Western Snake River Plain Research Articles

A global database of Mars-relevant hydrovolcanic environments on Earth with potential biosignature preservation

Abstract Basaltic hydrovolcanic environments on Earth produce abundant glass (sideromelane), which readily alters and acts as an important source of chemical energy for lithotrophic microorganisms; as such, these sites are significant for potential origins-of-life and early life research. Similar environments were identified on Mars and should be considered potential targets for astrobiological investigation. Pleistocene to recent phreatomagmatic and glaciovolcanic structures on Earth include tuff cones, tuff rings, maars, tuyas, and tindars. Such hydrovolcanic deposits contain abundant glass that is variably hydrothermally altered, and some areas contain published evidence of putative microbial habitation and microbially mediated alteration, including microtubules and granular alteration. We analyzed the literature on terrestrial hydrovolcanic environments and created a global database of 45 volcanic fields on Earth with compositions, alteration histories, and structures relevant to Mars. These sites have geochemistry, mineralogy, and syn- and post-eruptive environmental conditions that make them suitable targets for Mars-analogue astrobiological research. Relevant alteration products include palagonite, zeolites, clays, and calcite. Seven of these sites have evidence of microbially mediated alteration, which could be considered a useful biosignature in a Mars-analogue context. The sites are Wells Gray–Clearwater Volcanic Field, Canada; Fort Rock Volcanic Field, Western Snake River Plain Volcanic Field, and Upsal Hogback, USA; Reykjanes Volcanic Field and Western Volcanic Zone, Iceland; and Carapace Nunatak, Antarctica. Based on the properties of these already confirmed sites, along with comparing the remaining 38 Earth volcanic fields to volcanic rocks on Mars, we recommend 11 volcanic fields in particular from our database for future investigations: Auckland and South Auckland volcanic fields, New Zealand; O’ahu, Black Rock Desert, and Black Point, USA; Tuya Volcanic Field, Canada; Karapınar Volcanic Field, Türkiye; Vestmannaeyjar Archipelago, Iceland; Llancanelo Volcanic Field, Argentina; São Miguel Volcanic Field, Azores; and Icefall Nunatak, Antarctica. We recommend reviewing palagonitized tuff samples from these sites for evidence of microbial alteration, in addition to performing geochemical and mineralogical analyses to constrain their magmatic and alteration properties. By studying the rock record of hydrovolcanic environments on Earth to infer habitability and biological alteration, we contribute to establishing the conditions favorable for the origination, survival, and proliferation of life in a Mars-relevant setting.

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.

U–Pb zircon ages, mapping, and biostratigraphy of the Payette Formation and Idaho Group north of the western Snake River Plain, Idaho: Implications for hydrocarbon system correlation

ABSTRACT Sedimentary deposits north of the western Snake River Plain host Idaho’s first and only producing oil and gas field. They consist of the lower to middle Miocene Payette Formation, the middle to upper Miocene Poison Creek and Chalk Hills Formations, and the Pliocene to lower Pleistocene Glenns Ferry Formation. Using new geochronology, palynomorph biostratigraphy, and geologic mapping, we connect updip surface features to subsurface petroleum play elements. The Payette Formation is a likely main source of the hydrocarbons, and acts as one of the reservoirs in the unnamed basin. Here, we redefine the Payette Formation as 0 to ~3,500 ft (0 to ~1,000 m) of mudstone, with lesser amounts of sandstone overlying and interbedded with the Columbia River Basalt Group and Weiser volcanic field. Index palynomorphs, including Liquidambar and Pterocarya, present in Idaho during and immediately after the middle Miocene climatic optimum, and new U–Pb ages of 16.39 and 15.88 Ma, help establish the thickness and extent of the formation. For the first time, these biostratigraphic markers have been defined for the oil and gas wells. The Poison Creek Formation is sandstone interbedded with mudstone that is ~800–1,800 ft (250–550 m) thick. The Chalk Hills Formation is a tuffaceous siltstone, claystone, and sandstone that is as much as ~4,200 ft (1,280 m) thick. New U–Pb ages are 10.1, 9.04, and 9.00 for the Poison Creek Formation, along with maximum depositional ages of 10.7 to 9.9 Ma for four samples from the Poison Creek Formation. A single U–Pb age of 7.78 Ma was determined from pumice low in the Chalk Hills Formation. Like the Payette Formation, the Poison Creek Formation can be a reservoir, whereas the Chalk Hills Formation acts as a sealing mudstone facies. The overlying sandstone, siltstone, and conglomerate of the Glenns Ferry Formation act as the overburden to the petroleum system in the subsurface, and were important for burial and hydrocarbon maturation. The Glenns Ferry Formation is up to 500 ft (150 m) thick in the study area, as much has been eroded. Whereas the Payette and Poison Creek Formations were deposited during the mid-Miocene climatic optimum amongst and above volcanic flows, the Chalk Hills and Glenns Ferry Formations were deposited within ancient Lake Idaho during an overall increase in aridity and cooling after the mid-Miocene climatic optimum.

