New Mexico Geological Society Annual Spring Meeting — Abstracts

Mechanical models of single layer drape folds in compressible elastic media can be extended into the realm of multi-layered drape folds by formulating semi-analytical matrix solutions. In this talk, the effects of compressibility and stiffness contrast on the stress and displacement fields in simple two-layer drape folds is investigated using a series of numerical experiments. The goal is to examine the transition from a welded to a freely slipping base, which has previously been simulated by changing boundary conditions along the infinitesimally thin of a single layer model (Haneberg, 1992, J. Simel. Geol., in press). All contacts are firmly bonded, so that no inter-layer slip can occur; however, free-slip between the two layers can be incorporated by rearranging the internal boundary condition equations. Basal slip can be induced by decreasing the shear modulus of the lower layer, so that the effects of a weak basal layer are distributed across a zone of finite thickness. A control case, in which there is neither a stiffness nor a compressibility contrast between layers. produces results identical to an analytical solution for single layer drape folding with a bonded lower contact. When lower layer stiffness is decreased by an order of magnitude, principal stress axes in the lower layer become more nearly parallel and perpendicular to the layer boundaries, and maximum shear stress decreases significantly. If lower layer stiffness is decreased by the same amount and lower layer compressibility is increased from a Poisson's ratio of v₂= 0.10 to v₂= 0.25, however, principal stress axes are not nearly so parallel to layering and the maximum shear stress decrease is not nearly so great. Finally, if the lower layer stiffness is again decreased and lower layer compressibility is increased from a Poisson's ratio of v₂= 0.10 to v₂ = 0.49, principal stress orientations exhibit a wide range of orientations relative to layering, and a zone of increased maximum shear stress begins to develop in the upper layer. Decreased lower layer stiffness and increased lower layer compressibility also give rise to non-zero horizontal displacement vectors in the lower layer, which means that folded passive markers in the lower layer will be slightly asymmetric. This is exactly as predicted by the analytical solutions for welded and freely slipping basal contacts. These results suggest that oversimplified field criteria such as fold form or orientation of minor structures relative to layering are fallible, and cannot be used alone to infer the mechanical conditions under which real folds developed.