Date of Award
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During Cretaceous time, a Cordilleran arc formed the length of the North American margin. The arc intruded accreted terranes and continental crust, indicated by the 87Sr/86Sr = 0.706 isopleth. A magmatic flux from 110-85 Ma, and a trend to become younger and more felsic to the east characterize this arc. Further inland, the Idaho batholith has a continental isotopic signature and mostly dates <87 Ma. However, within the batholith, 98-87 Ma xenocrysts may mark an older arc assimilated into younger magmatism and/or eroded away. The Western Idaho Shear Zone (WISZ) forms a ~10 km wide western border of the batholith and has an abnormally steep 87Sr/86Sr gradient. Reducing the gradient to “normal” (modeling after Sierra Nevada) restores the width of the WISZ deformational area to ~100 km. Limited WISZ data indicate plutons intruded 118-104 Ma, then deformed 105-90 Ma. West of the WISZ, Cretaceous basins, such as the Ochoco basin, have well-defined geochronological data. The Ochoco basin is a Cretaceous overlap sequence deposited on the Blue Mountains Province. Estimates of its latitudinal translation range from >2000 to <400 km. Determining provenance of the Ochoco basin will help discern terrane translation distance.
This study augments previous work to create a geochronological framework to understand WISZ evolution, and compare to western Cretaceous basins. In the McCall segment of the WISZ, we collected eight samples. The Hazard Creek complex yields zircon ages of ca. 162 Ma, 128 Ma (with a 203 Ma inherited population), and 120-109 Ma; the Little Goose Creek complex sample dates of ca. 113 Ma. In the South Mountains segment, a possible extension of the WISZ, Council Mountain and No Business plutons are ca. 113 and 180 Ma. While age and composition data show few spatial trends, Hf data suggest samples become less radiogenic to the east (εHf =+10.1 to +4.7), consistent with previous Nd and Sr studies. The WISZ contains more complexity and a larger range of ages than previously understood. Our data are consistent with the “missing” Idaho arc hypothesis: from 120-100 Ma an arc similar in scale to the Sierra Nevadan arc stood the length of the WISZ data. Zircon age and Hf data from the Ochoco basin overlap with the WISZ, indicating a possible sediment source. Linking the Idaho batholith and Blue Mountains Province would limit translation of accreted terranes and provide a location for sediment eroded from the “missing” arc.
Patzke, Mollie, "Characterizing the Hazard Creek Complex of the Western Idaho Shear Zone: Analysis of a Middle Cretaceous Idaho Arc" (2017). Geosciences Student Honors Theses. 17.