Age and structure of the fish hatchery stock, lower Rapid River Canyon, west-central Idaho

Abstract

The Salmon River Suture Zone of west-central Idaho records Mesozoic accretion of island arc terranes to the western margin of Laurentia. Located west of the suture zone, exposures near Riggins, Idaho, represent accreted island arc rocks. Ages and deformation of intrusive rocks within and west of the suture zone help constrain timing of regional metamorphic events. One such intrusion, the Fish Hatchery Stock, is located in the lower Rapid River Canyon. The Rapid River Canyon -- situated ~11 km (7 miles) SW of Riggins, Idaho -- contains Late Triassic carbonate rocks of the Martin Bridge Formation, overlying the Middle and Late Triassic, volcanogenic Wild Sheep Creek Formation. The Fish Hatchery Stock, a mediumgrained, phaneritic hornblende tonalite, intrudes the Wild Sheep Creek Formation. Like the host rocks, the Fish Hatchery Stock is metamorphosed to the greenschist facies. New U-Pb geochronology (LA-ICP-MS; CA-TIMS) indicates a magmatic crystallization age of ~136 Ma for the Fish Hatchery Stock. This age is discordant with the existing 40Ar/39Ar hornblende cooling age of ~145 Ma (Snee et al., 1995). Fracture orientations (e.g., N35W, 43NE) in the stock suggest a kinematic relationship with the faulted Martin Bridge/Wild Sheep Creek contact, as well as with regional west-directed faults of the Riggins region (e.g., Rapid River Thrust). Metamorphism attributed to accretion in the Riggins region includes events occurring ~130 Ma and ~118 Ma, as recorded by 40Ar/39Ar cooling ages (Snee et al., 1995). Garnet growth also transpired in the hanging wall of the Rapid River Thrust between ~124-113 Ma (McKay et al., 2017). Deformation (Mode II fracturing) in the Fish Hatchery Stock adds to this regional data set, constraining faulting in the lower Rapid River Canyon to post-136 Ma.