LATE QUATERNARY SLIP RATES AND SURFACE RUPTURE OF THE BITTERROOT FAULT, WESTERN MONTANA: AN ENIGMATIC SEISMOGENIC FAULT IN THE NORTHERN ROCKIES BASIN AND RANGE

The Bitterroot fault is a 100-km-long 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. New detailed mapping using LiDAR along the southern Bitterroot Range documents multiple generations of fault scarps in Holocene-Pleistocene deposits with vertical offsets that increase in magnitude with age. Fault mapping indicates a complex fault geometry characterized by an en echelon pattern of discontinuous segments of 45–70° east-dipping normal faults that appear to cut the older Eocene detachment fault, and locally 70–80° west-dipping antithetic normal faults. ¹⁰Be cosmogenic radionuclides surface exposure dating technique provides age control for 32 boulders sampled in glacial deposits. Near Como Dam, a dated 16–17 ka Pinedale moraine offset by the Bitterroot fault scarp with a vertical separation of 3.5 ± 0.1 m, yields a fault slip rate of 0.2–0.3 mm/yr. Glacial Lake Missoula shorelines inset into a dated ~15 ka Pinedale moraine and vertically offset 4.6 ± 1.5 m by an antithetic strand of the Bitterroot fault, yield fault slip rates of 0.2–0.4 mm/yr. In the Ward Creek Fan located ~15 km to the north of Como Dam, two dated ~17 ka and 63–70 ka fan surfaces offset by the Bitterroot fault with vertical separations of 2.4 ± 0.2 m and 4.5 ± 0.1 m, yield fault slip rates of 0.1–0.2 mm/yr and 0.1 mm/yr, respectively. Our results indicate broadly consistent fault slip rates with an along-strike preferred average of 0.2–0.3 mm/yr for the southern Bitterroot fault. Fault scaling relations, structural model constraints and our slip rate results indicate both a seismogenic low angle and high angle fault geometry are possible at depth, which could generate a Mw ~7.2 earthquake or larger. We speculate the Bitterroot fault is likely characterized by millennia-timescale earthquake recurrence interval. Forthcoming paleoseismic trench results on the Bitterroot fault will aim to develop a Holocene-Pleistocene paleoearthquake chronology. Data from this study suggest seismic hazards from the Bitterroot fault potentially pose a risk to the rapidly growing Missoula metropolitan area, and to major infrastructures across the Missoula-Bitterroot valleys.