Experimental folding of rocks under confining pressure: Part VII. Partially scaled models of drape folds

Abstract

Drape folds are created experimentally at 0.5 kb confining pressure and displacement rates of 6.6 · 10 −3 cm/sec by displacement of a steelblock forcing-assembly (2 by 3 by 12 cm) along lubricated, arcuate, precut surfaces into a veneer of two to six initially intact layers (variable thicknesses to nearly 1.3 cm by 11 cm long) of chalk (ductile), dolomite (brittle), lead (ductile), limestone (ductile), and sandstone (brittle). Various forcingmember configurations produce a wide variety of fault-fold systems, several of which are produced during a single test. The various materials used for the veneer were chosen so that their relative-ductility contrasts would be similar to those of the three main “mechanical”, stratigraphie units of the Rocky Mountain foreland: the Cambrian shales, Paleozoic carbonate rocks, and Mesozoic clastic rocks. The layers are machined to scaled thickness of 1 mm ∼- 300 m. Deformations of the layered sequences are studied macroscopically and compared to previously published descriptions of experimental drape folds. Variations of rock type, layer thickness, and relative-ductility contrast in the multilithologic, layered veneer produce drape folds, the overall geometry of which is nearly identical to that of some natural structures. Rigid-body translations and rotations of brittle layers, cataclastic flow, faulting, fracture, and bedding-plane slip are present in all ductile-brittle-very ductile (top to bottom), layered sequences. In addition to dip-section movements, displacements parallel to the fold axis occur in all specimens. The role of very ductile material within the layered sequence is primarily to damp high stress-concentrations and broaden the area of folding.