Experimental folding of rocks under confining pressure: Part III. Faulted drape folds in multilithologic layered specimens

Abstract

Drape folds and reverse faults are produced experimentally at confining pressures to 2.0 kb and shortening rates of 10 −3 to 10 −6 sec −1 by displacing a block of brittle sandstone (2 by 3 by 12.6 cm) along a lubricated saw cut into one to five initially intact layers (0.2 to 1.0 cm thick and as much as 12.6 cm long) of limestone, sandstone, and rock salt. The saw cut is inclined at from 30° to 90° to the layer boundary. The deformation is characterized from studies of fault geometry, displacements and sequence, bedding-plane slip, layer-thickness changes, and the development of fault gouge, fold hinges, microfractures, calcite twin lamellae, and dimensional orientations of grains (in the rock salt). Stress trajectories are inferred from faults, microfractures, and calcite twin lamellae, and strains are calculated from layer-thickness changes and from calcite twin lamellae. Reverse faults curving concave downward propagate upward from the saw cut in the forcing block. With increasing displacement along the precut faults, the faults and associated gouge zones in the layer steepen and become progressively younger toward the upthrown block as displacement increases. The faults are preceded by swarms of extension microfractures that form throughout the deformation and that are the best clues to the stress trajectories. The downthrown layers are thickened by uniform flow and by repetition caused by the faulting. They are displaced away from the faults by bedding-plane slip. Trajectories of the greatest principal compressive stress (σ 1 ) are inclined at low angles to the layer boundaries near the faults and become perpendicular to these boundaries away from the fault. The maximum deformation of the downthrown block occurs when the saw cut is inclined at about 65° to the layering. The upthrown layers are all extended parallel to the layering and perpendicular to the fold axes, as indicated by extension fractures, thinned layers, and calcite twin lamellae and the development of graben zones and low-angle normal faults that are conjugate to the reverse faults. The layers are translated by bedding-plane slip away from the fault zone. Trajectories of σ 1 are inclined from 45° to 90° to the layering. The fabric data are internally consistent, and inferred stresses are in good agreement with those calculated from an elastic solution of the experimental boundary conditions. Principal strains calculated from calcite twin lamellae are within an average of 0.01 of those calculated from layer-thickness changes and permit clear resolution of individual events in domains of superposed deformations.