Owyhee Plateau Research Articles

Lithospheric structure beneath the High Lava Plains, Oregon, imaged by scattered teleseismic waves

We compute high‐resolution seismic images from scattered wavefield to detect discontinuities beneath the High Lava Plains (HLP), using data recorded at a dense broadband array. Our images of the HLP and surrounding regions reveal (1) a prominent Moho discontinuity with varying depth, with thinnest crust of 35 km beneath the volcanic track, and thickened crust of ∼45 km beneath the Owyhee Plateau (OP); (2) distinct intracrustal velocity reversals beneath regions of pre‐2.0 Ma volcanism and within the OP; and (3) intermittent negative velocity discontinuities in the uppermost mantle beneath regions of Holocene volcanism and volcanic centers near Steens Mountain and Newberry volcano. These features exhibit remarkable similarity with those seen in the surface wave tomography and Ps receiver functions. We fail to find evidence for a ubiquitous regional lithosphere‐asthenosphere boundary (LAB). In concert with petrological constraints on the equilibration depths of primitive basaltic melts, our results suggest that the present‐day HLP mantle lithosphere is thin or absent, perhaps a consequence of episodes of extensive mantle inflow, lithospheric extension, and possibly melting induced by rapid slab rollback and trench retreat. It remains possible, however, that strong E‐W seismic anisotropy reported across this region may reduce the effective S‐wave velocity contrast to render the LAB less detectable. In contrast, the Owyhee Plateau exhibits a clear LAB, consistent with it being a block of older preexisting lithosphere. Our images demonstrate the complexity of mantle dynamics in the Cascadian back‐arc and the close casual link between subduction‐related processes and the origin of HLP volcanism.

Crust and upper mantle structure beneath the Pacific Northwest from joint inversions of ambient noise and earthquake data

We perform a joint inversion of phase velocities from both earthquake and ambient noise induced Rayleigh waves to determine shear wave velocity structure in the crust and upper mantle beneath the Pacific Northwest. We focus particularly on the areas affected by mid‐Miocene to present volcanic activity. The joint inversion, combined with the high density seismic network of the High Lava Plains seismic experiment and data from the EarthScope Transportable Array, provides outstanding resolution for this area. In Oregon, we find that the pattern of low velocities in the crust and uppermost mantle varies between the High Lava Plains physiographic province and the adjacent northwestern Basin and Range. These patterns may be due to the presence of the Brothers Fault Zone which separates the clockwise rotating northwest Basin and Range from the relatively undeformed areas further north. Further to the east, the Owyhee Plateau, Snake River Plain (SRP) and northeastern Basin and Range are characterized by high crustal velocities, though the depth extent of these fast wave speeds varies by province. Of particular interest is the mid‐crustal high velocity sill, previously only identified within the SRP. We show this anomaly extends significantly further south into Utah and Nevada. We suggest that one possible explanation is lateral crustal extrusion due to the emplacement of the high density mafic mid‐crustal sill structures within the SRP.

Constraints on the causes of mid‐Miocene volcanism in the Pacific Northwest US from ambient noise tomography

We use data from the 118‐station High Lava Plains (HLP) seismic experiment together with other regional broadband seismic data to image the 3D shear wave velocity structure in the Pacific Northwest using ambient noise tomography. This extensive data set allows us to resolve fine‐scale crustal structures throughout the HLP area in greater detail than previous studies. Our results show 1) a high velocity cylinder in the crust and average velocities in the upper mantle beneath the Owyhee Plateau; 2) a mid‐crustal high velocity anomaly along the Snake River Plain that also extends south into Nevada and Utah; 3) a low velocity anomaly directly beneath Yellowstone throughout the crust; and 4) low velocities beneath the HLP both in the crust and uppermost mantle, possibly indicating very thin or absent mantle lithosphere in the area. These features provide important constraints on possible models for Miocene to recent volcanism in the Pacific Northwest.

