THE JURASSIC-CRETACEOUS HAZARD CREEK ARC, WESTERN IDAHO

In Idaho, the boundary between rocks of Laurentia affinity (to the east) and rocks of ocean island arc affinity (to the west) is currently demarcated by the ⁸⁷Sr/⁸⁶Sr = 0.706 isopleth and the western Idaho shear zone (WISZ). Numerous Jurassic-Cretaceous granitic intrusions are present along this boundary. We focus on the igneous rocks of the Hazard Creek complex, which are located west of the WISZ. The northward extent of the Hazard Creek complex is located ~10 km south of Riggins, ID. U-Pb zircon data records crystallization ages ranging from 180 to 109 Ma, indicating up to a 70 m.y. span of magmatism. Recent mapping and U-Pb zircon data indicate that the Hazard Creek complex extends southward to Emmett butte, ID, suggesting that the complex is longitudinally continuous for nearly ~150 km.

Rocks of the Hazard Creek complex and Little Goose Creek complexes exhibit εHf isotope compositions from +12.1 in westernmost samples to +4.7 in easternmost samples, with a decrease in the radiogenic signature from west to east. The decreasing radiogenic signature suggests that there was a subduction-related magmatic arc west of the ⁸⁷Sr/⁸⁶Sr = 0.706 isopleth, that includes both the Hazard Creek complex and Little Goose Creek complex. This arc was disrupted by deformation in the transpressional WISZ at ~100 Ma and I-type magmatism ceased by ~85 Ma. The Hazard Creek complex section of the arc that was only slightly deformed while the younger Little Goose Creek complex was significantly deformed by the WISZ. We interpret that these events were caused by the collision of the Insular terrane with North America at these latitudes. The WISZ also resulted in significant strike-slip tectonism, estimated at ~400 km of northward translation of the Hazard Creek complex. We anticipate that detrital zircons from this Early Cretaceous magmatic arc should be present in basins throughout the northern Cordillera.