Removal of the Northern Paleo-Teton Range along the Yellowstone Hotspot Track

Classically held mechanisms for removing mountain topography (e.g., erosion and gravitational collapse) require 10-100 Myr or more to completely remove tectonically generated relief. Here, we propose that mountain ranges can be completely and rapidly (<2 Myr) removed by a migrating hotspot. In western North America, multiple mountain ranges, including the Teton Range, terminate at the boundary with the relatively low relief track of the Yellowstone hotspot. This abrupt transition leads to a previously untested hypothesis that preexisting mountainous topography along the track has been erased. We integrate thermochronologic data collected from the footwall of the Teton fault with flexural-kinematic modeling and length-displacement scaling to show that the paleo-Teton fault and associated Teton Range was much longer (min. original length 190-210 km) than the present topographic expression of the range front (~65 km) and extended across the modern-day Yellowstone hotspot track. These analyses also indicate that the majority of fault displacement (min. 11.4-12.6 km) and the associated footwall mountain range growth had accumulated prior to Yellowstone encroachment at ~2 Ma, leading us to interpret that eastward migration of the Yellowstone hotspot relative to stable North America led to removal of the paleo-Teton mountain topography via posteruptive collapse of the range following multiple supercaldera (VEI 8) eruptions from 2.0 Ma to 600 ka and/or an isostatic collapse response, similar to ranges north of the Snake River plain. While this extremely rapid removal of mountain ranges and adjoining basins is probably relatively infrequent in the geologic record, it has important implications for continental physiography and topography over very short time spans.

Postglacial slip distribution along the Teton normal fault (Wyoming, USA), derived from tectonically offset geomorphological features

Along the eastern front of the Teton Range, northeastern Basin and Range province (Wyoming, USA), well-preserved fault scarps that formed across moraines, river terraces, and other geomorphological features indicate that multiple earthquakes ruptured the range-bounding Teton normal fault after the Last Glacial Maximum (LGM). Here we use high-resolution digital eleva­tion models derived from lidar data to determine the vertical slip distribution along strike of the Teton fault from 54 topographic profiles across tectonically offset geomorphological features along the entire Teton Range front. We find that offset LGM moraines and glacially striated surfaces show higher vertical displacements than younger fluvial terraces, which formed at valley exits upstream of LGM terminal moraines. Our results reveal that the tectonic off­sets preserved in the fault scarps are post-LGM in age and that the postglacial slip distribution along strike of the Teton fault is asymmetric with respect to the Teton Range center, with the maximum vertical displacements (27–23 m) being located north of Jenny Lake and along the southwestern shore of Jack­son Lake. As indicated by earlier three-dimensional numerical models, this asymmetric slip distribution results from postglacial unloading of the Teton fault, which experienced loading by the Yellowstone ice cap and valley glaciers in the Teton Range during the last glaciation.

Alpine glacier resilience and Neoglacial fluctuations linked to Holocene snowfall trends in the western United States

Geological evidence indicates that glaciers in the western United States fluctuated in response to Holocene changes in temperature and precipitation. However, because moraine chronologies are characteristically discontinuous, Holocene glacier fluctuations and their climatic drivers remain ambiguous, and future glacier changes are uncertain. Here, we construct a continuous 10-thousand-year (ka) record of glacier activity in the Teton Range, Wyoming, using glacial and environmental indicators in alpine lake sediments. We show that Teton glaciers persisted in some form through early Holocene warmth, likely as small debris-covered glaciers or rock glaciers. Subsequent Neoglacial ice expansion began ~6.3 ka, with two prominent glacier maxima at ~2.8 and 0.1 ka that were separated by a multicentennial phase of ice retreat. Comparison with regional paleoclimate records suggests that glacier activity was dominantly controlled by winter precipitation variability superposed on long-term Holocene temperature trends, offering key insights into western U.S. glacier resilience and vulnerability to future warming.

