Major reorganization of the Snake River modulated by passage of the Yellowstone Hotspot

The details and mechanisms for Neogene river reorganization in the U.S. Pacific Northwest and northern Rocky Mountains have been debated for over a century with key implications for how tectonic and volcanic systems modulate topographic development. To evaluate paleo-drainage networks, we produced an expansive data set and provenance analysis of detrital zircon U-Pb ages from Miocene to Pleistocene fluvial strata along proposed proto-Snake and Columbia River pathways. Statistical comparisons of Miocene-Pliocene detrital zircon spectra do not support previously hypothesized drainage routes of the Snake River. We use detrital zircon unmixing models to test prior Snake River routes against a newly hypothesized route, in which the Snake River circumnavigated the northern Rocky Mountains and entered the Columbia Basin from the northeast prior to incision of Hells Canyon. Our proposed ancestral Snake River route best matches detrital zircon age spectra throughout the region. Furthermore, this northerly Snake River route satisfies and provides context for shifts in the sedimentology and fish faunal assemblages of the western Snake River Plain and Columbia Basin through Miocene−Pliocene time. We posit that eastward migration of the Yellowstone Hotspot and its effect on thermally induced buoyancy and topographic uplift, coupled with volcanic densification of the eastern Snake River Plain lithosphere, are the primary mechanisms for drainage reorganization and that the eastern and western Snake River Plain were isolated from one another until the early Pliocene. Following this basin integration, the substantial increase in drainage area to the western Snake River Plain likely overtopped a bedrock threshold that previously contained Lake Idaho, which led to incision of Hells Canyon and establishment of the modern Snake and Columbia River drainage network.

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.

Supplemental Material: Major reorganization of the Snake River modulated by passage of the Yellowstone Hotspot

Figure S1: Detrital zircon age spectra from modern rivers.; Figure S2: Detrital zircon age spectra from fluvial and lacustrine sandstones; Figure S3: Shepard plots from Multi-Dimensional scaling (MDS) analysis comparing distance and disparity for four metrics of detrital zircon similarity; Figure S4: DZmix results for three hypothesized river networks; Figure S5: SRP sample location map and detrital unmixing results; Table S1: Modern and ancestral river detrital zircon sample locations, ages, and references; Table S2: U-Pb zircon age results for new modern and ancestral river sands; Table S3: Intercomparison results between modern and ancestral river sediments; Table S4: Best-fit DZmix results estimating the relative contribution of hypothesized sources to measured detrital zircon age spectra of ancestral river sands; Table S5: Best-fit DZMix results that estimate the relative contribution of Snake River Plain tributaries to Miocene-Pliocene Lake Idaho strata.

Supplemental Material: Major reorganization of the Snake River modulated by passage of the Yellowstone Hotspot

Figure S1: Detrital zircon age spectra from modern rivers.; Figure S2: Detrital zircon age spectra from fluvial and lacustrine sandstones; Figure S3: Shepard plots from Multi-Dimensional scaling (MDS) analysis comparing distance and disparity for four metrics of detrital zircon similarity; Figure S4: DZmix results for three hypothesized river networks; Figure S5: SRP sample location map and detrital unmixing results; Table S1: Modern and ancestral river detrital zircon sample locations, ages, and references; Table S2: U-Pb zircon age results for new modern and ancestral river sands; Table S3: Intercomparison results between modern and ancestral river sediments; Table S4: Best-fit DZmix results estimating the relative contribution of hypothesized sources to measured detrital zircon age spectra of ancestral river sands; Table S5: Best-fit DZMix results that estimate the relative contribution of Snake River Plain tributaries to Miocene-Pliocene Lake Idaho strata.

Patterns of incision and deformation on the southern flank of the Yellowstone hotspot from terraces and topography

Understanding the dynamics of the greater Yellowstone region requires constraints on deformation spanning million year to decadal timescales, but intermediate-scale (Quaternary) records of erosion and deformation are lacking. The Upper Snake River drainage crosses from the uplifting region that encompasses the Yellowstone Plateau into the subsiding Snake River Plain and provides an opportunity to investigate a transect across the trailing margin of the hotspot. Here, we present a new chronostratigraphy of fluvial terraces along the lower Hoback and Upper Snake Rivers and measure drainage characteristics through Alpine Canyon interpreted in the context of bedrock erodibility. We attempt to evaluate whether incision is driven by uplift of the Yellowstone system, subsidence of the Snake River Plain, or individual faults along the river’s path. The Upper Snake River in our study area is incising at roughly 0.3 m/k.y. (300 m/m.y.), which is similar to estimates from drainages at the leading eastern margin of the Yellowstone system. The pattern of terrace incision, however, is not consistent with widely hypothesized headwater uplift from the hotspot but instead is consistent with downstream baselevel fall as well as localized deformation along normal faults. Both the Astoria and Hoback faults are documented as active in the late Quaternary, and an offset terrace indicates a slip rate of 0.25−0.5 m/k.y. (250−500 m/m.y.) for the Hoback fault. Although tributary channel steepness corresponds with bedrock strength, patterns of χ across divides support baselevel fall to the west. Subsidence of the Snake River Plain may be a source of this baselevel fall, but we suggest that the closer Grand Valley fault system could be more active than previously thought.

Supplemental Material: Patterns of incision and deformation on the southern flank of the Yellowstone hotspot from terraces and topography

Item 1: Surficial map of Alpine Canyon, Item 2: OSL data, Item 3: Bedrock Strength.

Supplemental Material: Patterns of incision and deformation on the southern flank of the Yellowstone hotspot from terraces and topography

Item 1: Surficial map of Alpine Canyon, Item 2: OSL data, Item 3: Bedrock Strength.

Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): Is the Yellowstone hotspot waning?

Super-eruptions are amongst the most extreme events to affect Earth’s surface, but too few examples are known to assess their global role in crustal processes and environmental impact. We demonstrate a robust approach to recognize them at one of the best-preserved intraplate large igneous provinces, leading to the discovery of two new super-eruptions. Each generated huge and unusually hot pyroclastic density currents that sterilized extensive tracts of Idaho and Nevada in the United States. The ca. 8.99 Ma McMullen Creek eruption was magnitude 8.6, larger than the last two major eruptions at Yellowstone (Wyoming). Its volume exceeds 1700 km3, covering ≥12,000 km2. The ca. 8.72 Ma Grey’s Landing eruption was even larger, at magnitude of 8.8 and volume of ≥2800 km3. It covers ≥23,000 km2 and is the largest and hottest documented eruption from the Yellowstone hotspot. The discoveries show the effectiveness of distinguishing and tracing vast deposit sheets by combining trace-element chemistry and mineral compositions with field and paleomagnetic characterization. This approach should lead to more discoveries and size estimates, here and at other provinces. It has increased the number of known super-eruptions from the Yellowstone hotspot, shows that the temporal framework of the magmatic province needs revision, and suggests that the hotspot may be waning.

Supplemental Material: Discovery of two new super-eruptions from the Yellowstone hotspot track (USA): Is the Yellowstone hotspot waning?

Unit summaries, previous nomenclature, methodologies, and all raw data. <br>