ONTOGENY AND OROGENY: HADEAN TO HOLOCENE EVOLUTION IN WESTERN LAURENTIA

Exposures of Archean crystalline basement, Proterozoic metamorphic belts, and Phanerozoic accreted terranes in western Laurentia reveal one of the most protracted sequences of craton formation, modification, and ongoing deconstruction on Earth. The complex interplay of magmatic and tectonic forces that have shaped this part of Laurentia are evident along a <300 km transect from southwestern Montana to West-Central Idaho. The sequence of events recorded ranges from formation of a rare segment of Earth’s Eoarchean crust to Holocene, plume-driven volcanism and generally conforms to the earliest ideas about the marginal accretion and stabilization of continents through orogeny, i.e., the oldest rocks are exposed toward the interior of the craton by Laramide uplifts in the eastern part of the transect. In the central part of the transect, this oldest crust is overprinted by a Proterozoic mobile belt that is a collage of mostly Paleoproterozoic rocks ranging from 1.7 to 2.5 Ga exposed along the soles of Sevier thrust faults. These Paleoproterozoic rocks are dominantly orthogneisses with protoliths derived from melts generated during subduction of the Medicine Hat Ocean (~1.8 Ga) and as a result of the crustal thickening that occurred during the continent-continent collision that terminated the Great Falls Orogeny. This multi-component Archean-Proterozoic crust forms the basement infrastructure of the Belt basin, a Mesoproterozoic basin with one of the thickest sedimentary accumulations ever formed. After a hiatus of over a billion years, which included Windermere rifting and shelf deposition, this ancient continental margin was reactivated by the addition of new crust and mobilization of older crust. The western part of the transect terminates in a collage of terranes comprised of mostly Phanerozoic rocks accreted during the Mesozoic, but containing remnants of much older crust; all intruded by Cretaceous and younger batholiths (e.g., Idaho and Sawtooth) that have recycled parts of the older crust into initially more juvenile melts. The final, ongoing event is impingement of the Yellowstone plume on the Proterozoic and Phanerozoic mobile belts. At present, the plume may be trapped between the younger mantle lithosphere of the mobile belts and the robust Archean keel underlying the northern Wyoming craton.