NEW CONSTRAINTS ON THE SOUTHERN EXTENT OF THE WESTERN IDAHO SHEAR ZONE AND MAGMATISM AND METAMORPHISM THEREIN

The western Idaho shear zone (WISZ) is a lithospheric scale structure that records Cretaceous transpressional deformation between the accreted Blue Mountains province and cratonal North America. Understanding the extent of this structure provides critical details to large-scale Cordilleran tectonic models. WISZ fabrics occur principally in mid-Cretaceous intrusive rocks that are exposed north of Sage Hen Reservoir (~44.3°N), but correlative rocks with similar fabrics are also found in the Owyhee Mountains located ~140 km to the south. The area in between is largely covered with Cenozoic volcanics. Here, we report on a newly recognized exposure of the WISZ between these two segments along with new geochronological constraints for the previously documented exposures near Sage Hen.

An intrusive unit, herein named the Eagle Eye tonalite, contains WISZ fabrics and is located a short distance north of Black Rock Canyon of the Payette River. This unit is predominately a garnet-biotite tonalite and is exposed in a small window below the Columbia River Basalts. The fabric here strikes ~020º, consistent with change in orientation observed in Sage Hen to the north. Microstructures are like those observed in the WISZ further north. LA-ICP-MS U-Pb analysis of zircon from the Eagle Eye tonalite yielded ²⁰⁶Pb/²³⁸U dates between 111 and 116 Ma. A crystallization age of 112.3 +2.1/-0.3 Ma was extracted using the TuffZirc of Ludwig (2001). The age and lithology of this unit suggests a clear correlation with the Mesa Creek orthogneiss, exposed 35 km to the north around Sage Hen.

Additional constraints on the timing of metamorphism and deformation in the WISZ are provided by monazite U-Th-Pb dates obtained from the Muir Creek orthogneiss, a unit near the Sage Hen. Previously reported zircon results for this unit are highly complex but suggest igneous crystallization around 110 Ma followed by a prolonged period at high temperature conditions. Monazites from this unit yield a ²⁰⁸Pb/²³²Th age of 101.9 +/- 0.9 Ma, which is broadly consistent with existing evidence on the timing of WISZ initiation. Monazites were also obtained from a paragneiss sliver within the orthogneiss, which record highly variable ages ranging from 130 to 94 Ma. These ages suggest that these minerals record growth during both accretion and subsequent WISZ deformation.