Crustal structure beneath the High Lava Plains of eastern Oregon and surrounding regions from receiver function analysis

[1] We analyze teleseismic P-to-S receiver functions to image crustal structure beneath the High Lava Plains (HLP) of eastern Oregon and surrounding regions. Coverage from 206 broadband seismic stations provides the first opportunity to resolve variations in crustal composition, thickness, and heterogeneity on scales of a few km in depth and tens of km laterally across the HLP region. We utilize both H − κ stacking and a new Gaussian-weighted common conversion point stacking technique. We find crust that is ≥40 km thick beneath the Cascades, Idaho Batholith, and Owyhee Plateau and thinner (∼31 km) crust beneath the HLP and northern Great Basin. Low Poisson's ratios of ∼0.240 characterize the granitic crust beneath the Idaho Batholith, while the Owyhee Plateau exhibits values of ∼0.270, typical of average continental crust. The Owyhee Plateau is a thick simple crustal block with distinct edges at depth. The western HLP exhibits high average values of 0.304, typical for regions of widespread basaltic volcanism. Combined with other geological and geophysical observations, the areas of abnormally high Poisson's ratios (∼0.320) and low-velocity zones in the crust beneath north-central and southern Oregon are consistent with the presence of partial melt on either side of the HLP trend, suggesting a central zone where crustal melts have drained to the surface, perhaps enabled by the Brothers Fault Zone. Thicker crust and an anomalous N-S band of low Poisson's ratios (∼0.252) skirting the Steens Mountain escarpment is consistent with residuum from a midcrustal magma source of the massive flood basalts, supporting the view of extensive mafic underplating and intraplating of the crust from Cenozoic volcanism.

Relationships between landscape patterns and fire occurrence within a successional gradient in sagebrush steppe–juniper woodland

Expansion of western juniper (Juniperus occidentalis Hook. var. occidentalis) has altered vegetation composition, fire behaviour and fire potential throughout south-western Idaho and eastern Oregon. Utilising GIS-derived products and fire-simulation software, the influence of the spatial arrangement of different woodland developmental stages on simulated surface fire occurrence was evaluated. Custom fuel models and a recent vegetation map processed in FARSITE under moderate fire conditions were used to create a fire-occurrence grid in three sixth-order watersheds on the Owyhee Plateau of south-western Idaho. Landscape pattern metrics were selected to quantify the spatial arrangement of plant communities within a neighbourhood around points from each successional stage randomly placed within each watershed. Linear regression analysis of fire occurrence and each of the selected landscape metrics was compared for four successional stages of western juniper encroachment to assess the effect of landscape-scale vegetation arrangement on fire occurrence. The landscape structure had little influence on whether an early-successional area burns in a surface fire, whereas the surrounding landscape structure influenced whether a late-successional or mature woodland area burned. Landscape metrics that showed significance in late-successional and mature woodland stages include patch density, mean area and Simpson’s diversity.

Quantifying successional rates in western aspen woodlands: Current conditions, future predictions

Stands of quaking aspen ( Populus tremuloides) rank among the most biologically diverse plant communities across the intermountain region of western North America. Marked declines of aspen have occurred in recent decades, likely due to a combination of effects from changes in fire regimes, herbivory, climate (e.g. drought), and interspecific competition with conifer species. However, it is poorly understood how the effects of these factors are manifested at a landscape scale over decadal time periods. Analysis of field data combined with topographic information collected across the 500,000 ha Owyhee Plateau in southwestern Idaho revealed that aspen in the area occur in three different biophysical settings; First, aspen stands exist at high altitudes on south-facing slopes where local conifer species are not likely to occur because of limiting temperature or precipitation levels under current climate conditions. In these areas aspen is the potential vegetation type rather than conifers. Second, aspen grow on anomalously wet microsites (e.g. near springs), and third, aspen grow within upland mixed aspen/conifer stands, which are experiencing rapid rates of conifer establishment. Based on a paired t-test ( α = 0.05) we conclude that stands growing on wet microsites show significantly slower successional rates of conifer establishment relative to upland aspen stands. We developed a conceptual state-and-transition model for upland aspen/conifer stands occurring across a range of topographic positions. We then parameterized the model using extensive field data in the vegetation dynamics computer simulation model Vegetation Dynamics Development Tool (VDDT), and examined the current and future aspen distribution under varying fire regimes. Model results indicate that average fire return intervals of 50–70 years are desirable for maintenance of aspen in upland areas where conifers are present. Under the current fire regime in the area many upland aspen/conifer stands will likely be lost within 80–200 years. Thresholds for the effect of conifer encroachment and browsing on aspen regeneration identified through this research are similar to those described by others across the West. We therefore suggest that the results presented for the Owyhee Plateau are likely applicable to semi-arid aspen woodlands across the American West where succession to conifers is a cause of aspen decline.

