RF/φ ANALYSIS OF MYLONITIC ROCKS IN THE COUGAR CREEK COMPLEX, OREGON – IDAHO, AND POTENTIAL USE OF SC'-TYPE EXTENSIONAL SHEARBANDS AS QUANTITATIVE VORTICITY INDICATORS

Mylonitic rocks of the Cougar Creek Complex of northeastern Oregon and west-central Idaho provide an opportunity to document the deformational structures produced during general noncoaxial shear within quartz-feldspar mylonites and to explore the potential role of SC'-type extensional shear bands in vorticity analysis. Well-developed porphyroclasts within six mylonite zones were utilized to determine bulk kinematic vorticity (W_k) using Rf/φ analysis. Estimated W_k values in the Courgar Creek mylonites range from W_k = 0.258 to W_k = 0.375. Synthetic and antithetic shear band inclinations were measured relative to shear zone boundaries and compared to the determined general noncoaxial flow fields and eigenvector orientations. In each mylonite zone, synthetic shear band populations exhibit a range of inclination with maximum inclination lying approximately parallel to the Acute Bisector (AB) of the eigenvectors. Similarly, antithetic shear band populations show a range of inclination with a maximum inclination lying near the Obtuse Bisector (OB) of the eigenvectors. We infer that extensional shear bands form initially parallel to the bisectors of the eigenvectors and rotate towards the flow plane with progressive deformation, decreasing their inclination relative to the shear zone boundary. AB and OB have significance in the strain field in that they represent orientations of maximum angular strain rate. Thus, planes perpendicular to AB and OB are mechanically favorable for small zones of localized simple shear (shear bands) within the heterogeneous bulk strain of the mylonite. Orientation analysis of populations of SC'-type shear bands may provide a direct, quantitative means of estimating W_k.