Seasonality and evaporation of water resources in Reynolds Creek Experimental Watershed and Critical Zone Observatory, Southwestern Idaho, USA

AbstractThe Reynolds Creek Experimental Watershed (RCEW) and Critical Zone Observatory (CZO), located south of the western Snake River Plain in the Intermountain West of the United States, is the site of over 60 years of research aimed at understanding integrated earth processes in a semi‐arid climate to aid sustainable use of environmental resources. Meteoric water lines (MWLs) are used to interpret hydrologic processes, though equilibrium and nonequilibrium processes affect the linear function and can reveal seasonal and climatological effects, necessitating the development of local meteoric water lines (LMWLs). At RCEW‐CZO, an RCEW LMWL was developed using non‐volume‐weighted, orthogonal regression with assumed error in both predictor and response variables from several years of precipitation (2015, 2017, 2019, 2020, and 2021) primarily at three different elevations (1203, 1585, and 2043 m). As most precipitation is evaporated or intercepted by vegetation in the driest months, an RCEW LMWL for groundwater recharge (RCEW LMWL‐GWR) was also developed using precipitation from the wettest months (November through April). The RCEW LMWL (δ2H = 7.41 × δ18O – 3.09) is different from the RCEW LMWL‐GWR (δ2H = 8.21 × δ18O + 9.95) and compares favorably to other LMWLs developed for the region and climate. Comparative surface, spring, and subsurface water datasets within the RCEW‐CZO are more similar to precipitation during the wettest months than dry months, illustrating that some semi‐arid hydrologic systems may most appropriately be compared to MWLs developed from precipitation only from the wettest season.

Linking exhumation, paleo-relief, and rift formation to magmatic processes in the western Snake River Plain, Idaho, using apatite (U-Th)/He thermochronology

Abstract The western Snake River Plain (WSRP) in southwest Idaho has been characterized as an intracontinental rift basin but differs markedly in topography and style from other Cordilleran extensional structures and structurally from the down-warped lava plain of the eastern Snake River Plain. To investigate mechanisms driving extension and topographic evolution, we sampled granitoid bedrock from Cretaceous and Eocene-aged plutons from the mountainous flanks of the WSRP to detail their exhumation history with apatite (U-Th)/He (AHe) thermochronometry. AHe cooling dates from seventeen samples range from 7.9 ± 1.4 Ma to 55 ± 10 Ma. Most cooling dates from Cretaceous plutons adjacent to the WSRP are Eocene, while Eocene intrusions from within the Middle Fork Boise River canyon ~35 km NE of the WSRP yield Miocene cooling dates. The AHe dates provide evidence of exhumation of the Idaho batholith during the Eocene, supporting a high relief landscape at that time, followed by decreasing relief. The Miocene AHe dates show rapid cooling along the Middle Fork Boise River that we take to indicate focused river incision due to base level fall in the WSRP. Eocene AHe dates limit magnitudes of exhumation and extension on the flanks of the WSRP during Miocene rift formation. This suggests extension was accommodated by magmatic intrusions and intrabasin faults rather than basin-bounding faults. We favor a model where WSRP extension was related to Columbia River Flood Basalt eruption and enhanced by later eruption of the Bruneau-Jarbidge and Twin Falls volcanic fields, explaining the apparent difference with other Cordilleran extensional structures.

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.

