Idor Idaho-Oregon Earthscope Project: Deformation and Modification of a Steep Continental Boundary

The IDOR project is an integrated study of the tectonic boundary between Precambrian North America and accreted terranes (island arcs of the Blue Mountains) in the Idaho-Oregon region. Its goal is to determine how the accretionary edge of the continental margin formed and was subsequently modified by deformation and magmatism. The margin contains unusually sharp isotopic gradients (Sr, O) representing the edge of Precambrian continental crust. This isotopic break correlates with a lithospheric-scale shear zone (western Idaho shear zone or WISZ) that formed in the mid Cretaceous in western Idaho. The WISZ is a major deformation zone characterized by a sequence of gneisses ~5 km thick and records transpressional deformation. The sharp isotopic gradient results, at least in part, from modification of the original Late Jurassic-Early Cretaceous suturing by the extreme shortening associated with the WISZ. The Idaho batholith, based on recent geochronology, was emplaced in multiple phases and is entirely east of the WISZ. The main batholith phase has no easily recognized mantle component, which contrasts markedly from other Cordilleran batholiths. Batholith emplacement occurred simultaneously with Sevier thrusting further east. Eocene (Challis) magmatism occurred throughout the region, both as localized centers and dike swarms. Miocene deformation and magmatism occurs throughout the region. A second sharp isotopic break is denoted by Miocene basaltic rocks, and is parallel to the sharp isotopic boundary in western Idaho (the exposed WISZ) but located ~120 km to the west. One hypothesis invoked to explain the repeated isotopic boundary is that a Sevier-age detachment cut and offset the WISZ, beheading it and leaving the WISZ’s root zone in eastern Oregon. Miocene magmatism, including the extrusion of the voluminous Columbia River basalts, appears localized on the sharp isotopic boundary in eastern Oregon. Extensional deformation, in contrast, appears partly localized on the WISZ in western Idaho. The IDOR project will investigate these issues through coordinated passive and active seismic (~500 km long line) deployments, geochemistry, geochronology, and structural geology.