Yellowstone Hotspot Research Articles

Geothermal play fairway analysis, part 1: Example from the Snake River Plain, Idaho

The Snake River Plain (SRP) volcanic province overlies the track of the Yellowstone hotspot, a thermal anomaly that extends deep into the mantle. Most of the area is underlain by a basaltic volcanic province that overlies a mid-crustal intrusive complex, which in turn provides the long-term heat flux needed to sustain geothermal systems. Previous studies have identified several known geothermal resource areas within the SRP. For the geothermal study presented herein, our goals were to: (1) adapt the methodology of Play Fairway Analysis (PFA) for geothermal exploration to create a formal basis for its application to geothermal systems, (2) assemble relevant data for the SRP from publicly available and private sources, and (3) build a geothermal PFA model for the SRP and identify the most promising plays, using GIS-based software tools that are standard in the petroleum industry.The study focused on identifying three critical resource parameters for exploitable hydrothermal systems in the SRP: heat source, reservoir and recharge permeability, and cap or seal. Data included in the compilation for heat source were heat flow, distribution and ages of volcanic vents, groundwater temperatures, thermal springs and wells, helium isotope anomalies, and reservoir temperatures estimated using geothermometry. Reservoir and recharge permeability was inferred from the analysis of stress orientations and magnitudes, post-Miocene faults, and subsurface structural lineaments based on magnetics and gravity data. Data for cap or seal included the distribution of impermeable lake sediments and clay-seal associated with hydrothermal alteration below the regional aquifer. These data were used to compile Common Risk Segment maps for heat, permeability, and seal, which were combined to create a Composite Common Risk Segment map for all southern Idaho that reflects the risk associated with geothermal resource exploration and identifies favorable resource tracks.Our regional assessment indicated that undiscovered geothermal resources may be located in several areas of the SRP. Two of these areas, the western SRP and Camas Prairie, were selected for more detailed assessment, during which heat, permeability, and seal were evaluated using newly collected field data and smaller grid parameters to refine the location of potential resources. These higher resolution assessments illustrate the flexibility of our approach over a range of scales.

Late Miocene or older canyon incision in the northern U.S. Cordillera shown by erosion rates, incision models, and basalt flow ages

Deep canyons along the Salmon, Snake, and Clearwater rivers in central Idaho, USA suggest long-lasting transient incision, but the timing and drivers of this incision are not well understood. The perturbation of the Yellowstone hotspot, eruption of flood basalts, and drainage of Lake Idaho all occurred within or near to this region, but the relationship among these events and incision is unclear. Here, we utilized in situ 10Be cosmogenic radionuclide concentrations for 46 samples (17 new) of fluvial sediment across the region to quantify erosion rates, calibrate stream power models, and estimate incision timing. We estimate that transient incision along the Salmon River began prior to ca. 10 Ma. However, canyon age decreases to ca. 5 Ma or earlier farther to the north. For a group of tributaries underlain by basalt, we use the age of the basalt to estimate that local transient incision began between ca. 11.5 and 5 Ma. Based on these timing constraints, the canyons along the Salmon and Clearwater rivers predate the drainage of Lake Idaho. We argue that canyon incision was triggered by events related to the Yellowstone hotspot (e.g., basalt lava damming, subsidence of the Columbia Basin, reactivation of faults, and/or lower crustal flow). Furthermore, our models suggest basalt may be more erodible than the other rock types we study. We show that lithology has a significant influence on fluvial erosion and assumptions regarding river incision model parameters significantly influence results. Finally, this study highlights how geodynamic processes can exert a significant influence on landscape evolution.

Multi-scale imaging of 3-D electrical conductivity structure under the contiguous US constrains lateral variations in the upper mantle water content

The magnetotelluric (MT) component of the USArray survey is a unique data set that enables imaging the electrical conductivity distribution from the surface down to astenospheric depths. Here, we present a new 3-D electrical conductivity model (MECMUS-2022) derived by inverting data from 1291 USArray MT stations covering ∼80% of the contiguous United States on a quasi-regular 70-km grid. The inversion was performed using a novel multi-scale imaging approach that can consistently incorporate a large range of spatial scales and perform 3-D modeling directly in the spherical frame, completely avoiding the conventional flat-Earth assumption. Furthermore, the use of locally refined meshes allows us to take into account the complex coastline and mimic the natural resolution footprint of broadband MT transfer functions. We find conductivity variations that correlate with known continental structures such as due to the active tectonic processes within the western United States (e.g., Yellowstone hotspot, Basin and Range extension, and subduction of the Juan de Fuca slab) as well as the presence of deep roots beneath cratons. We further interpreted conductivity variations in terms of the upper mantle water content by coupling electrical conductivity with constrains on mantle thermo-chemical structure derived from the analysis of seismic data (in the form of P-to-s and S-to-p receiver functions). Our results suggest the existence of a relatively dry upper mantle beneath the United States, with a weak trend in the upper mantle water content from North to South.