Late Pliocene to early Pleistocene climate dynamics in western North America based on a new pollen record from paleo-Lake Idaho

Marked by the expansion of ice sheets in the high latitudes, the intensification of Northern Hemisphere glaciation across the Plio/Pleistocene transition at ~ 2.7 Ma represents a critical interval of late Neogene climate evolution. To date, the characteristics of climate change in North America during that time and its imprint on vegetation has remained poorly constrained because of the lack of continuous, highly resolved terrestrial records. We here assess the vegetation dynamics in northwestern North America during the late Pliocene and early Pleistocene (c. 2.8–2.4 Ma) based on a pollen record from a lacustrine sequence from paleo-Lake Idaho, western Snake River Plain (USA) that has been retrieved within the framework of an International Continental Drilling Program (ICDP) coring campaign. Our data indicate a sensitive response of forest ecosystems to glacial/interglacial variability paced by orbital obliquity across the study interval, and also highlight a distinct expansion of steppic elements that likely occurs during the first strong glacial of the Pleistocene, i.e. Marine Isotope Stage 100. The pollen data document a major forest biome change at ~ 2.6 Ma that is marked by the replacement of conifer-dominated forests by open mixed forests. Quantitative pollen-based climate estimates suggest that this forest reorganisation was associated with an increase in precipitation from the late Pliocene to the early Pleistocene. We attribute this shift to an enhanced moisture transport from the subarctic Pacific Ocean to North America, confirming the hypothesis that ocean-circulation changes were instrumental in the intensification of Northern Hemisphere glaciation.

Petrogenesis of Pleistocene Basalts from the Western Snake River Plain, Idaho

Abstract We present new geochemical, Sr, Nd, and Pb isotope, and 40Ar/39Ar data from Pleistocene basalts of the Western Snake River Plain (WSRP), Idaho, USA to explore their petrogenesis and to investigate the nature of the lithosphere at the western boundary of the North American craton. The basalts are divided into three groups based on their geochemical and isotopic characteristics. Prior to ∼1 Ma, volcanoes in the WSRP erupted iron-rich tholeiites (FeB1), but subsequent volcanism was dominated by concurrent eruptions of mildly alkaline, alumina-rich lavas (AlB) and iron-rich tholeiites (FeB2) with isotopic signatures similar to the AlB lavas. New 40Ar/39Ar dates of AlB and FeB2 basalts range from 0·920 ± 0·049 to 0·287 ± 0·014 Ma. MELTS models of FeB1 differentiation trends indicate that the range of compositions in this suite can be produced by 10–15 % crystallization of olivine and plagioclase at low pressure using the least evolved FeB1 composition as a parental magma; isotopic ratios can be produced via combined assimilation of a Miocene rhyolite and fractional crystallization. Additional modeling suggests that parental magmas at AlB centers were produced by 3–12 % equilibrium melting of a garnet–spinel-enriched mantle source, slightly different from that proposed for the youngest mildly alkaline lavas of the eastern and central Snake River Plain. Our new geochemical, isotopic, and geochronological data for the FeB2 basalts suggests that they are related to AlB-type magmas via a combination of fractional crystallization and assimilation of evolved mafic crust. MELTS models suggest that crystallization of an AlB parental melt at a depth of 6–8 km (2·5 kbar) could produce residual liquids having many of the major oxide characteristics of FeB2 ferrobasalts. Sr–Nd–Pb isotopic signatures of these three suites indicate a dominant contribution from an enriched plume source. FeB1 lavas are probably products of mixing between melts of an enriched plume mantle source (represented by Imnaha and Steens Basalts of the Columbia River Basalt Group) and isotopically heterogeneous sub-continental lithospheric mantle (SCLM) that has been isolated from the convecting mantle since the Archean. Isotopic ratios of FeB2 and AlB lavas capture mixing between enriched plume mantle and a more isotopically homogeneous ancient SCLM domain characteristic of the eastern and central Snake River Plain, with a coupled decrease in lithospheric contribution and degree of partial melting through time to the present. Mixtures of enriched asthenospheric reservoirs with lithospheric mantle have been proposed for neighboring volcanic fields to the east along the strike of the Yellowstone–SRP hotspot track, and to the west owing to differences in the mantle underlying the boundary of the North American craton and accreted terranes. Our petrogenetic model for the Pleistocene WSRP basalts suggests that there is also a lateral, across-strike gradient in the geometry and interaction of enriched plume mantle and ancient lithosphere. We reiterate suggestions that the WSRP is a lithosphere-scale conduit connecting initial plume-head impingement in east–central Oregon with the subsequent Yellowstone–SRP hotspot plume-tail track.

