Abstract:
The Salmon River suture zone of western Idaho (USA) records mid-crustal metamorphism and deformation associated with orogenesis
during Mesozoic accretion of volcanic arc terranes to western Laurentia. We present petrographic and microstructural observations, garnet
geochemistry, pressure-temperature isochemical phase diagrams, and Sm-Nd garnet and U-Pb zircon ages to investigate the timing and
conditions of metamorphism in the Salmon River suture zone. The Salmon River suture zone is comprised of three thrust sheets: from east
to west, the amphibolite facies Pollock Mountain plate, upper greenschist to amphibolite facies Rapid River plate, and greenschist facies
Heavens Gate plate. The Pollock Mountain plate was isothermally loaded from 6 to >8 kbar at ~700 °C between 141 and 124 Ma during
northwest-southeast crustal shortening. The underlying Rapid River plate was isothermally loaded from 7 to ~10 kbar at 600–650 °C during
ca. 124–112 Ma metamorphism, which is contemporaneous with late- to post-peak metamorphism and ca. 118 Ma exhumation of the overlying
Pollock Mountain plate. In the Rapid River plate, thrust sheet emplacement induced high-strain ductile deformation and led to regional
development of linear-planar fabrics. The 206Pb/238U zircon ages for syndeformational to postdeformational magmatism record ca. 117 Ma or
younger juxtaposition of the two plates on the southeast-dipping Pollock Mountain thrust fault. Coeval 124–112 Ma metamorphism of the
Rapid River plate, ca. 118 Ma exhumation of the Pollock Mountain plate, and ca. 117 Ma or younger movement along the Pollock Mountain
fault suggest that metamorphism of the Rapid River plate was possibly driven in part by thrust juxtaposition and loading along the Pollock
Mountain fault. In this context, we interpret that metamorphism records diachronous thrust stacking during prolonged (>30 m.y.) accretionary
orogenesis in western Idaho.