Paper No. 3
Presentation Time: 8:30 AM


FAYON, Annia K., Earth Sciences, University of Minnesota, Minneapolis, MN 55455, TIKOFF, Basil, Department of Geoscience, University of Wisconsin, 1215 W Dayton St, Madison, WI 53706, KAHN, Maureen J., Geology, Carleton College, 300 North College St, Northfield, MN 55057, GASCHNIG, Richard M., Department of Geology, University of Maryland, College Park, MD 20742 and VERVOORT, Jeff D., School of Earth and Environmental Sciences, Washington State University, Webster Physical Science Building 1228, Pullman, WA 99164-2812,

New (U-Th)/He zircon ages were obtained from the western Idaho shear zone (WISZ) and the Atlanta lobe of the Idaho batholith, primarily along the EarthScope IDOR seismic transect (~44.5° N latitude). The WISZ is the boundary between the North American craton to the east and accreted terranes to the west, and is demarcated by intense deformational fabrics, a rapid change in wallrock lithology, and steep isotopic (Sr, O) gradients. The Atlanta lobe occurs immediately east of the WISZ. Rocks exposed within the WISZ yielded the oldest (U-Th)/He zircon ages (indicating cooling below ~200 °C), ranging from 75 to 70 Ma. The plutons immediately to the east of the WISZ exhibit ages of ~ 65 Ma, despite being locally only slightly younger in crystallization age (based on U/Pb in zircon ages). Further east in the Atlanta lobe, cooling ages are more variable, ranging from 60 Ma to as young as 25 Ma, the youngest documented cooling ages within the Atlanta lobe. The variability of cooling ages within the Atlanta lobe east of WISZ can be attributed to either differential exhumation along normal faults and/or resetting due to Challis volcanic/magmatic activity. While the latter may account for Eocene cooling ages, it does not explain the Miocene ages obtained in two of the samples. These younger ages are more likely associated with younger normal fault activity.

In summary, the new (U-Th)/He zircon ages indicate a variable cooling history - spanning ~50 m.y. – for the igneous rocks of central Idaho. Results from this preliminary study are consistent with prior 40Ar/39Ar thermochronology studies within the WISZ that indicate cooling below 325 °C at ~ 75 Ma (Ar/Ar on biotite) and 110 °C at ~40 Ma (apatite fission-track). All these combined data show that rocks within the WISZ were at or near the Earth's surface while the Atlanta lobe remained at temperatures greater than 200 °C. Variability of ages across the region illustrates more complex cooling paths of the Atlanta lobe during Late Cretaceous to Miocene time, while the WISZ remained thermally undisturbed.