EXTENT AND KINEMATICS OF THE WESTERN IDAHO SHEAR ZONE WITHIN THE 7.5’ SAGE HEN RESERVOIR QUADRANGLE, IDAHO

New geologic mapping of the bedrock within the Sage Hen Reservoir Quadrangle, Idaho, provides insight into the deformation in the mid-Cretaceous western Idaho shear zone and subsequent magmatism and deformation. Our mapping is based on that of N. Braudy (Braudy, N., 2017, Master’s thesis, University of Wisconsin-Madison). Foliation in the Sage Hen Reservoir region dips steeply east and strikes NNE, consistent with a recorded orientation change to the north. Lineations are generally downdip and defined by alignment of biotite. Shear sense indicators occurs on the fabric perpendicular to the lineation, as confirmed by microstructural analysis. The western Idaho shear zone records dextral transpressional kinematics and is interpreted as a lithospheric-scale plate boundary that occurred simultaneously with magmatism.

A series of granitic units, some containing garnets, are the primary units impacted by deformation of the western Idaho shear zone in the Sage Hen Reservoir area. A ~7 km screen of metamorphosed sedimentary rocks that have been deformed by shearing is also exposed locally. Geochronology work is currently being performed on detrital zircons from quartzites within the sedimentary screen to provide a better constraint on the age of the host rocks. The Payette River tonalite, on the eastern edge of the quadrangle contains the same overall trend of western Idaho shear zone fabric, but the solid-state deformation is minimal. The western part of the map area is overlain by Columbia River basalts. These basalts are tilted shallowly westward, suggesting that the underlying granites are also rotated westward.

In addition to traditional mapping, we collected relevant metadata about the uncertainty of measurements and observations in the field area. We employed a 5-point ranking system, which is applicable to both outcrop measurements and unit contacts. The uncertainty metadata, orientation data, and field photos are all available via the StraboSpot digital database.