TRANSTENSION ALONG THE NORTHEASTERN BOUNDARY OF THE SNAKE RIVER PLAIN

We propose that the GPS horizontal velocity field of the Northern Basin and Range Province and the adjacent Snake River Plain results in active faulting and transtension in the Centennial and Red Rock Valleys, southwestern Montana. Geologic mapping, modeling of fault scarp morphology, and mountain front landforms in the Centennial Tectonic Belt, which extends westward from Yellowstone National Park, reveals youthful, underfilled grabens bounded by seismogenic normal faults of limited strike-length and throw. Quaternary faults have discontinuous surface ruptures, are in part en echelon, and follow the Sevier thrust sheets including the Tendoy and Mckenzie imbricate fans along the thin-skinned emergent thrust front where it abuts with the foreland Laramide Blacktail-Snowcrest uplift in the Montana recess. Quaternary fault geometries and kinematics attest to the importance of contractional inheritance on neotectonic extension in generating mountain front relief. Age progressive alluvial fan deposits record cumulative offsets along the Quaternary faults and record a maximum slip rate of 1.5mm/yr on the Red Rock faults. Monument Hill fault strands exhibit broad clustering in surface rupture ages thought to require M>6.0-6.6 events. Faults of the Centennial graben represent a localized displacement transfer zone between the eastern Snake River Plain and the northeastern Basin and Range Province. The E-W striking, north-dipping Centennial fault zone and companion south-dipping Lima Dam fault zone accommodate right-lateral transtension within Centennial Valley. In contrast the NW-SE striking NE-dipping Red Rock fault zone and related SW-dipping Monument Hill fault zone accommodate linked left-lateral transtension within Red Rock Valley. The geometry, interconnectivity and kinematics of these active faults in southwestern Montana display the processes and products of early Basin and Range extension along the northern arm of the Yellowstone plume wake.