WHOSE FAULT IS IT? A KINEMATIC ANALYSIS OF HIGH ANGLE FAULTS ADJACENT TO THE EASTERN SNAKE RIVER PLAIN NEAR ARCO, ID

The Eastern Snake River Plain (ESRP) is a northeast-trending volcanic province formed during the North American plate’s migration over the stationary Yellowstone hotspot. The Idaho National Laboratory (INL) is located in the west-central portion of the ESRP and overlies one of the most productive aquifers in the United States. The U.S. Geological Survey has collected borehole information at the INL in order to investigate the subsurface stratigraphy of the ESRP and characterize the ESRP aquifer with respect to the migration of radioactive and chemical wastes. The majority of the ESRP aquifer flows through basalt. However, the basalts thin near the northern margin of the ESRP, and groundwater is in contact with underlying rhyolites that have been tectonically tilted and faulted in a manner that may significantly affect the hydrogeology of the aquifer system.

This Idaho State University research, done in cooperation with the USGS, examines patterns of faulting in the Arco Hills as a basis for inferring the subsurface architecture of the adjacent ESRP, which is covered by Quaternary basalt. In addition to the predominantly northwest-striking normal faults that accommodate Basin and Range extension northwest of the Snake River Plain, small-offset, northeast-striking, steeply-dipping faults also occur near the margin of the ESRP. At a regional scale, GPS data suggest an active dextral component of slip along a similarly northeast-trending shear zone between the Basin and Range and Snake River Plain near the Centennial Mountains ~120 km to the northeast. To evaluate the hypothesis that these small faults could be the expression of a similar dextral component of displacement during an earlier phase of development of the Snake River Plain, we conducted a field-based kinematic analysis of small faults northeast of Arco, Idaho. Collectively, these faults record nearly pure dip-slip offset and a northwest-southeast extension direction that is slightly oblique to the ~55° northeast trend of the ESRP. Our preliminary interpretation of these faults is that 1) they result from crustal flexure adjacent to the margin of the ESRP; or 2) they formed as a result of a gradual, along-strike decrease in slip of the Lost River Fault toward the ESRP.