Basaltic phreatomagmatic fissure at 71 Gulch Part 1: sediment magma mingling and eruptive behavior

71 Gulch Volcano, located in the western Snake River Plain, southwestern Idaho (USA), was formed by a basaltic fissure eruption into Pliocene Lake Idaho. Deposits at and below the eruptive surface record the nature of explosive and non-explosive sediment-magma and water-magma interactions. The paleoenvironment and the volcanic plumbing system of 71 Gulch eruption were reconstructed from a detailed study of the now-exposed eruptive and subsurface deposits. Estimated water levels during the eruption range from 1 to 53 m across the entire volcano based on the distribution of pillow lavas. Billowed dikes and fluidal peperites are two types of intrusive features found ≤ 12 m below the syn-eruptive surface that have similar continuous margins representing ductile behavior and emplacement into soft, deformable sediment. In comparison, tabular dikes and blocky peperites occur throughout 41 m thick of exposed deposits below the syn-eruptive surface. A conical, diatreme-like vent structure dominates the subsurface exposure, indicating excavation by phreatomagmatic explosions. Localized spatter within the main subsurface vent suggests temporary exclusion of water and thus a period of brief vent growth above the lake level. Evidence of sediment-magma mingling through blocky and fluidal peperites shows how differing subsurface depths impact the resulting mingling textures.

Upper mantle tomography beneath the Pacific Northwest interior

The Pacific Northwest has undergone extensive tectonic and magmatic construction in the Cenozoic, and many clues to its history are in a well-resolved upper mantle structure. We present new P, S, and VP/VS models that benefit from improved algorithms and additional data from recently deployed flexible array experiments in the region of the Columbia River Basalts, western Snake River Plain, and Idaho Batholith. We refine the geometries of previously imaged structures and identify new anomalies beneath the Pacific Northwest. A clear separation is resolved between the Wallowa and Idaho structures that extends from the base of the crust to the depth extent of these structures. The separation follows the Precambrian margin of North America, possibly indicating separate tectonic histories for the two structures. A shallow fast anomaly beneath Idaho Batholith may be responsible for the observed curvature of the Yellowstone track around this batholith. And a strong and unusually deep (180–250 km) slow anomaly is present east of the apparent Juan de Fuca slab gap. Resolving these structures well is important in understanding the origin of the lithospheric fragments in the mantle, and key to any reconstruction of the evolution of the Pacific Northwest-Farallon-Kula-Juan de Fuca tectonic system. Of most relevance, understanding the origin of the imaged high-velocity mantle structures provides strong constraint on their density, that otherwise would be largely unknown, which controls the evolution of vertical and horizontal stresses acting at the base of the lithosphere and hence the evolution of topography. Also, resolving the volumes of partially molten upper mantle shapes our understanding on the origin of the abundant volcanism in the Pacific Northwest interior.

Evaluation of the geothermal potential of the western Snake River Plain based on a deep corehole on the Mountain Home AFB near Mountain Home, Idaho

A geothermal exploration corehole was drilled to a total depth of 1821.5 m on the Mountain Home Air Force Base near Mountain Home, Idaho. The corehole was used to collect an unusually large amount of data, including uniaxial compressive stress (UCS) experiments on core samples, to evaluate the geothermal potential of the western Snake River Plain. In addition, unlike many exploration holes in this region, a fluid entry was encountered at 1745.3 m and flowed artesian to the surface. A maximum temperature of 149.4 °C was calculated for the entry. A temperature log run on the corehole from 3 to 1675 m is nearly linear with little variation. The average geothermal gradient is 73 °C/km, and the average heat flow between 200 and 1500 m is 102 ± 15 mW/m2. Chemical analyses of a sample from the fluid entry suggest that a significant proportion of the water is not meteoric. Five geothermometers show equilibrium temperature in the range of 133–157 °C. Furthermore, based on the unconfined UCS experiments on basalt core samples, a brittle unit was found to comprise the fractured reservoir that the geothermal water flows from, while an overlying ductile unit acts as a hydrothermal caprock. This implies that the reservoir/caprock pair may be a target for future exploration wells drilled to delineate the extent of the potential resource and the boundaries of the connected fracture network.

