Geotectonics and geochemistry of the Sandpoint quadrangle, N. Idaho and N.E. Washington

Recent geological and geophysical studies support the idea that the North American Cordillera is a collage of numerous discrete crustal fragments or terranes. It is also documented that most of these terranes are allochthonous relative to the North American craton (NAC). The Sandpoint quadrangle is located in the southeastern most portion of the Cordilleria, where the many conflicting interpretations have been proposed for the ancient western edge of the NAC as well as for the timing and the sequences of the collisions between the allochthonous terranes and the NAC. Placing the Sandpoint quadrangle into its proper tectonic setting, therefore, is critical to any tectonic interpretation of the whole Cordillera. This study provides structural, geochemical and isotopic evidences which permit some qualified statements to be made on the tectonic settings of the Sandpoint quadrangle.; The geotectonic setting of the Sandpoint quadrangle is described in terms of a model compiled from several previously proposed models. Major tectonic elements in this model include; (a) an amalgamation of all allochthonous terrans, and (b) an oblique westward subduction between NAC and these allochthonous terranes around mid to late Cretaceous time. This westward oblique subduction is held responsible for more than 2,000 km of northeastward migration of the allochthonous terranes towards NAC as indicated by paleomagnetic and paleontologic evidence. It also explains the kinematics capable of causing the major differences in structure observed between the western and eastern parts of the study area. Mid to late Cretaceous and early Tertiary plutonism in the study area can also be attributed to this subduction. Large scale imbrication of the crust due to the collision between the allochthonous terranes and the NAC around late Cretaceous time resulted in crustal thickening and consequent partial melting of the crustal material at different crustal levels. (Abstract shortened with permission of author.)...