Structural and Metamorphic History of the Leech River Shear Zone, Vancouver Island, British Columbia

AbstractThe Leech River Shear Zone (LRSZ) on southern Vancouver Island separates the metasedimentary schists of the Leech River Complex (LRC) from the accreted oceanic plateau of the Siletz‐Crescent terrane. The juxtaposition of these rock units suggests a possible origin as a subduction plate boundary but tectonic context of the LRSZ has yet to be fully established. We present field, microstructural, petrological, and geochronological observations that constrain the structural and metamorphic history of LRSZ. The mylonite zone of the LRSZ straddles the lithologic contact between the schists of the LRC and basalts of the Siletz‐Crescent terrane. Foliation orientations, a steeply plunging stretching lineation, and kinematic indicators all suggest reverse‐sinistral motion. Compositions of garnet in the schist and amphibole in the metabasalt record synkinematic growth at temperature and pressure conditions of 550°C–570°C and 450–490 MPa. These metamorphic conditions require elevated geotherms that are consistent with plate models that position the Kula‐Farallon Ridge and Yellowstone Hotspot in the region in the Eocene (∼50 Ma). Detrital zircon U‐Pb age distributions for the Leech River Schist have Paleocene maximum depositional ages and are similar to the Upper Nanaimo Group that unconformably overlies the Wrangellia terrane. Our ages support early Paleocene deposition of the schist in a subduction trench/slope environment followed by underthrusting and underplating. These results establish the exhumed mylonite zone of the LRSZ as a paleo‐plate interface that was active during Eocene subduction.

Polybaric magmatic evolution of the eocene penticton group alkaline rocks, south-central British Columbia

Volcanic rocks of the Challis-Kamloops belt erupted during the Eocene (55–45 Ma) throughout south-central British Columbia, Idaho, and Wyoming of the North American Cordillera. The Eocene volcanic rocks of southern British Columbia can be subdivided into three distinct groups (Kamloops, Princeton, Penticton) based on geography and composition. The southernmost Penticton Group is unique as it is composed of volcanic and intrusive alkaline rocks (e.g., Marron Formation, Terrace Mountain outlier, Allendale Lake pluton) as opposed to calc-alkaline rocks that are typical of the Kamloops and Princeton groups to the north. The rocks of this study are from the Penticton Group, are compositionally variably (SiO 2 = 51.2–64.7 wt%), and have highly fractionated chondrite-normalized REE patterns (La N /Yb N = 71–104) without noticeable negative Eu-anomalies (Eu/Eu* = 0.81–1.00). The rocks have similar Sr-Nd isotopic ( 87 Sr/ 86 Sr i = 0.706422–0.706563; ε Nd ( t ) = −4.3–6.1) ratios and Neoproterozoic depleted mantle model ages (T DM = 794–873 Ma). Petrological modeling demonstrates that the primary process of chemical differentiation within the Penticton Group is fractional crystallization. However, it appears that the Marron, Allendale Lake, and Terrace rocks underwent fractional crystallization at different pressures (i.e., 4 GPa, 1.5 GPa, and 1 GPa). The polybaric differentiation of the Penticton Group indicates that the alkaline system evolved at different crustal depths in a similar manner as magmas related to the Yellowstone hotspot and markedly different from the Kamloops and Princeton groups. We suggest that the Penticton Group is underlain by ancient lithospheric mantle that was modified by Precambrian subduction processes and was isolated from the convecting mantle whereas the northern part of the Eocene volcanic belt is underlain by exotic accreted terranes (Intermontane Superterrane) and was derived from a different, younger lithospheric mantle. • The Penticton Group alkaline rocks were emplaced during the Eocene (∼50 Ma). • The primary process of chemical differentiation is fractional crystallization. • Differentiation occurred under variable pressure at a post-collisional setting. • The magmas were derived from the ancient lithospheric mantle of North America.