Geology and in situ stress of the MH-2 borehole, Idaho, USA: Insights into western Snake River Plain structure from geothermal exploration drilling

Project HOTSPOT, the Snake River Scientific Drilling Project (International Continental Scientific Drilling Program), tested for deep geothermal resources and examined the petrology of volcanic rocks with three drillholes in the central and western Snake River Plain (western USA). The MH-2 drillhole targeted fractured crystalline and hydrothermally altered basalt in the area of the Mountain Home Air Force Base (Idaho) to a total depth of 1821 m. At 1745 m depth the drillhole encountered flowing artesian hydrothermal fluids of at least 150 °C. We integrate geological analyses of core, image log, and borehole geophysical data, and in situ stress analyses to describe the structural environment that produces permeability for artesian flow. The rocks in the lower 540 m of the drillhole consist of basalt flows as much as 30 m thick, altered basalt, and thin sedimentary horizons. The mechanical stratigraphy is defined by nine mechanical horizons that are in three ranges of rock strength on the basis of experimentally determined strength data, core logging, and geophysical log signatures. Hydrothermal alteration products and mineralization in the core are associated with three highly faulted sections; the lowermost section is associated with the zone of flowing thermal water. Shear slip indicators on faults observed in core indicate slip ranging from pure strike slip to normal failure mechanisms in the stronger horizons. The borehole breakouts indicate that the maximum horizontal stress, SH, is oriented 047° ± 7°, and drilling-induced tensile fractures indicate that SH is oriented at 67° ± 21°. The in situ stress orientations exhibit little variation over the depth of the measured interval, but the SH magnitude varies with depth, and is best explained by an oblique normal fault stress regime. The geomechanical model indicates that if pore pressures at depth are elevated above the normal hydrostatic gradient, as observed here, the system has the potential to deform by mixed normal and strike-slip failure. Our observations and interpretations suggest that the MH-2 borehole was drilled into oblique normal faults that intersect a buried 300°-trending fault block masked by the basaltic volcanic complex. These data indicate that the transition from the central to western Snake River Plain is characterized by complex structures developed in response to a transitional stress state related to Snake River Plain and western Basin and Range stress regimes. The western Basin and Range stress and tectonic regime may extend from northern Nevada into western Idaho and may enhance the potential for geothermal resources by creating interconnected fracture and fault-related permeability at depth.

A crustal-scale integrated geophysical and tectonic study of the Snake River Plain region, northwestern U.S.A.

ABSTRACTThe Pacific Northwest region of North America is a site of very complex tectonomagmatic activity. This activity is due to subduction of the Pacific plate, the associated Cascade chain of volcanoes, micro-plate interactions, and mantle plume activity to the east of the plate margin that produced the Yellowstone hotspot track along the Eastern Snake River Plain (ESRP). A number of recent geophysical and geological studies have produced new results that have drawn attention to the complex tectonic setting of the region east of the Cascade Range, and its tectonic evolution is the subject of considerable scientific interest and debate. Numerous seismic studies have specifically focused on the crustal and upper mantle structure of the ESRP and Yellowstone area. However, crustal-scale studies of the Western Snake River Plain (WSRP) are limited. We undertook an integrated analysis of new and existing geophysical data and geologic constraints to study the crustal structure of the WSRP and generated two-dimensional crustal models across it. We observed both differences and similarities in the structural and tectonic evolution of the eastern and western arms of the SRP based on our integrated analysis. From a broader perspective based on recent geological and geophysical studies in the surrounding region, the intersection of the two arms of the SRP emerges as a major element of a complex tectonic intersection that includes the High Lava Plains of eastern Oregon, the Northern Nevada rift, a southwestern extension of the ESRP into northern Nevada, as well as, faulting and volcanism extending north-westward to connect with the Columbia River basalt plateau region. Thus, the goal of this study is to advance our understanding of the tectonomagmatic evolution of the region and to encourage further studies in the region.

