Abstract

As the Idaho batholith was emplaced just before the beginning of the Laramide orogeny, it formed a part of the hinterland on the west side of the Rocky Mountain geosyncline. Acting as a strong, rigid mass, it transmitted the orogenic forces into the thick succession of relatively weak sediments in the bordering trough, where the strata were forced into long regular folds of the Appalachian type, and broken by great low-angle thrusts.Structural studies in scattered mining districts, within as well as outside the borders of the batholith, show that, in transmitting the forces, the batholith was not only locally deformed but also controlled deformation of the bordering formations. In many places the batholith was broken by minor thrusts of west-northwest trend and southwest (exceptionally northeast) dip. The granitic mass was also broken by many extensive transverse strike-slip faults trending northeast, apparently in response to horizontal shearing. The batholith influenced deformation of the thick Belt series for scores of miles to the north, where, during late stages of crustal shortening, transverse zones of faulting of general east-southeast trend were formed.These transverse zones of structural weakness both within and outside the batholith facilitated and thus localized igneous intrusion and mineralization at the close of the orogeny. They also influenced deformation, igneous activity, and mineralization during a mid-Tertiary disturbance.

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