TECTONIC HISTORY OF CRETACEOUS-PALEOGENE WESTERN IDAHO: EVIDENCE FOR INTERMONTANE AND INSULAR TERRANE COLLISIONS

The Salmon River suture zone – including western Idaho, eastern Oregon, and eastern Washington – records a sharp boundary between North America and the accreted terranes of the Blue Mountains. The Blue Mountain terranes correlate well to the Intermontane terrane of Canada, including the offshore assembly of two magmatic arcs (Wallowa & Olds Ferry terranes) with an intervening mélange (Baker terrane). The Blue Mountain terranes that are presently outboard of western Idaho accreted to North America in northern Nevada (and were moved northward by western Idaho shear zone (WISZ) – see below). However, the northward continuation of the Blue Mountain terranes also accreted in western Idaho, as evidenced by Blue Mountain terrane material interlayered within the North American craton. Collision appears to be latest Jurassic to earliest Cretaceous, and temporally correlates with the Nevadan orogeny. The more significant tectonic event, however, appears to be the activation of the dextral, transpressional WISZ that completely overprints the suturing event. Seismic data indicates that the WISZ continues downward and offsets the Moho by ~7-8 km, indicating a significant transcurrent component. Although the WISZ records deformation of the eastern Blue Mountain (Intermontane) terranes and western North America, the timing is consistent with collision of the Insular terrane. A prior restoration of known offsets on strike-slip faults in western Canada indicates that the Insular terrane lay offshore Idaho at this time. In this model, the Blue Mountains collide with North American, like a bumper car, pushed by Insular terrane collision. Moreover, deformation mostly resulted from northward movement of the Blue Mountains terranes into a transpressional syntaxis. The seismic results indicate the lack of a sub-horizontal crustal decollement under western Idaho, despite significant shortening eastward in the Montana fold-and-thrust belt. Inboard deformation, therefore, appears to result from endloading on the western edge of North America.