The smectite to corrensite transition: X-ray diffraction results from the MH-2B core, western Snake River Plain, Idaho, USA

AbstractThe MH-2B borehole, a part of Project HOTSPOT, was drilled to a depth of 1821 m in late Cenozoic basalts, hyaloclastites and interbedded lake sediments, on the Mountain Home Air Force Base in southern Idaho, USA. Drillers encountered hot water (145°C) under artesian pressure at 1745 m in a narrow zone of highly fractured rock associated with a major sub-surface fault. X-ray diffraction (XRD) analysis identified corrensite (with and without smectite) between 1700 and 1800 m, but only smectite above 1700 m and below 1800 m. This corrensite horizon contains a relatively narrow zone of fracturing and hot artesian water near its centre but for the most part occurs in relatively massive basalt flows. No evidence was found for randomly interstratified chlorite-smectite.

Yellowstone hotspot–continental lithosphere interaction

The Snake River Plain represents 17 m.y. of volcanic activity that took place as the North American continent migrated over a relatively fixed magma source, or hotspot. We present new Pb, Sr, and Nd data for a suite of 25 basalts collected from Western and Central Snake River Plain (SRP). The new isotope data, combined with previously published data from the SRP, provide a traverse of the Wyoming craton margin, from the 87Sr/86Sr = 0.706 line boundary of western SRP with Phanerozoic accreted terranes, east through the central and eastern SRP, to the Yellowstone Plateau. Low-K basalts from the western SRP, overlain by high-K basalts, provide a temporal record of regional source variation from ∼16.8 to 0.2 Ma. Principal Component Analysis (PCA) of the new and previously published SRP basalt Pb isotopes reveals that >97% of the total variability is accounted for by mixing between three end-members and is consistent with a sublithospheric Yellowstone hotspot mantle source with a radiogenic isotope composition similar to the mantle source of the early Columbia River Basalt Group (CRBG) and two continental lithosphere end-members, heterogeneous in age and composition. We use the SRP Pb, Sr, and Nd isotope data to model the Yellowstone Hotspot–continental lithosphere interaction by three component mixing between two continental lithospheric components, Archean lithosphere (CL1) that represents older lithosphere underlying the Yellowstone Plateau in the east, and Paleoproterozoic lithosphere (CL2) representing the younger lithosphere underlying the SRP in the west near the craton margin, and a sublithospheric end-member, representing the Yellowstone hotspot (PL). The results suggest a continuous flow of PL material westward as the NA continental lithosphere migrated over the upwelling hotspot along a shoaling gradient in the sub-continental mantle lithosphere. The model shows a decrease in Total Lithosphere end-members (CL1 + CL2) and the Lithosphere Ratio (CL1/CL2), from the craton interior at Yellowstone toward its western margin, consistent with geologic and geophysical evidence that the continental lithosphere beneath the SRP decreases in age and thickness from east to west. The Lithosphere Ratio shows step-like decreases from Yellowstone in the east to the 87Sr/86Sr = 0.706 line in the west, indicating that the SRP cuts across geochemically distinct parcels of lithospheric mantle, consistent with terrane accretion models for the craton margin. In the western SRP, young high-K basalts have a lower mass fraction of Total Lithospheric compared to the underlying low-K tholeiites, but the same Lithosphere Ratio, consistent with a recent (700–900 ka) decrease in lithosphere contribution between eruption of early low- and younger high-K basalts.

Receiver function imaging of upper mantle complexity beneath the Pacific Northwest, United States

