meeting — Abstracts

Theoretical displacement, finite strain, and stress fields, along with calculated fold geometry, were compared in order to better understand the development of single-layer drape folds with bonded and frictionless lower boundaries. The upper boundary of the layer was a free surface, and vertical displacement along the lower boundary was specified by a Fourier series incorporating a step of variable height and width. Analytical solutions-obtained using a commercial computer package for symbolic mathematical manipulation-show that the mechanics of single-layer drape folding are controlled by dimensionless layer thickness, step width, step height, and to a lesser degree, lower boundary conditions. Layer compressibility, to which the model was largely insensitive, was described by the dimensionless ratio of Lame' constants. For layers with compressibility similar to that of soil or rock. variations in step geometry were accommodated near the lower boundary and the resulting folds were virtually identical. Ukewise, principal finite strain plots show little discernible difference except in isolated areas. Principle stress trajectory and displacement fields for layers with bonded and frictionless lower boundaries, however, are strikingly different.

Complicating factors such as far-field extension or compression. layering, and nonlinear material properties were not investigated. However, for cases in which the present models are applicable. it would appear that field studies of large-scale fold geometry alone can yield little information about the mechanics of drape folding. Fmite strain analyses, if carefully conducted and focussed on critical portions of folds, may have some utility. The nature and orientation of minor structures, controlled by principal stress magnitude and orientation. is apparently much more sensitive to different lower boundary conditions. For example, a layer with a bonded lower contact should be characterized by extensional fractures oblique to bedding, whereas a layer with a frictionless lower contact should be characterized by extensional fractures parallel or perpendicular to bedding, depending on location. Andersonian fault orientation may or may not be a useful tool, depending on details of step geometry. In areas of active deformation, for example associated with groundwater withdrawal and land subsidence. it should also be possible to infer lower boundary conditions by observing near-surface displacement fields.