MESOSCALE MAPPING OF SHEAR SENSE, WESTERN IDAHO SHEAR ZONE, MCCALL IDAHO

The western Idaho shear zone is a late Cretaceous shear zone located in west central Idaho. The purpose of this study is to determine the shear sense of the western Idaho shear zone. Shear sense is typically examined at the hand sample or the microscopic scale. This shear zone presents conflicting shear sense indicators at these scales, however dextral indicators appear to dominate. The out-crop scale of this project is significant because it is different than is typically investigated, and provides a new perspective on shear sense indicators in shear zones. In 2002, a grid of 180 squares was set up across an outcrop in the western Idaho shear zone. Pictures were taken of each square, as well as a core sample for anisotropy of magnetic susceptibility (AMS) analysis from every corner, collecting data across an area of approximately 165 square meters. We produced a picture mosaic of the outcrop, creating a mesoscale map with a scale of approximately 1:100. This map allows the identification of folded or offset layers to show a sense of shear. Thin sections provide insight about microscopic structure of the rock and the detailed petrology of the area. Combining these forms of information shows that right lateral shear affected this area of the western Idaho shear zone. This study has shown that mesoscale mapping has the potential for determining sense of shear.