LATE QUATERNARY SLIP RATES OF THE BITTERROOT FAULT IN WESTERN MONTANA FROM BE-10 DATING OF OFFSET PINEDALE AND BULL LAKE GLACIAL DEPOSITS

The Bitterroot fault is a 100-km-long Quaternary active normal fault that bounds the eastern margin of the north-south trending Bitterroot Mountains and accommodates extension within the Intermountain Seismic Belt. Earthquake and fault history are unknown for the Bitterroot fault, although the seismic risk is potentially high given the proximity of the rapidly growing towns in the Missoula–Bitterroot valleys. New detailed mapping using LiDAR along the southern Bitterroot Range documents multiple generation of fault scarps with vertical offsets that increase in magnitude with age in Holocene-Pleistocene deposits. Fault mapping indicates a complex fault geometry characterized by en echelon pattern of discontinuous segments of 45-60° east-dipping normal faults, and locally 60°-80° west-dipping antithetic normal faults that cut the older Eocene Bitterroot detachment fault. Be-10 cosmogenic radionuclides exposure dating technique provides in situ age control for 32 surface boulders (>1m) sampled in glacial deposits. Near the Lake Como Dam, two Pinedale glacial moraine sequences yield peak age distributions of 15.0 ± 0.4 ka and 16.4 ± 0.6 ka. Vertical separation of 5.0 ± 0.5 m across the dated ~16 ka moraine yields a fault slip rate of 0.3–0.5 mm/yr. Highstand Lake Missoula shorelines (4250-4230 ft) inset into the dated ~15 ka moraine and vertically offset by 4.5 ± 1.5 m, yield fault slip rates of 0.3–0.5 mm/yr. In the Ward Creek fan located ~15 km to the north of Lake Como, two dated glacial debris fan sequences yield peak age distributions of 16.6 ± 0.4 ka and 62.8 ± 1.7 ka. Vertical separations of 2.7 ± 0.3 m and 6.6 ± 0.3 m across the ~17 ka and ~63 ka surfaces, yield fault slip rates of 0.2–0.3 mm/yr and 0.1–0.2 mm/yr, respectively. Reported exposure ages are calculated using LSDn nuclide-dependent scaling age model and assume negligible erosion rates. Our results indicate consistent long-term fault slip rates for fault segments at Lake Como (0.3–0.5 mm/yr) and Ward Creek fan (0.1–0.3 mm/yr) with an along-strike average of 0.3 ± 0.2 mm/yr for the southern Bitterroot fault. Fault scaling relations and evidence of multiple surface ruptures suggest the Bitterroot fault could produce a Mw ~7.2 earthquake. Earthquake ground motion from Bitterroot fault poses a significant risk to the Missoula metropolitan area, the State’s second most populous region, and major infrastructures across the Missoula-Bitterroot valleys.