LANDSLIDE HAZARD MAPPING AND ASSESSMENT OF POTENTIAL COSEISMIC LANDSLIDES ALONG THE NINEMILE FAULT, MISSOULA COUNTY, MONTANA

A belt of large, deep-seated landslides follows the trace of the Ninemile fault for over 30 km on the northwest side of the Missoula Valley, along the urban corridor of the city of Missoula. The fault is a major tectonic structure of the northwest-trending Lewis and Clark Line fault zone, with prominent displacements during the Sevier Orogeny and Cenozoic extension. The Ninemile fault is considered a poorly constrained Quaternary fault within a network of active faults to both the north and south that accommodate continental extension associated with the Northern Rockies Basin and Range. Landslides are distributed along both the footwall and hanging wall rock units of the Ninemile fault. Landslide occurrence exploits the structurally weak Paleogene Renova Formation's pebbly claystone and overlying Miocene Sixmile Creek Formation's gravel on the hanging wall of the southwest-dipping normal–oblique fault. Landslides also displaced structurally strong Precambrian Belt Supergroup quartzites on the mountainous footwall of the fault. The area of the landslide belt is well known for being flooded by late Pleistocene Glacial Lake Missoula and provides relative age control on landslide timing. The glacial lake had inscribed dozens of prominent strandlines on the valley sides, at elevations from 3,200 to 4,200 feet. The final drainage of the lake, ~13,000 years ago, scoured bedrock channels, eroded lake-bottom beds, and left prominent flood-stage gravel terraces. These features provide relative timing constraints for the landslides, many of which cut glacial lake strandlines, and some dammed creek beds or ran out on glacial lake beds or floodplains. The coincidence of large, deep-seated landslides and the trace of the Ninemile fault implies that some landslides may have been seismically induced. Preliminary mapping suggests two major landslide populations, based on their relative surface roughness and morphology along this fault zone in the past ~13,000 years, that could represent coseismic landslides generated during large earthquake events. Alternatively, landslide generation along the Ninemile fault may be concentrated along the belt because of the landslide-prone Renova Formation and highly fractured Belt rocks against the fault, which have been eroded into steep slopes.