Convective Melting and Water Behavior around Magmatic-Hydrothermal Transition: Numerical Modeling with Application to Krafla Volcano, Iceland

Abstract Water is an essential component of rhyolitic magmas and nearly universally, silicic magmatism in the upper crust includes a transition from magma to water-saturated roof rocks. We have numerically simulated the effect of the addition of geothermal fluids to an intruded rhyolitic sill from the hydrothermal system contained within the porous felsitic roof rocks. Water uptake in the melt proceeds via its thermodynamically estimated saturation with a partial melt, corresponding to the fugacity of hydrothermal water in the melt-fluid zone at particular T–P-${X}_{H_2O}$ conditions. It is assumed that the exchange occurs until the melt fraction increases to the threshold melt fraction value εb ≈ 0.3–0.45. In this approximation, the amount of added water is the product of its solubility and the critical melt fraction εb. In two series of numerical experiments run at pressures of 200 and 50 MPa, the interaction of water-filled porous felsite with near liquidus rhyolite magma resulted in water absorption, induced partial melting creating a narrow several meters wide mushy zone, and sluggish convection below that distributed water across the intruded sill. At P = 200 MPa, the addition of about 1.5 wt% water results in stronger volume convection, causing the melting rate to increase to 20 m/year. However, the addition of <0.22 wt% water induced no melting on the magma/contact mush interface, and the intruded sill crystallizes without convection. We apply the results of these numerical experiments to hot and dry rhyolites of the Yellowstone hot spot track magmas and then to the 2009 AD rhyolite sampled by the IDDP-1 exploration well in Krafla (Iceland). An active contact between the hydrothermal system within felsite and hot 963°C rhyolite magma was accidentally crossed at the depth of 2100 m, with a very thin (<30 m) transition providing information for a partial verification of our theoretical model. With the parameters observed in 2009, including the water concentrations in the melt (1.8 wt%) and felsite (0.92 wt%) and the high temperature of the intruded magma (945°C–980°C), we obtained slow melting of the preheated felsite roof at a rate of about 1 m/year. This seems reasonable if the 2009 magma was intruded during the 1975–1984 Krafla Fires eruption. We additionally present new δD values (−118‰) and H2Otot (1.6–2.0 wt%) concentration and reinterpret published δD and H2Otot values for felsitic fragments from IDDP-1; we find these to be lower than the δD in the mantle-derived magmas or hydrothermal systems. We demonstrate that, in theory, the formation of a fluid with such a low δD can be provided by the addition of low-δD water from OH-bearing minerals in hydrothermally altered roof rock. This may happen during dehydration of epidote from the altered roof rocks, and, alternatively, may also proceed by the mechanism of thermal diffusion (the Soret effect) through the partially molten/hydrothermal transition zone controlled by fluid fugacity. The high-temperature gradient in the contact zone between magma and the geothermal system of about 15–17°/m with conditions at the cold end close to the critical point for the aqueous fluid further decreases the expected δD value at the hot end of the contact zone to less than −110‰.

Deciphering the State of the Lower Crust and Upper Mantle With Multi‐Physics Inversion

AbstractThe composition of the lower crust is a key factor in understanding tectonic activity and deformation within the Earth. In particular, the presence or absence of melt or fluids has strong control on tectonic evolution. Multi‐physics inversion results from the western United States indicate that tectonic inheritance plays a much stronger role in determining the location of melt in the lower crust than previously thought. Even in a currently active area such as the Yellowstone Hotspot, the results suggest that fluid dominated structures and fluid free regions are located directly next to each other. This is incompatible with the commonly used model of recent tectonic activity as a main controlling factor for the presence of fluids or melt. These results have global implications for how geophysical models are interpreted and how they can be related to geodynamic simulations.

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

ABSTRACT 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

Abstract 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.

