| In southeastern Idaho, the Sevier fold-thrust belt records the geometry, timing, and kinematics of contractional structures that accommodated horizontal crustal shortening during growth of the North American Cordillera. However, these structures have been intensely overprinted by Cenozoic extension and volcanism. Detailed geologic mapping and U-Pb zircon analysis of Miocene basin fill in the Portneuf Range has identified a phase of horizontal extension between 6.421 and 6.032 Ma, which was contemporaneous with extension observed south of the ESRP that may represent ongoing Basin and Range extension that was accelerated by passage of the Yellowstone volcanic system. The Putnam thrust sheet, a major Sevier-aged thrust fault that occurs near the foreland-hinterland transition of the fold-thrust belt, has been complexly dismembered by normal faulting, contributing to differing interpretations for its development. Restoration of slip along normal faults and construction of stratigraphic separation diagrams has allowed for a revised model for the Putnam sheet. |