The Greater Yellowstone region is riddled with Quaternary faults — those that have been active in the last 2.6 million years. While the largest faults and those that have moved in historic times get most of the attention, smaller, inconspicuous faults may also have an impact on the local seismic hazard and can give clues about when surface-rupturing earthquakes have occurred in the past.
As part of recently completed 1:100,000-scale surficial geologic mapping on the east half of the Jackson Lake 30’ x 60’ quadrangle, Wyoming State Geological Survey (WSGS) geologists mapped faults that offset Quaternary-age units in the area south of Yellowstone National Park and near the eastern border of Grand Teton National Park. Two of these Quaternary faults were not previously documented. Their discovery was aided by the availability of light detection and ranging (lidar) data, which give geologists a high-resolution digital image of the ground surface beneath the vegetation. In the field and through examination of lidar data, WSGS geologists focused on locating, tracing and measuring topographic profiles across fault scarps. The profiles are two-dimensional “slices” measured perpendicular to the fault trace that show the overall shape of a scarp. The data derived from these profiles can be used to calculate the vertical surface offset across a scarp — an important metric that gives information about past ruptures along a fault.
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One of the notable takeaways from this work comes from the newly recognized Uhl Hill fault, an east-dipping normal fault in eastern Grand Teton National Park. The scarp of this fault cuts through sediment that was deposited in two distinct pulses during the most recent glaciation, when ice from the Yellowstone Plateau flowed south into Jackson Hole. These glacial pulses are locally called the “Pinedale-1” and “Pinedale-2” advances, and they are known to have occurred roughly 20,000 and 15,000 years ago, respectively, based on nearby cosmogenic surface exposure dating.
Where the Uhl Hill fault cuts through the older Pinedale-1 deposit, its scarp has an average vertical surface offset of 2.5 meters (about 8 feet). Where the fault scarp cuts the younger Pinedale-2 deposit, its average surface offset is only 1 meter (about 3 feet). This general pattern is to be expected, as older deposits have experienced more earthquakes than younger deposits and have therefore accumulated more displacement. More specifically, these observations suggest that the Uhl Hill fault has likely experienced one surface-rupturing earthquake (which are relatively rare in the Intermountain West, with only three historical events) in the last 15,000 years, and that at least one additional surface-rupturing earthquake occurred between 20,000 and 15,000 years ago. Thus, it’s possible to piece together an approximate earthquake history of a fault by mapping and measuring scarps, numerically dating deposits, and applying the principle of cross-cutting relationships. This is one reason why geologists are so interested in fault scarps.
The faults on the east half of the Jackson Lake quadrangle are different from many of the more famous faults in the region, such as the Teton fault, in that they don’t bound mountain ranges. Instead, these faults form modest scarps in otherwise gently rolling terrain. In a foundational 1992 paper, USGS geologists Ken Pierce and Lisa Morgan hypothesized that these and similar faults are in their infancy, or are recently reactivated from once-dormant older faults. The proposed mechanism for this recent faulting is the passage of the North American plate over the Yellowstone hotspot which, like a ship’s bow wave, has initiated uplift along the leading edge of the hotspot track. Additional mapping and paleoseismic studies are needed to further test this hypothesis, and the release of lidar data promises to fuel future work. Stay tuned in the coming years as geologists continue to piece together the story of geologically young faulting in the Greater Yellowstone region.
Yellowstone Caldera Chronicles is a weekly column written by scientists and collaborators of the Yellowstone Volcano Observatory. This week’s contribution is from James Mauch, geologist with the Wyoming State Geological Survey.