Abstract

Part of the Pioneer Mountains of central Idaho is a metamorphic core complex in which Mesozoic(?) fabrics have been over-printed by early Tertiary extension. The earlier deformation (Mesozoic?) affected all pre-Tertiary rocks in the core and resulted in macroscopic, northeast-vergent folding and complex mesoscopic folding of a Precam-brian basement gneiss complex and an overlying upper Proterozoic and lower Paleozoic metasedimentary sequence. A synkinematic granodiorite pluton intruded between the gneiss complex and the metasedimentary sequence records a relatively simple strain indicative of east-northeast-directed ductile thrusting of the hanging wall. The pre-Tertiary structures are probably a deep-seated manifestation of Sevier/Laramide compression. The early Tertiary extension is demonstrated by (1) a conjugate set of ductile shear zones showing normal offset; (2) a lineated "carapace" of mylonitic quartzite at the structurally highest levels of the core; and (3) a brittle, oblique-slip fault system along the northern edge of the core. Kinematic indicators from all three types of Tertiary structures suggest northwest/southeast transport. The lineated mylonitic quartzites, which are cut by the brittle fault system, are interpreted as the basal portion of a large ductile shear zone created during extension which subsequently passed into brittle deformation conditions. Exposure of the metamorphic core apparently occurred prior to Eocene Challis volcanism as shown by quartzite clasts apparently derived from the metasedimentary sequence volcanics within the core, however, suggests that extension continued during volcanism. Neogene Basin and Range extension is super-imposed on the older core-complex-type extension; the two extension directions are nearly 90° apart. The early Tertiary extension is coeval and coaxial with regional Eocene extension observed in northern Idaho, Washington, and British Columbia and may have been caused by strike-slip complications or escape tectonics during rapid Farallon/North American plate convergence.

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