9317395 Janecke This project will test the theory that in Eocene and Oligocene times there was a fundamental tectonic boundary, between what is how SW Montana and east-central Idaho, in the form of a narrow, N- trending half-graben. Structural and stratigraphic analyses of the half-graben and its sedimentary fill combined 40Ar39Ar geochronology will ascertain the regional extent of the structure (s) and whether or not they formed due to fraintational collapse of previously thickened crust in the Sevier orogenic belt. The results will have wide application to the evolution of orogenic systems world-wide ***
