NEW MAPPING OF INCONSPICUOUS QUATERNARY FAULTS IN EASTERNMOST JACKSON HOLE, WYOMING

Active normal faults in the Greater Yellowstone region reflect the influence of uplift associated with the Yellowstone hotspot superimposed on Basin and Range lithospheric extension. Along the northeast (leading) edge of the hotspot track are inconspicuous and enigmatic faults believed to be in an early stage of development. Because these faults lack the topographic signature of longer-lived range-bounding structures, their detection is difficult and they remain lightly studied. We present detailed mapping and scarp profile measurements of several such faults in easternmost Jackson Hole, Wyoming, which displace Pinedale-1 (~20 ka) and Pinedale-2 (~15 ka) glacial till and other Quaternary deposits.

The proposed Uhl Hill fault is a 4.5-km-long, southeast-dipping normal fault in eastern Grand Teton National Park with newly recognized Quaternary activity. Scarps in Pinedale-1 moraines have vertical surface offset (VSO) values two-to-three times greater than scarps in Pinedale-2 moraines, suggesting post-15 ka displacement and the possibility of multiple surface rupturing events from 20–15 ka. The Uhl Hill fault is expressed at its southern end by a 1-m-high scarp across part of the Spread Creek alluvial fan. A 1.5 km-long scarp in Pinedale-1 till ~4 km to the northeast across the Buffalo Fork valley may be a northern extension of the fault. 20 km east, the Togwotee Lodge faults form a complex system of sharp, east-facing, en echelon scarps in Bull Lake till (~150 ka), Pinedale-1 till, and colluvium. Scarps in Pinedale-1 till have highly variable VSO values with some of the largest measurements occurring near the northern terminus of the mapped fault system, suggesting a greater fault length than currently recognized. Field observations of scarps beyond the mapped extent are difficult due to obstruction by landslides and dense forest cover, and mapping of the Togwotee Lodge faults should be revisited once lidar data become available.

Our work highlights the need for future paleosiesmic studies on these faults to answer unresolved questions of rupture timing, relation to regional structures like the Teton fault, and seismic hazard from other incipient faults in the Greater Yellowstone region.