DISMEMBERING THE PUTNAM THRUST: MIOCENE EXTENSION WITHIN THE FORELAND-HINTERLAND TRANSITION OF THE IDAHO-WYOMING SALIENT AND A REVISED KINEMATIC MODEL FOR THE PUTNAM THRUST SYSTEM

The Sevier fold-thrust belt of southeastern Idaho records the geometry, timing, and kinematics of contractional structures that accommodated horizontal crustal shortening during growth of the North American Cordillera in the Mesozoic. 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 that resulted in ~13-25% extension and ~15° of eastward tilt of Mesozoic structures. Basin and Range extension in southeastern Idaho is time-transgressive and contemporaneous with eastward migration of the Yellowstone Hotspot system. The extension documented here represents a transitional period where the locus of rapid extension which occurred between ~9 to 7.5 Ma in the Portneuf Valley migrated northeast to the Blackfoot Mountains between ~7.5 and 4 Ma. Restoration of slip along normal faults and construction of stratigraphic separation diagrams has allowed for a revised model for the evolution of the Putnam thrust sheet. The Putnam thrust sheet, a major Sevier-aged contractional structure found near the foreland-hinterland transition of the fold-thrust belt has been complexly dismembered by normal faulting, contributing to differing interpretations for its development and geometry. The Putnam thrust sheet contains three major structures: the Putnam, Bear Canyon, and Toponce thrusts. Previous interpretations considered the Toponce thrust to be an eastern equivalent of the Bear Canyon thrust, yet the amount of lateral ramping required for these to be equivalent is unlikely given their proximity. In our revised model these two thrusts are separate, but related structures, with the Toponce thrust forming as a minor out-of-sequence imbricate excised from the Bear Canyon thrust. Overall, the Putnam thrust sheet evolved as a dominantly in-sequence system, with thrusts propagating eastward and younging with depth.