Electrical resistivity imaging of continental United States from three-dimensional inversion of EarthScope USArray magnetotelluric data

Long-period magnetotelluric (MT) sites, covering much of the continental United States (US) have been collected through the EarthScope USArray MT Transportable Array (TA) project. Previous regional studies using subsets of these data suggest large-scale variations in deep (sub-lithospheric) resistivity, often of significant amplitude. Here, we present a range of three-dimensional (3D) continental-scale electrical resistivity models from 3D inversion of the MT TA data, with a focus on testing robustness and resolution of this deep structure. The main features of our initial model, obtained with no constraints, are quite similar to those from previous studies, with significant (±1 order of magnitude) lateral variations in deep resistivity. We show that data fit, as measured by global normalized root-mean-square misfit, is not increased by replacing structures below 200 km depth with layer averages. A more careful examination of residuals leads to further refinements, with a slight improvement in data fit. These include a moderately conductive mantle transition zone (13Ω.m), and subtle (±1/4 order of magnitude) lateral variations in resistivity below 200 km. In contrast to the initial results, these variations can be explained by reasonable variations in mantle temperature and hydration. Overall, our results suggest that the asthenospheric mantle at 200–275 km is nearly dry on average and at greater depths contains 150–300 ppm water. Lateral variations may be explained by this range of water content, or by temperature variations of 80–120∘C. Patterns of resistivity variations below 200 km are consistent with known continental structure, and suggest deep roots (∼250 km) beneath cratons, and perhaps also beneath the mid-continent rift. Reduced resistivity, likely requiring greater hydration (or higher temperatures), occurs beneath the North-central part of the continental US, the oldest part of the continent, and beneath the Yellowstone hotspot.

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

Abstract 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.

Quantifying Normal Fault Evolution from River Profile Analysis in the Northern Basin and Range Province, Southwest Montana, USA

Abstract Over the past few decades, tectonic geomorphology has been widely implemented to constrain spatial and temporal patterns of fault slip, especially where existing geologic or geodetic data are poor. We apply this practice along the eastern margin of Bull Mountain, Southwest Montana, where 15 transient channels are eroding into the flat, upstream relict landscape in response to an ongoing period of increased base level fall along the Western North Boulder fault. We aim to improve constraints on the spatial and temporal slip rates across the Western North Boulder fault zone by applying channel morphometrics, cosmogenic erosion rates, bedrock characteristics, and calibrated reproductions of the modern river profiles using a 1-dimensional stream power incision model that undergoes a change in the rate of base level fall. We perform over 104 base level fall simulations to explore a wide range of fault slip dynamics and stream power parameters. Our best fit simulations suggest that the Western North Boulder fault started as individual fault segments along the middle to southern regions of Bull Mountain that nucleated around 6.2 to 2.5 Ma, respectively. This was followed by the nucleation of fault segments in the northern region around 1.5 to 0.4 Ma. We recreate the evolution of the Western North Boulder fault to show that through time, these individual segments propagate at the fault tips and link together to span over 40 km, with a maximum slip of 462 m in the central portion of the fault. Fault slip rates range from 0.02 to 0.45 mm/yr along strike and are consistent with estimates for other active faults in the region. We find that the timing of fault initiation coincides well with the migration of the Yellowstone hotspot across the nearby Idaho-Montana border and thus attribute the initiation of extension to the crustal bulge from the migrating hotspot. Overall, we provide the first quantitative constraints on fault initiation and evolution of the Western North Boulder fault, perhaps the farthest north basin in the Northern Basin and Range province that such constraints exist. We show that river profiles are powerful tools for documenting the spatial and temporal patterns of normal fault evolution, especially where other geologic/geodetic methods are limited, proving to be a vital tool for accurate tectonic hazard assessments.

The Case for a Long-Lived and Robust Yellowstone Hotspot

The Case for a Long-Lived and Robust Yellowstone Hotspot

Regionally continuous Miocene rhyolites beneath the eastern Snake River Plain reveal localized flexure at its western margin: Idaho National Laboratory and vicinity