Performing horizontal to vertical ratio testing in stiff soils in and around Grand Teton National Park

Horizontal to vertical spectral ratio (HVSR) testing was completed at two cross sections in and around GTNP. The HVSR testing produced reliable estimates of the fundamental frequencies for many of the sites tested. The goal of the testing was to determine a depth of soil above competent bedrock. However the fundamental frequencies recorded yielded predicted depths that are much shallower than expected. Also the predicted depths did not increase at greater distance from the Teton Range, which would be expected at these sites. Based on these predictions the authors do not believe the frequencies recorded are a good indication of the depth of the soil above bedrock but instead it is believed that the depths correspond with a layer of softer topsoil/overburden above a stiffer gravel layer. Although the goal of measuring the depth of soil above bedrock was not met, HVSR produced results that may be useful to others for determination of a fundamental frequency of resonance at our testing locations. Featured photo by Anna Cressman, taken from the AMK Ranch photo collection. https://flic.kr/p/2jjWZGT

Long-term alpine stream monitoring in the Teton Range: Investigating multi-year patterns and thermal physiology

Alpine streams and the biotic communities they contain are imperiled worldwide due to climate warming and the rapid decline of ice. The loss of glaciers and permanent snowpack may drive local populations to extinction, especially organisms with narrow habitat tolerances. We have been monitoring alpine streams in the Teton Range since 2015 that originate from three hydrological sources: surface glaciers, snowfields, or subterranean ice (e.g., rock glaciers). We call these stream types glacier-fed, snowmelt-fed, and icy seeps, respectively. We hypothesize that icy seeps may persist on the landscape longer than other hydrologic sources and that these features may act as a refuge for cold-adapted organisms such as the stoneflies Zapada glacier and Lednia tetonica. In November 2019, Z. glacier and a sister species of L. tetonica, Lednia tumana, were listed under the U.S. Endangered Species Act. This decision was based in part on work funded by the UW-NPS and highlights the pressing nature of our efforts. In 2019, we collected a 5th year of long-term data to begin investigating multi-year signals in the data. Our second 2019 objective was to further explore how thermal regimes affect tolerance of potentially imperiled insects. Because our annual data collection occurs in late summer with sample processing and analysis extending into the following year, this report will be a broad update on the project as whole, rather than 2019-specific. Through long-term monitoring of streams from different hydrological sources, we are building a dataset that will allow us to understand changes as air temperatures warm and permanent ice is lost in the alpine zone. Featured photo by Nicole Y-C on Unsplash. https://unsplash.com/photos/9XixVlnUCbk

Holocene earthquake history and slip rate of the southern Teton fault, Wyoming, USA

The 72-km-long Teton normal fault bounds the eastern base of the Teton Range in northwestern Wyoming, USA. Although geomorphic surfaces along the fault record latest Pleistocene to Holocene fault movement, the postglacial earthquake history of the fault has remained enigmatic. We excavated a paleoseismic trench at the Buffalo Bowl site along the southernmost part of the fault to determine its Holocene rupture history and slip rate. At the site, ∼6.3 m of displacement postdates an early Holocene (ca. 10.5 ka) alluvial-fan surface. We document evidence of three surface-faulting earthquakes based on packages of scarp-derived colluvium that postdate the alluvial-fan units. Bayesian modeling of radiocarbon and luminescence ages yields earthquake times of ca. 9.9 ka, ca. 7.1 ka, and ca. 4.6 ka, forming the longest, most complete paleoseismic record of the Teton fault. We integrate these data with a displaced deglacial surface 4 km NE at Granite Canyon to calculate a postglacial to mid-Holocene (14.4–4.6 ka) slip rate of ∼1.1 mm/yr. Our analysis also suggests that the postglacial to early Holocene (14.4–9.9 ka) slip rate exceeds the Holocene (9.9–4.6 ka) rate by a factor of ∼2 (maximum of 3); however, a uniform rate for the fault is possible considering the 95% slip-rate errors. The ∼5 k.y. elapsed time since the last rupture of the southernmost Teton fault implies a current slip deficit of ∼4–5 m, which is possibly explained by spatially/temporally incomplete paleoseismic data, irregular earthquake recurrence, and/or variable per-event displacement. Our study emphasizes the importance of minimizing slip-rate uncertainties by integrating paleoseismic and geomorphic data sets and capturing multiple earthquake cycles.