Abstract

Specimens composed of as many as five layers of various combinations of dry Coconino Sandstone (brittle) and Indiana Limestone (ductile) are folded at 1-kb confining pressure. Stress-shortening curves for specimens with aspect ratios ≥20 are nearly linear up to a maximum stress (critical buckling stress) and then show pronounced postbuckling work-softening. For specimens of equal total thickness, the average maximum stress decreases with an increase in the number (and thickness) of limestone relative to sandstone layers. A single beam of limestone or sandstone has larger maximum stress than does the corresponding three-layer specimen of equal thickness. Curves for limestone specimens composed of from one to five layers and with aspect ratios ≤3 are monotonic and show poorly defined yield regions and pronounced work-hardening. Both thick-beam (aspect ratios of 7 to 13) and thin-beam (aspects ratios of 20 or more) folds tentatively are regarded as buckles because instabilities probably are involved in the folding.

Fold (anticlinal) shape depends on the mechanical behavior of the bottom layer. When the ductile limestone is lowest, the fold shape is nearly sinusoidal; when the brittle sandstone is lowest, the anticline has a chevron shape. “Bedding-plane” slip is a maximum in the inflection region on the flanks, and the sense of slip is the reverse of that in classical flexural-slip folding.

Dynamic petrofabric interpretations of small thrust and normal faults, macrofractures and microfractures, compression and extension axes derived from calcite twin lamellae, and the twin-lamellae spacing index indicate that although bedding-plane slip occurs, the entire layered specimen acts as a single mechanical beam throughout most of the folding. In thin, multilithologic folds, the apparent neutral surface is displaced from the center toward the compressed side. In thick, multilayered limestone specimens, large axial shortening prior to bending displaces the neutral surface toward the region of extension, so that even the uppermost layer in the anticlinal hinge can be in the zone of layer-parallel compression.

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