The eastern Snake River Plain (ESRP) is a northeast-trending topographic basin interpreted to be the result of the time-transgressive track of the North American plate above the Yellowstone hotspot. The track is defined by the age progression of silicic volcanic rocks exposed along the margins of the ESRP. However, the bulk of these silicic rocks are buried under 1 to 3 kilometers of younger basalts. Here, silicic volcanic rocks recovered from boreholes that penetrate below the basalts, including INEL-1, WO-2 and new deep borehole USGS-142, are correlated with one another and to surface exposures to assess various models for ESRP subsidence. These correlations are established on U/Pb zircon and 40Ar/39Ar sanidine age determinations, phenocryst assemblages, major and trace element geochemistry, δ18O isotopic data from selected phenocrysts, and initial εHf values of zircon. These data suggest a correlation of: (1) the newly documented 8.1 ± 0.2 Ma rhyolite of Butte Quarry (sample 17KS03), exposed near Arco, Idaho to the upper-most Picabo volcanic field rhyolites found in borehole INEL-1; (2) the 6.73 ± 0.02 Ma East Arco Hills rhyolite (sample 16KS02) to the Blacktail Creek Tuff, which was also encountered at the bottom of borehole WO-2; and (3) the 6.42 ± 0.07 Ma rhyolite of borehole USGS-142 to the Walcott Tuff B encountered in deep borehole WO-2. These results show that rhyolites found along the western margin of the ESRP dip ~20º south-southeast toward the basin axis, and then gradually tilt less steeply in the subsurface as the axis is approached. This subsurface pattern of tilting is consistent with a previously proposed crustal flexural model of subsidence based only on surface exposures, but is inconsistent with subsidence models that require accommodation of ESRP subsidence on either a major normal fault or strike-slip fault.

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

AbstractSuper-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.

Noble gases, dead carbon, and reinterpretation of groundwater ages and travel time in local aquifers of the Columbia River Basalt Group

Groundwater noble gas concentrations are used to model the temperature of aquifer recharge but also may be used to understand geologic influences on water chemistry. Aquifers in the South Fork Palouse River Basin are hosted within the fractured basalts of the Columbia River Basalt Group and the interbedded sediments of the Latah Formation. The travel time of groundwater, and the associated recharge rate, in this multiple aquifer system is of primary interest to water resource managers because of declining water levels. Prior investigations of the deeper groundwater indicated low values of percent modern carbon and elevated alkalinity and δ13C values compared to shallower groundwater. The groundwater travel time suggested by the uncorrected carbon-14 ages (up to 31,000 BP) implies a recharge rate much slower than hypothesized from identified recharge pathways, low storativity values, and relatively large groundwater withdrawals. No sources of dead carbon were previously identified, but the disconnect between old groundwater and likely recharge pathways suggest input from a dead carbon source. Groundwater collected for this study contained elevated alkalinity, δ13C, He, and 3He/4He values deeper in this aquifer system located in this young flood basalt province. Elevated alkalinity and δ13C values correlate with a mantle CO2 source that is reflected in oversaturated He and elevated 3He/4He (R/Ra) ratios. The flood basalts are Miocene expressions of the modern Yellowstone hotspot, which exudes relatively large concentrations of He (with elevated 3He/4He (R/Ra) ratios) and CO2. A deep and 14C-free geologic source of CO2 helps to explain the low percent modern carbon in the deeper groundwater. Correction of groundwater ages through incorporation of this dead carbon source produced ages up to 64% smaller than previous age estimates; although, greater age corrections may be necessary because of differences in sample and analysis methods for alkalinity, δ13C, and 14C that influence detection of the dead carbon input.

Yellowstone Plume Conduit Tilt Caused by Large‐Scale Mantle Flow

AbstractMantle plumes are hot upwellings of rock thought to originate at the core‐mantle boundary. As they rise through the mantle, their conduits may become tilted due to lateral large‐scale mantle flow. Recent tomographic images have revealed a strongly tilted plume conduit starting at the core‐mantle boundary beneath northern Baja California rising toward the Yellowstone hot spot from the southwest. Here we perform numerical computations of plumes deflected in large‐scale mantle flow with the aim of finding if realistic model parameter ranges exist that yield a good fit with the tomographically observed conduit. We restrict ourselves to models that yield reasonable results for plume conduit tilt and hot spot motion globally. These models require high viscosity ≈1023 Pa s at some lower mantle depths. For a plume head reaching the surface 17 Ma, corresponding to the start of the Columbia River Basalts, our models require rise times ≈80 Myr or longer to match the tilt of the conduit observed by tomography. We used several tomography models to determine mantle density with almost all models predicting southwestward flow in the lowermost mantle beneath the western United States. Exact details of the shape of the predicted conduit's southwesterly tilt vary, depending on the density and viscosity structures we used. In many cases we find comparatively strong tilts in two depth ranges, in the upper and lower portions of the mantle, which is also a characteristic of the tomographically observed conduit. We expect that future models may help to constrain large‐scale flow by matching these corresponding depth ranges.

