NEW AGE AND GEOMORPHIC CONTROLS ON THE BOULDER FRONT FAULT, WOOD RIVER BASIN, NORTHERN BASIN AND RANGE PROVINCE, IDAHO

The upper Wood River valley and the Sawtooth valley represent areas of high seismogenic potential in south-central Idaho, yet little is known about the rupture history of many of the faults in the region. The most recent seismic sequence, initiated by the March 31st 2020 Mw6.5 event at the northern termination of the Sawtooth fault and still ongoing, perfectly capture the complexity and the many unknowns of the tectonic setting driving active seismicity.

We present results from the analysis of a segment of the Boulder front fault along the Boulder Creek scarp sought to determine the age of past ruptures of this segment of the fault. Geologic mapping separated four surficial and glacial deposits along the Boulder Creek lower drainage. The segment of the fault was profiled with differential GPS along 6 transects orthogonal to the scarp. The diffusional degradation of the scarp was modeled and age inferred. Scarp heights are ~1m to ~ 4m and are higher in older deposits, demonstrating repeated ruptures along the segment. Stratigraphically controlled 14C ages of ~2720±30 and 3190±30 y are determined for the lowest terraced deposit, which still present offset. The slip rates on this rupture was ~0.6 m/my. Results of modeling the topographic development of the watershed predicts fault slip may have begun 0.6-1Ma.