Neotectonic evolution and fault geometry change along a major extensional fault system in the Mission and Flathead Valleys, NW-Montana

Analysis of 3.5 kHz high-resolution seismic data from Flathead Lake, combined with results from onshore geologic mapping and literature review from previous studies in the area, reveals a significant change in fault geometry and seismic activity along strike of the Mission Fault system in the Mission and Flathead Valleys of northwestern Montana. The Mission Fault system is composed of faults with normal sense of motion and faults with minor oblique-slip and strike-slip motion. It evolves from a single fault strand in the Mission Valley south of Flathead Lake into a multiple strand fault system in the Flathead Lake basin and north of the lake. Fault activity decreases to the north as suggested by northward decreasing fault scarp heights in the lake basin. North of the lake the Mission Fault system is truncated by oblique strike-slip faults and the extensional strain is accommodated by the Swan Fault, another major normal fault north and east of the study area. We observed five phases of increased tectonic activity in the lake basin during the last 15,000 years. The oldest phase (phase B), active between 15,000 and 13,000 cal yr BP, resulted in fault scarps with up to 14 m of relief along the Mission Fault and the Kalispell–Finley Point Fault. We calculated average displacement rates as high as 1 mm/yr for this oldest phase. Phases C–F represent smaller tectonic events in the lake basin during the last 10,000 cal yr BP. Offset of seismic reflectors during these younger events is generally at dm-scale, indicating relatively low average displacement rates.