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Timing of deformation and exhumation in the western Idaho shear zone, McCall, Idaho

Scott Giorgis^1,, William McClelland², Annia Fayon³, Brad S. Singer⁴ and Basil Tikoff⁴

¹ Department of Geological Sciences, State University of New York–Geneseo, 1 College Circle, Geneseo, New York 14454, USA
² Department of Geological Sciences, University of Idaho, Moscow, Idaho 83844-3022, USA
³ Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455-0219, USA
⁴ Department of Geology and Geophysics, University of Wisconsin, Madison, Wisconsin 53706, USA

Correspondence: E-mail: giorgis{at}geneseo.edu

The western Idaho shear zone is one of several Cretaceous high-strain zones in the Cordillera that are thought to have been associated with the northward translation and/or docking of terranes presently in British Columbia. Located in west-central Idaho, this zone of intense deformation consists of a mid-crustal exposure of a lithospheric-scale, intra-arc dextral shear zone that overprints the Salmon River suture zone along the western edge of the Idaho batholith. U/Pb zircon geochronology constrains the main phase of deformation to between ca. 105 and 90 Ma. Cessation of movement on the shear zone occurred by 90 Ma, as determined by dating of the syntectonic Payette River tonalite and a crosscutting pegmatite dike in the Little Goose Creek complex. The ⁴⁰Ar/³⁹Ar thermochronology indicates that the shear zone passed through both the hornblende (~550 °C) and biotite (~325 °C) closure temperatures between 85 and 70 Ma. The ⁴⁰Ar/³⁹Ar biotite dates from an outcrop-scale, crosscutting shear zone are indistinguishable from that of the host rock, indicating that deformation occurred above the closure temperature of biotite. Apatite fission-track analysis suggests that exhumation to shallow crustal levels occurred ca. 40 Ma during mid-Tertiary regional exhumation or renewed tectonic activity along the Salmon River suture zone. Taken together, the ⁴⁰Ar/³⁹Ar results and apatite fission-track analyses indicate a two-stage uplift history for the western Idaho shear zone. Overall, the geochronology indicates that dextral transpressional movement on the western Idaho shear zone was temporally distinct from the Early Cretaceous suturing event. Additionally, the first stage of exhumation recorded by the western Idaho shear zone immediately followed transpressional deformation. Cessation of displacement on the western Idaho shear zone by ca. 90 Ma indicates that the exhumation did not solely occur as a result of ductile deformation on the shear zone itself. Moreover, dextral strike-slip movement on the western Idaho shear zone had also ceased by 90 Ma, indicating that terrane translation models for the Cordillera can only use the western Idaho shear zone to accommodate northward translation up to ca. 90 Ma. Lastly, the timing of movement on the western Idaho shear zone and contractional deformation recorded in the Insular terrane suggests a correlation between these events. This hypothesis implies that the deformation recorded in the western Idaho shear zone may have been linked to the oblique collision of the Insular superterrane with North America.

Key Words: western Idaho shear zone • transpression • Late Cretaceous • Idaho batholith • U/Pb • ⁴⁰Ar/³⁹Ar • apatite fission-track analysis

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