RHEOLOGY AND DEFORMATIONAL HISTORY DERIVED FROM THE STUDY OF FOLDED GARNET LAYERS, NORTHERN SNAKE RANGE, NEVADA

Garnet-rich layers of varying thickness and inclusion density, some of which are folded, are used to provide insight into the rheology and deformational history of rocks in the footwall of the low-angle detachment of the northern Snake Range metamorphic core complex, Nevada. The rocks of interest are Precambrian staurolite-garnet schists, exposed in Hendry’s Creek on the east flank of the complex. These rocks experienced E-W crustal extension and vertical thinning associated with top-to-east shear. Folds were analyzed using field measurements and photo digitization. X-ray computed tomography was used to create 3D images of the folds. Images and photographs were used to quantitatively analyze wavelength, amplitude, and thickness, from which estimates of viscosity ratio and strain were derived. Garnet layers that successfully folded contain concentrations of garnet inclusions of greater than 30%. Quartz is concentrated between the garnet inclusions in such layers. This suggests that the rigid garnets enhanced the viscosity ratio of a quartz-rich layer to its matrix. Application of buckling theory indicates that the viscosity ratio layer/matrix is between about 15 and 40. Applying the Einstein-Roscoe equation shows the effect of the garnets, with average concentration in folded layers of approximately 35%, was to increase the effective viscosity of the layers by about 3; thus without the garnets the viscosity ratio would be about 9. Garnet layers that are completely unfolded have inclusion concentrations of less than 20% and original viscosity ratios less than 5. Concentrations above the threshold for unfolded layers and below the threshold for folded layers resulted in slightly perturbed layers that could not be analyzed for distinct fold properties. A linear relationship between wavelength and amplitude, normalized over thickness, among the garnet folds was identified. This can be explained by the predicted linear relationships between fold wavelength and viscosity contrast and fold amplitude and viscosity contrast at constant strain. Estimates of strain associated with folding, involving ca. 60% shortening perpendicular to the fold hinges, are consistent with estimates of strain based on stratigraphic thinning with a component of top-east shear.