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The Bitterroot dome-Sapphire tectonic block appears to be a well-developed example of a plutonic-core gneiss-dome complex or infrastructure separated from the adjacent suprastructure by a gently dipping zone of mylonitic shearing or an “Abscherungszone.” The suprastructural Sapphire block, on the order of 15 km thick, 100 km long, and 70 km wide, apparently moved eastward about 60 km, bulldozing rocks of the eastern Flint Creek Range ahead of it. Movement of the block must have occurred about 75 or 80 m.y. ago during late stages of consolidation of the Idaho batholith which, along with sillimanite-zone regional metamorphic rocks, makes up the infrastructure under the mylonitic detachment zone. Timing of movement in the Sapphire block matches that in the Bitterroot dome.

The Bitterroot dome must have risen after off-loading of the Sapphire block, because the shear foliation and lineation that formed during movement completely cross the dome; this indicates that the block must have moved eastward across the whole of the area now occupied by the dome rather than radially down the flanks of an existing dome. The shear lineation maintains its eastward trend even at the south end of the dome where the foliation dips southward. The lineation and shear foliation are strongest along the eastern flank of the dome, over which the greatest thickness of the block would have passed.

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