MODELING OF RIGID CLAST ROTATION IN TRANSPRESSION APPLIED TO THE WESTERN IDAHO SHEAR ZONE

The orientation of rigid markers within a shear zone can be used to constrain the amount of accumulated strain. Ghosh and Ramberg (1976) presented an analytical solution for rigid clast rotation for any combination of pure shear and simple shear. With minor modifications, these equations were applied to transpressional settings to create a 2D (horizontal) numerical model of rigid clast rotation. A 2D model of transpression is possible for the horizontal plane because material lines and rigid markers are metastable in this plane. We conducted physical experiments to test the validity of these calculations. Ellipses of various aspect ratios were subjected to transpressional deformations with varying angles of oblique convergence. There is a high degree of correlation between the orientation of the long axis of the ellipse, as predicted by the numerical model, and orientations observed in the physical experiments.

The application of these calculations to a population of clasts suggests that the degree of preferred clast orientation (the fabric ellipse ratio) is a function of the clast aspect ratio and the amount of strain. At low strains (Rf <10), this relationship is independent of the angle of oblique convergence. Using the aspect ratio of the clasts and the fabric ellipse ratio provides information about the minimum amount of recorded strain. In addition, the fabric ellipse ratio and the average orientation of the rigid markers should constrain the angle of oblique convergence.

We applied these results to the Western Idaho Shear Zone, which strikes roughly N-S near McCall, Idaho and is characterized by transpressional kinematics. The bedrock geology consists of three N-S trending granitic sills. The center sill, the Little Goose Creek Complex, contains most of the shear zone and has large, potassium feldspar megacrysts. Feldspar orientation data from planes perpendicular to lineation (the horizontal plane) yield average aspect ratio of 1.75. Using this aspect ratio and calculating fabric ellipse ratios from megacryst orientations for each outcrop, estimates of minimum horizontal strain in the Little Goose Creek Complex range from Rf=2.9 to 11.5. These data indicate a minimum offset of 10 km across the shear zone.