EXTENSIONAL FABRIC DEVELOPMENT ALONG AN ARC-CONTINENT BOUNDARY, SALMON RIVER CANYON, WEST-CENTRAL IDAHO

1:24,000-scale mapping of the Partridge Creek area (PCA) in the Salmon River Canyon structurally evaluates steeply east-dipping metasedimentary, metaplutonic, and plutonic units within the western Idaho shear zone. This fault marks the Blue Mountains oceanic arc and pre-Cenozoic North American continent boundary, and has been interpreted to be a late Mesozoic dextral transpression or intracontinental subduction zone using lithologic, geochemical, and fabric element constraints, and geothermobarometry.

In the PCA, the Kelly Mountain Schist defines the cratonic wall rock and is composed of muscovite + biotite ± garnet ± sillimanite schist. Fibrolitic mineral, and biotite aggregate and felsic stretch lineations have a down-dip plunge in the schistosity plane. This plane also contains tops-down-to-the-east, extensional kinematic indicators oriented parallel to the lineations including mica fish, rotated garnet porphyroblasts, feldspar porphyroclast trains, and C'-type shear bands. Asymmetric markers having conflicting shear sense occur perpendicular to the lineations. Five mapped units of tabular sills and dikes intrude the schist, and display a range of magmatic-state/Pre-Full Crystallization to solid-state/Crystal Plastic Strain fabrics and down-dip hornblende and biotite mineral and felsic stretch lineations. These include the Crevice Granodiorite, Looking Glass and Spring Creek Tonalites, and Partridge Creek and Van Ridge Gneisses. The latter intrudes the arc-continent boundary because it contains the westernmost screens of continental schist. Ductile extension is best preserved in the schist, although ductile and brittle extension is also recorded in the igneous units as stair-stepped sills and boudin trains, feldspar porphyroclast trains, and fault breccia and slickenlines.

This micro- and mesoscale kinematic analysis documents ductile extension along the arc-continent boundary not previously noted within the western Idaho shear zone. Both ductile and brittle extension overprint and mimic the orientation of earlier formed fabrics. The use of lineations to constrain compressional versus transpressional, or transtensional reconstructions along the shear zone remains problematic.