RHEOLOGICAL INFORMATION INFERRED FROM SMALL-SCALE FOLDS, NORTHERN SNAKE RANGE, NEVADA

Small-scale folds in garnet-rich layers in schists of the McCoy Creek Group, Hendry’s Creek, Northern Snake Range, NV, show classical properties of buckle folds. They have a preferred wavelength that varies with layer thickness, and they die out rapidly away from each folded layer. They are developed in rock that has experienced a homogenous bulk strain, as evidenced by homogeneity of fabric and constancy of layer orientation on the outcrop scale. The folds are extremely cylindrical, with hinges that are parallel to the maximum total extension direction (X), and their axial surfaces lie on average parallel to the schistosity, which is taken to be the XY plane of the total strain. Strain was determined in two samples of quartz-pebble conglomerates from the region, giving ratios (X:Y:Z) of 15:4:1 and 9:3:1. Images and photographs of the folds were used to quantitatively analyze wavelength, amplitude, and thickness, employing the Fold Geometry Toolbox of Adamuszek. The results suggest that viscosity ratios of layer to matrix varied from about 15 to 40 during folding. Crystal plastic deformation mechanisms likely responsible for the deformation would suggest non-linear rheology, but there is no indication of pinch-and-swell or boudinage parallel to the fold hinges. This is not surprising since theory indicates that the effective value of the power-law exponent in the flow law associated with either buckling or necking instability decreases as the stretch rate increases in the dimension perpendicular to that in which the instability develops. If the value of n in the power law controlling deformation in the layer was 3 and if strain rate can be inferred from the finite strain, the effective value of n would be 1.4-1.5. Fold shape is highly variable, reflecting the irregular nature of the layer boundaries and does not provide an independent estimate of layer rheology. The folds developed during deformation involving E-W crustal extension and vertical thinning associated with top-to-the-east shear in the Northern Snake Range detachment in Nevada.