212-3 THE BOULDER FRONT FAULT IN THE NORTHERN BASIN AND RANGE, PROVENCE, IDAHO

Session: Reconstructing Earth Surface Processes in Orogenic Systems

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Abstract:

The Northern Basin and Range Province in central Idaho hosts a series of active normal faults including the range-bounding Boulder Front fault along the SW slope of the Boulder Mountains, Idaho. Situated in the upper Wood River watershed, the SW-dipping Boulder Front fault cuts a series of Pinedale-age moraines and fluvial outwash terraces with clear evidence of surficial rupture. At least seven distinct fault scarps span more than 20 kilometers from Galena Summit to SE of Boulder Creek. Evidence of multiple ruptures of the Boulder Creek scarp during the Holocene is observed with inset deposits being less offset than older terraces. Rupture ages were calculated from diffusion modeling of the fault scarp as a single slip rupture event (~4.27±0.68kyr) and as continuous slip events (~2.99±0.41kyr) with additional constraints provided by six radiocarbon dates (~1.12-4.48±0.06kyr) in the hanging wall of offset fluvial terrace deposits. A recent rupture (<~2.81kyr±0.06kyr) is evident on the Boulder Creek scarp while neighboring drainages suggest that other segments of the Boulder Front fault may have ruptured within the last ~1,100 years. Calibration of a diffusion coefficient for glacial sediment near Boulder Creek was determined locally to be 2.09x10^-3 m²/yr. Recurrence intervals of ~2.3kyr and earthquakes of M5.4-M6.7 are calculated for the Boulder Front fault based on rupture-length relationships, diffusional modeling, and radiometric dating techniques. Physical extension of the fault system may exist to the south towards the tourist destinations of Ketchum and Sun Valley, Idaho, and poses a seismic hazard to infrastructure and local populations.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-10532

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