Drape folding of compressible elastic layers-I. Analytical solutions for vertical uplift

Two new analytical solutions allow the effects of layer thickness, vertical fault zone width and throw, layer compressibility, and basal slip on drape fold geometry to be investigated. Welded (Case I) and non-welded (Case II) lower contacts with step vertical displacement functions and an upper free surface are used as boundary conditions. Comparison of fold profiles, displacement fields and principal stress trajectory fields suggests that fold form yields relatively little information about lower boundary geometry or stress conditions. Drape folds with non-welded contacts, however, are distinctly asymmetric along their lower boundaries. Displacement and principal stress trajectory fields, on the other hand, can be used to distinguish which of the two boundary conditions was used to produce a given theoretical fold. Values of maximum shear stress are much higher along welded contacts than along non-welded contacts, suggesting that faulting and brecciation should be more prevalent along welded contacts. These findings are consistent with published field observations, and suggest that some attributes of theoretical folds-for example, lower boundary geometry and minor fracture orientation may be used to infer the conditions under which real drape folds were formed.