The Ruby Mountains and adjoining East Humboldt Range form a North American Cordilleran metamorphic core complex called the Lamoille Quadrangle. In brief, the core complex in the quadrangle exposes Neoproterozoic and lower Paleozoic miogeoclinal strata that were thrust-faulted, deeply buried, metamorphosed, recumbently folded, and intruded pervasively by Cretaceous to Oligocene granitoids. Cenozoic unroofing accompanied footwall rise below an inclined down-to-the-west extensional fault system. That fault system imprinted a ductilely sheared mylonitic carapace on the now-exposed deep footwall in Oligocene and Miocene time, and has continued into Holocene time to move as a gently dipping near-surface brittle fault.
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