Small-scale topographic variations on the upper mantle seismic discontinuities provide important constraints on the thermal influences of upwellings and downwellings in geodynamically complex regions. Subduction of the Juan de Fuca plate and other tectonic processes dominating the Pacific Northwest, United States in the Cenozoic involve massive thermal flux that likely result in an upper mantle that has strong 3-D temperature variations. We address the interaction of such processes in the region using receiver functions to image the upper mantle seismic discontinuities at 410 and 660 km. We utilized over 15 000 high quality receiver functions gathered from 294 teleseismic earthquakes recorded at 277 regional broadband seismic stations, primarily those of the Earthscope/USArray Transportable Array. We find the average depths of the discontinuities to be 412 km and 658 km, respectively, with no obvious 520 km discontinuity detected. The peak-to-peak range is greater on the ‘410’ than the ‘660’, suggesting the possibility of more significant regional dynamic processes at upper mantle depths. Our results are not consistent with a mantle plume below central Oregon in the High Lava Plains region. Our observation of a thinner transition zone beneath the western Snake River Plain region, however, is consistent with a regional increase in mantle temperatures, perhaps due to either asthenospheric flow from beneath and around the southern edge of the Juan de Fuca plate, or to vertical flow in the form of regional mantle upwelling related to the Snake River Plain / Yellowstone hotspot track. Further, our results are not consistent with a simple subducting Juan de Fuca slab morphology, but rather suggest similar levels of significant complexity in slab structure found by recent regional tomographic studies. We find evidence for a thickened and therefore cooler mantle transition zone beneath the Wallowa / Idaho Batholith region, consistent with tomographic models which suggest broad scale increased seismic velocities to transition zone depths. We speculate that this region may be the location of mantle downwelling at larger scales than previously proposed.

Early occurrence of sagebrush steppe, Miocene (12 Ma) on the Snake River Plain

Pollen analysis of soils overlain by welded tuffs of the Yellowstone hotspot provides a radiometrically-dated chronology of the vegetation 12–7 Ma for central Snake River Plain. The welded tuffs, produced as the Yellowstone hotspot migrated eastward, overlie soils containing perfectly-preserved pollen, occasionally containing cytoplasm. The pollen assemblage is dominated by Artemisia (sagebrush; ave. 32%, range 11–61%) and Poaceae (grass; ave. 21.3%, range 4–55%). This assemblage is very similar to that found in the area today, except the welded-tuff samples contain low percentages (< 5%) of arboreal taxa not found in the current flora: Podocarpus, Cedrus, Taxus, Aesculus, and Ulmus. The values of Artemisia and Poaceae are much higher than those found in samples of the same age (12–7 Ma) from the Pacific Northwest (which are dominated by tree pollen) and the Great Salt Lake basin (which are dominated by pine [ Pinus], and greasewood [ Sarcobatus]). Furthermore, the welded-tuff samples from the Snake River Plain contain a diverse herbaceous taxa characteristic of modern sagebrush steppe, and a diverse fungal spore assemblage including the root symbiont Glomus and the dung fungus Sporormiella. These results show that sagebrush steppe was widespread on the western Snake River Plain 12 Ma, at least 5 Ma earlier than previously recognized.

The Pliocene Lost River found to west: Detrital zircon evidence of drainage disruption along a subsiding hotspot track

SHRIMP analysis of U/Pb ages of detrital zircons in twelve late Miocene to Pleistocene sand samples from six drill cores on the Snake River Plain (SRP), Idaho, suggests that an ancestral Lost River system was drained westward along the northern side of the SRP. Neoproterozoic (650 to 740 Ma, Cryogenian) detrital zircon grains from the Wildhorse Creek drainage of the Pioneer Mountains core complex, with a source in 695 Ma orthogneiss, and which are characteristic of the Big Lost River system, are found in Pliocene sand from cores drilled in the central SRP (near Wendell) and western SRP (at Mountain Home). In addition to these Neoproterozoic grains, fluvial sands sourced from the northern margin of the SRP contain detrital zircons with the following ages: 42 to 52 Ma from the Challis magmatic belt, 80 to 100 Ma from the Atlanta lobe of the Idaho batholith, and mixed Paleozoic and Proterozoic ages (1400 to 2000 Ma). In contrast, sands in the Mountain Home Air Base well (MHAB) that contain 155-Ma Jurassic detrital grains with a source in northern Nevada are interpreted to represent an integrated Snake River, with provenance on the southern, eastern and northern sides of the SRP. We propose that late Pliocene and early Pleistocene construction of basaltic volcanoes and rhyolitic domes of the Axial Volcanic Zone of the eastern SRP and the northwest-trending Arco Volcanic Rift Zone (including the Craters of the Moon volcanic center), disrupted the paleo-Lost River drainage, confining it to the Big Lost Trough, a volcanically dammed basin of internal drainage on the Idaho National Laboratory (INL). After the Axial Volcanic Zone and Arco Volcanic Rift Zone were constructed to form a volcanic eruptive and intrusive highland to the southwest, sediment from the Big Lost River was trapped in the Big Lost Trough instead of being delivered by surface streams to the western SRP. Today, water from drainages north of the SRP enters the Snake River Plain regional aquifer through sinks in the Big Lost Trough, and the water resurfaces at Thousand Springs, Idaho, about 195 km to the southwest. Holocene to latest Pliocene samples from drill core in the Big Lost Trough reveal interplay between the glacio-fluvial outwash of the voluminous Big Lost River system and the relatively minor Little Lost River system. A mixed provenance signature is recognized in fine-grained sands deposited in a highstand of a Pleistocene pluvial-lake system.

