SMALL-SCALE FOLDING RELATED TO DETACHMENT TECTONICS - NORTHERN SNAKE RANGE, NV

Local small-scale folding occurs within and below the detachment zone of the eastern flank of the Northern Snake Range in east-central Nevada. Overall sense of shear in the detachment zone is top-to-the-ESE, as is well displayed by a mylonitic lineation in Cambrian quartzite in the footwall of the detachment and by various kinematic indicators. Fold style in marble in the detachment zone varies from similar to complex, and reflects the complexity of flow and deformation and changing conditions in space and time, from ductile to semi-brittle. Cherty layers in the marble are both buckled and boudinaged. Fold hinges are broadly distributed in a plane sub-parallel to the detachment. A stretching lineation is generally parallel to the lineation in the footwall, but locally is affected by late stage folding. In Precambrian McCoy Creek Group rocks of the footwall in Hendry’s Creek, some 300m below the detachment, is a 100m scale recumbent fold, with hinge parallel to the extension direction. In garnet-staurolite schist within this fold are a number of cm-scale intensely buckled garnet-rich layers with hinges also parallel to the extension direction. Schistosity is axial planar to these folds. Fold analysis indicates an effective viscosity ratio (layer to matrix) of between 25 and 50 and shortening along the layer of between 30 and 60%. Applying the Einstein-Roscoe equation to estimate the effective viscosity of a layer containing rigid spheres suggests the viscosity ratio should be about 8. The discrepancy is attributed to the fact that the garnets are non-spherical and in contact with their neighbors, thus stiffening the layer. The most likely deformation that produced the folds involves both vertical and lateral shortening, and with 40 to 150% extension parallel to the hinges. This is consistent with the kinematics implied by fabric and boudinage in rocks away from the folds.