Paleoseismic Evidence for Climatic and Magmatic Controls on the Teton Fault, WY

AbstractGeologic records of past earthquakes are rare but critical for identifying long‐term patterns in fault behavior and assessing modern earthquake hazards. We present a continuous 14,000‐year paleoearthquake reconstruction using precisely dated lacustrine sediments and landslide deposits from a lake basin positioned directly on the Teton normal fault, which cuts across Grand Teton National Park, WY, and is among the most hazardous intraplate faults in the western United States. We show that beginning immediately after deglaciation, a series of at least seven major fault ruptures occurred at regular intervals of ~1,050 years (± ~250 years), followed by >5,000 years of inactivity. These results are consistent with trench data and model simulations and suggest that faulting was variably influenced by climate‐controlled glacial fluctuations and magmatic activity of the nearby Yellowstone hotspot.

Experimental Melting of Hydrothermally Altered Rocks: Constraints for the Generation of Low-δ18O Rhyolites in the Central Snake River Plain

Abstract Quantifying the relative contributions of crustal versus mantle-derived melt is important for understanding how silicic magmas are generated, stored, and interact with country rock in trans-crustal magmatic systems. Low-δ18O rhyolitic ignimbrites and lavas erupted during Miocene volcanic activity in the central Snake River Plain (14–6 Ma) have been inferred to be the result of large-scale partial or bulk melting of pre-existing hydrothermally altered lithologies of the Idaho batholith and Challis volcanic field. In this study, we assess the melting behaviour of heterogeneously altered source materials via partial melting experiments over a range of run times at conditions of 750–1000°C and 1–2 kbar, and apply our observations to current models for the petrogenesis of low-δ18O rhyolites along the Yellowstone hotspot track. Partial melt produced in the experiments inherits the bulk oxygen isotope composition from hydrothermally altered peraluminous source materials independent of the melt fraction, excluding the possibility for preferential, disequilibrium melting of 18O-depleted mineral phases during incipient melting. We propose a new model to explain the generation of low-δ18O rhyolites in the central Snake River Plain, whereby mantle-derived magmas assimilate ∼30–40% of crustal material that was hydrothermally altered at high temperatures in two stages: (1) a preceding episode of hydrothermal alteration during intrusion of Eocene plutons (‘pre-existing source’); (2) syn-magmatic hydrothermal alteration within a nested caldera complex. During assimilation, dilution of peraluminous crustal lithologies with mantle-derived magma maintains the metaluminous character of rhyolites erupted along the Yellowstone hotspot track. These results link previous models favouring melting of either pre-existing or syn-magmatically altered lithologies for the generation of low-δ18O rhyolites along the Yellowstone hotspot track and provide direct experimental observation of the chemical processes occurring during assimilation processes in magmatic environments.

Accommodation, slip inversion, and fault segmentation in a province-scale shear zone from high-resolution, densely spaced wide-aperture seismic profiling, Centennial Valley, MT, USA

We acquired a ~9-km long, high-resolution reflection seismic profile in the Centennial Valley, Montana, to better understand the kinematics of basin bounding faults and their role in accommodating proposed right-lateral shear in the Northern Basin and Range adjacent to the Yellowstone hotspot. In pursuing these goals, our findings have also shed light on the development of hanging wall stratigraphy and seismic hazards for this part of the SW Montana seismic belt. Here we present the profile and a working interpretation that identifies fault inversion, and an oblique, anticlinal accommodation zone linking the Centennial and Lima Reservoir faults in the Centennial Valley. These interpretations are consistent with seismicity and GPS-geodetically observed right-lateral shear aligned with the Centennial Valley north of the Yellowstone hotspot. Data were acquired using dense, wide-aperture arrays and illuminate the subsurface stratigraphy and faults down to ~1200 m, showing that the basin is a half-graben with a southern depocenter driven by the listric geometry of the north-dipping Centennial fault. Reflectors onlap basement highs with growth geometry against these faults. Our interpretation of a bright basal reflection as the Timber Hill Basalt (~6 Ma) or related flow, is consistent with a late Miocene – Pliocene inception of the basin proposed by other research. We also note a small inversion structure that we interpret as local evidence of transpression in the shear zone. This transpression is part of the accommodation zone and seismogenic faults including the Lima Reservoir fault that has well-expressed Holocene surface ruptures a few kilometres west of the seismic line along the northern edge of the Centennial basin.