THE WESTERN IDAHO SHEAR ZONE, WEST MOUNTAINS, IDAHO: PRELIMINARY STRUCTURAL GEOLOGY RESULTS OF IDOR PROJECT

The Idaho-Oregon project is an integrated study of the tectonic boundary between the North America craton (East) and the Blue Mts (West). The boundary is marked by discontinuities in lithology, geochemistry (Sr, O isotopes), and the western Idaho shear zone (WISZ).

The IDOR seismic transect cuts orthogonally through the WISZ in the West Mts. Five different units are found (E to W): 1) Plutonic rocks with biotite and muscovite (Idaho Batholith); 2) An undeformed pluton (granodiorite of Rat Creek); 3) A tonalite sill (Payette River tonalite); 4) Orthogneisses and paragneisses (orthogneisses correlative to Little Goose Creek complex); and 5) weakly deformed plutonic rocks (tentatively correlated with Hazard Creek complex). Solid-state deformation characterizes the WISZ, with a weakening of WISZ fabric toward the eastern margin; the western margin is mostly covered by Miocene (Columbia River) basalts.

The WISZ is dominated by orthogneisses, with paragneisses localized along and near plutonic boundaries. Gneisses have a steeply east-dipping, solid-state foliation defined by ribboned quartz grains and biotite stringers and a sub-vertical lineation defined by biotite stringers and, locally, aligned hornblendes or garnet porphyroblasts. Shear sense is defined by mafic enclaves, feldspar augens, boudinaged dikes, garnet porphyroblasts, and isoclinal sub-vertical plunging folds. The kinematics indicate dextral shear sense on faces perpendicular to foliation and lineation. Thus, the WISZ exhibits dextral transpression with E-W shortening and vertical elongation.

Geochronological dates (U/Pb on zircon) from this area are consistent with previous works. The undeformed granodiorite of Rat Creek is dated at 89 Ma to 84 Ma. The Payette River tonalite is dated at 91 Ma, providing a minimum age of deformation associated with the WISZ. Zircons from paragneisses show detrital ages of Proterozoic and Neoarchean, with a few grains showing metamorphic (?) rims of mid-Cretaceous age. Zircons from orthogneisses vary from 107 Ma to 91 Ma and inherited ages of between over 200 Ma to 106 Ma. A weakly deformed pluton outside the WISZ on the W side is ~101 Ma, indicating that WISZ was not active at this time. Thus timing of deformation in the WISZ is constrained to be 101-91 Ma.