STRUCTURAL ANALYSIS OF THE POTTERS POND MIGMATITE DOMAIN, WESTERN IDAHO SHEAR ZONE: RELATIONSHIPS BETWEEN PARTIAL MELTING, DEFORMATION, AND EXHUMATION DURING TRANSPRESSION

The Potters Pond migmatite domain (PPMD) is a structurally complex zone of heterogeneous migmatites and intrusive rocks within the western Idaho shear zone (WISZ). The PPMD is located ~10 km southwest of Cascade, Idaho, where the orientation of the WISZ changes from a trend of 020 in the south to 000 in the north. Dextral transpression resulted in the exhumation of migmatites in a region extending 1.5 to 2 km E-W and ~5 km N-S from Snowbank Peak to south of Blue Lake. Structural mapping within the migmatite domain, which is comprised of metatexite and melanocratic host gneisses and zones of diatexite, has identified multiple generations of leucosome and varied structural fabrics. Early, melt-rich diatexites in the PPMD are characterized by wispy schileren structures with disaggregated blocks of amphibolite, tonalite, and screens of deformed metasedimentary units. Structural measurements within the diatexite reveal two primary orientations of cross-cutting leucosome (020 and 050), defined by (1) melanocratic diatexite containing peritectic garnet; and (2) garnet-free diatexite of granitic to granodioritic composition. Evidence for melt-present deformation (e.g., leucosome in dilatant structural sites) and solid-state deformation (e.g., shear fabrics, stretched quartz) are observed within the PPMD. Observable contacts between the PPMD and surrounding units (e.g., Payette River tonalite, Sage Hen orthogneiss) range from sharp (western PPMD) to inter-fingering intrusive lobes, or co-mingled melt phases. Structurally late tonalitic intrusions at the periphery of the PPMD contain hornblende-bearing leucosomes, suggesting a change in melting conditions. High temperatures deformation fabrics in the PPMD indicate that melt-present deformation played a role in the evolution of the WISZ. The exhumation of migmatites within the transpressional restraining bend in the region of the PPMD further suggests that deformation enhanced, or was enhanced by, partial melting resulting in strain partitioning along the WISZ.