THE MONUMENT HILL FAULT ZONE AS A NATURAL LABORATORY FOR STUDYING THE BOUNDARY OF BASIN AND RANGE EXTENSION NORTH OF THE SNAKE RIVER PLAIN, RED ROCK VALLEY, SOUTHWESTERN MONTANA

Ongoing research in the northern Red Rock Valley, SW Montana, defines an active Quaternary normal fault system which helps characterize the eastern boundary of the Basin and Range north of the Snake River Plain. The Monument Hill fault zone consists of a continuous, 11 km long upper strand and two discontinuous, ~1 km long lower strands. The seismically active Red Rock graben, bounded by the west-dipping Monument Hill and the east-dipping Red Rock fault zones, has a geometry consistent with left lateral transtension. Quaternary hanging wall deposits of the Monument Hill fault zone are subdivided into four alluvial fan units, characterized by soil and surface geomorphic criteria. The distribution and geometry of surface ruptures are consistent with diffusion modeling of scarps in alluvium, obtained using diffusivity values calculated for dated Red Rock scarps. The lower strand cuts late and middle Pleistocene fans and has modeled rupture ages of ~24 ka whereas the upper strand cuts late Pleistocene fans and Holocene channels and has a modeled rupture age of 12 ka. The integrated, long term effect of Quaternary activity is manifest in mountain front landforms and stream long profiles. Active fault segments have elongate drainage basins, irregular hypsometries, and channel long profiles with anomalously steep reaches not coincident with rock-type changes when modeled in slope-area space. We interpret the Monument Hill fault to be the northern extension of the active Red Rock fault, which steps east and reverses polarity across a mid-basin accommodation zone near Kidd. This youthful fault and mountain front geometry may represent the early evolutionary stages of Basin and Range grabens, like the nearby and morphometrically similar Madison River Valley. We conclude that the Quaternary activity of the Monument Hill fault represents Basin and Range extension projecting north of the Snake River Plain and east into the Sevier belt, locally influenced by the passage of the Yellowstone hotspot.