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Age and structure of the fish hatchery stock, lower Rapid River Canyon, west-central Idaho
Mann, Audra N.
Mann, Audra N.
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2018-05
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Electronic dissertation
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Electronic dissertations
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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.
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Thesis (M.S.)-- Wichita State University, College of Liberal Arts and Sciences, Dept. of Geology
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Wichita State University
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Copyright 2018 by Audra N. Mann
All Rights Reserved