A SPECULATIVE MODEL FOR EARLY CRETACEOUS DEFORMATION IN THE SOUTHERN IDAHO SECTION OF THE NORTH AMERICAN CORDILLERA

A significant issue in the northern U.S. Cordillera is the timing of collision of the Blue Mountain terranes to the western edge of North America, along what is known as the Salmon River suture zone. This zone is nearly completely overprinted by the spatially overlapping but temporally distinct mid-Cretaceous western Idaho shear zone. Workers have interpreted the timing of terrane collision anywhere from Late Jurassic, earliest Cretaceous (150 Ma), middle Early Cretaceous (~130 Ma), and continuous Early Cretaceous-Late Cretaceous.

We propose the following speculative model, based on incorporation of existing and new data from the region, that accounts for the variability in timing. The Wallowa terrane accreted to the Olds Ferry terrane in the latest Jurassic, but offshore from the continental margin. At ~130 Ma, the amalgamated Blue Mountain terranes, which is part of the Intermontane superterrane, collides with North America. This collision is a result of southward movement of the northern section of the Intermontane superterrane. The issue, however, is that movement is constrained by the irregular plate geometry inherited from Precambrian rifting. The Blue Mountain terranes are effectively caught in an embayment (McCall) between two promontories (Palouse promontory to the North; California promontory to the south). The southward (sinistral along the margin) movement of the northern Intermontane superterrane has the following major effects: 1) It causes moderate (~30°) clockwise rotation of the Blue Mountain terranes because it is pushed against the buttress of the northern part of the California promontory; 2) It results in regional contractional deformation in northernmost Nevada (Luning Fencemaker belt) and in the adjacent part of Sevier fold-and-thrust belt (e.g., Willard thrust sheet); 3) It pushes the Klamath Mountains westward, from its previous position directly along strike with the Jurassic Sierra Nevada batholith. This model further explains a series of granitic rocks found on the Blue Mountain terranes, which record a complex history of Late Jurassic to early Late Cretaceous arc and slab breakoff magmatism.