Intra-vent peperites related to the phreatomagmatic 71 Gulch Volcano, western Snake River Plain volcanic field, Idaho (USA)

The western Snake River Plain volcanic field in SW Idaho contains up to 400 basaltic vents and centers that produced lava shields, pahoehoe lava fields, scoria cones, and a great variety of phreatomagmatic volcanoes between late Miocene and middle Pleistocene time. Tephra deposits produced by phreatomagmatic eruptions are particularly well exposed in the walls of the Snake River canyon, where thick accumulations of pyroclastic rocks indicate widespread phreatomagmatic eruptive events throughout most of the volcanic history of the region. Previously, many of the phreatomagmatic deposits were considered to be the products of subaqueous eruptions that took place on the floor of one or more large freshwater intra-continental lakes. Recent field based observations confirm the presence of widespread phreatomagmatic pyroclastic rocks; however, some that had been interpreted as being subaqueous exhibit textural features that are more consistent with subaerial depositional environments. Intrusive and extrusive magmatic bodies with features associated with peperite formation have also been identified. Most of these peperites can be attributed to magma–sediment mixing in intra-crater/conduit or vent settings, and therefore they can only be used as widespread paleoenvironmental indicators with limitations to demonstrate magma and surface water (e.g. lake) non-explosive interaction. One of the studied sites (“71 Gulch Volcano”) was previously used to indicate the presence of a shallow lake. At this site there is clear field evidence that peperitic feeder dykes contacted muddy, sandy siliciclastic sediments forming globular peperite. The peperitic feeder dykes transition to pillowed, ponded lava up section. The ponded lavas are partially surrounded by a ~ 5-m-thick unit composed of gently dipping, dune bedded, volcanic glass shard-rich, unsorted, tuff and lapilli tuff containing abundant impact sags caused by volcanic lithics. We suggest that the 3D architecture of the erosional remnant of “71 Gulch Volcano” does not require the presence of a lake at the time of its formation; it is equally possible that that it represents a subaerial phreatomagmatic upper conduit — crater filling succession. This interpretation opens up many questions about the Mio/Pliocene evolution of SW Idaho, the timing of the volcanism, and its association with the evolution of the lacustrine systems in the region. In addition, re-evaluations of the volcanic features in SW Idaho have some general implications for the usage of phreatomagmatic pyroclastic rocks for paleoenvironmental reconstruction.

High-K alkali basalts of the Western Snake River Plain (Idaho): Abrupt transition from tholeiitic to mildly alkaline plume-derived basalts

Basaltic volcanism in the western Snake River Plain underwent an abrupt change circa ~ 700 ka to 900 ka, from low-K tholeiitic basalt and ferrobasalt to high-K transitional alkali basalt. The low-K tholeiitic basalts share major element, trace element, and isotopic characteristics with olivine tholeiites of the eastern Snake River Plain, and must have been derived by similar processes from similar sources. In contrast, the younger high-K alkali basalts share major element, trace element, and isotopic characteristics with plume-derived alkali basalts of ocean islands suites like Hawaii. We conclude that this abrupt transition reflects either or both the erosion of pre-existing mantle lithosphere in the wake of the Yellowstone–Snake River plume, or the depletion of this lithosphere in fusible components so that it no longer contributed to the overall mass flux of magma. The abruptness of the transition implies that it may have a catastrophic origin, such as lithospheric delamination caused by a Rayleigh–Taylor instability beneath the Idaho batholith.