Structure of the arc-continent transition in the Riggins region of west-central Idaho
Gray, Keith D.
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New U-Pb zircon geochronology from the Riggins region of west-central Idaho refines the timing of synmetamorphic deformation across the Salmon River suture zone, a broad NNE-striking belt (>25 km-wide) of high strain recording Jura-Cretaceous arc--continent collision. Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry (LA-ICP-MS) yields mid-Cretaceous crystallization ages on formerly undated intrusive rocks sampled from the northeastern Seven Devils Mountains and Salmon River canyon. Ages range from ~136 Ma (Heavens Gate stock) in accreted oceanic crust to ~92 Ma (Looking Glass pluton) along the western margin of Laurentia. This study integrates existing U-Pb, Ar-Ar, and Sm-Nd isotopic data from the region in efforts to constrain structural fabrics developed across the arc-continent transition. L-S tectonites are mapped along a 50-km, E-W transect extending from the Snake River into the Salmon River canyon (Crevice area). From west-to-east across the transect, a systematic decrease in the age deformation is observed. Although modified by late-stage folding, L1-S1 fabrics are continuous across the arc-continent boundary and record a progressive history of accretion-related deformation (post-136 Ma to post-92 Ma). This interpretation contrasts with earlier tectonic models invoking a two-phase evolution for western Idaho's accretionary margin.