CRYSTALLOGRAPHIC VORTICITY AXES ANALYSES FROM THE WESTERN IDAHO SHEAR ZONE: EVIDENCE FOR OROGENIC-SCALE TRANSPRESSION

The western Idaho shear zone marks the western boundary of the North American craton at the latitude of south-central Idaho. The North America margin is presently juxtaposed against the Blue Mountain terranes along a sharp boundary marked by a distinct change in geology and abrupt change in isotopic (Sr, Nd, and O) values: These demarcations are coincident with the western Idaho shear zone. Steeply E-dipping, N-NNE oriented foliations and vertical lineations characterize the fabric in the western Idaho shear zone. Fault reconstructions result in vertical foliations and vertical lineations; these patterns are consistent with dextral transpressional deformation. A wallrock screen was recently mapped near Sage Hen reservoir, Idaho, which contains schists, marbles, and quartzites. In addition to providing constraining quantitative constraints on metamorphic conditions during WISZ deformation, these rocks can be used to provide information about shear zone kinematics. We applied the new technique of Crystallographic Vorticity Axes (CVA) analysis to the quartzite samples. CVA analysis calculates a rotational (vorticity) axis that best describes the intragranular dispersion and rotation of crystallographic axes in a single deformed grain. A population of CVA axes – each from a different grain - are used together to constrain the position of a preferred, bulk vorticity axis for a sample. In the quartzites within the WISZ, the vorticity axes are parallel to the lineation direction (e.g., vertical). In contrast, simultaneous deformation near Orofino, ID, in the Ahsahka shear zone was interpreted as dominantly reverse-sense motion. CVA analyses at this location indicate rotational (vorticity) axes that lie in the foliation plane but are perpendicular to the lineation; this pattern is consistent with simple shear dominated deformation. These patterns make sense in the context of a curved orogenic margin, and illustrate the power of CVA analyses to help unravel three-dimesnional deformation.