Mantle source provinces beneath the Northwestern USA delimited by helium isotopes in young basalts

We report new He, Nd and Sr isotope results for basalts from the northwestern United States. The new 3He/ 4He results for olivine phenocrysts in basalts from the eastern Snake River Plain (SRP), the Owyhee Plateau (OP) and the Oregon High Lava Plains (HLP), together with published He isotope data for Yellowstone and the Cascades volcanic arc, delineate distinct mantle sources for each of these sub-provinces. All basalts from the eastern SRP (8 Quaternary localities plus 1 Miocene locality) have 3He/ 4He ratios higher than observed in normal mid-ocean ridge basalts, but overlapping with ranges observed in hotspot-related oceanic islands. For a lateral distance of some 400 km along the SRP, 3He/ 4He ranges from ~ 11 R A in the west to > 19 R A adjacent to Yellowstone. Such high ratios have not been observed elsewhere in the western U.S., and are consistent with the presence of a mantle plume. The lateral gradient in 3He/ 4He suggests that the proportion of plume-derived He decreases westward, but this interpretation is complicated by possible addition of crustal helium during open-system crystal fractionation in some SRP basaltic magmas. Although crustal contamination may modulate 3He/ 4He in basalts along the SRP, the effect is not strong and it does not obscure the elevated 3He/ 4He mantle source signature. In contrast, young basalts from the HLP and the OP have 3He/ 4He values of 8.8–9.3 R A, within the range for mid-ocean ridge basalts; these data reflect a shallow asthenospheric source with no discernible influence from the Yellowstone hotspot. Basalts from Newberry volcano have slightly lower 3He/ 4He (7.6–8.3 R A), within the range for other Cascades arc lavas (7.0–8.4 R A). Three alternative explanations are possible for the origin of the high 3He/ 4He signature along the SRP: (1) multi-component mixing of (a) magmas and/or CO 2-rich fluids derived from plume mantle having high 3He/ 4He, (b) continental lithosphere having low 3He/ 4He, and (c) shallow asthenospheric mantle (MORB source); (2) a mantle plume beneath Yellowstone that has an unusual combination of He, Nd and Sr isotope characteristics; or (3) a continental lithospheric mantle that experienced ancient enrichment of 3He relative to (U + Th). The isotope relations between He–Nd and He–Sr, along with other considerations, generally favor the first explanation, but the other possibilities cannot be ruled out at the present time.

A spatially explicit model to predict future landscape composition of aspen woodlands under various management scenarios

Quaking aspen ( Populus tremuloides) is declining across the western United States. Aspen habitats are among the most diverse plant communities in this region and loss of these habitats can result in shifts in biodiversity, productivity, and hydrology across a range of spatial scales. Western aspen occurs on the majority of sites seral to conifer species, and long-term maintenance of these aspen woodlands requires periodic fire. Over the past century, fire intervals, extents, and intensities have been insufficient to regenerate aspen stands at historic rates; however the effects of various fire regimes and management scenarios on aspen vegetation dynamics at broad spatial and temporal scales are unexplored. Here we use field data, remotely sensed data, and fire atlas information to develop a spatially explicit landscape simulation model to assess the effects of current and historic wildfire regimes and prescribed burning programs on landscape vegetation composition across two mountain ranges in the Owyhee Plateau, Idaho. Model outputs depict the future structural makeup and species composition of the landscape at selected time steps under simulated management scenarios. We found that under current fire regimes and in the absence of management activities, loss of seral aspen stands will continue to occur over the next two centuries. However, a return to historic fire regimes (burning 12–14% of the modeled landscape per decade) would maintain the majority of aspen stands in early and mid seral woodland stages and minimizes the loss of aspen. A fire rotation of 70–80 years was estimated for the historic fire regime while the current fire regime resulted in a fire rotation of 340–450 years, underscoring the fact that fire is currently lacking in the system. Implementation of prescribed burning programs, treating aspen and young conifer woodlands according to historic fire occurrence probabilities, are predicted to prevent conifer dominance and loss of aspen stands.

Miocene to Holocene landscape evolution of the western Snake River Plain region, Idaho: Using the SHRIMP detrital zircon provenance record to track eastward migration of the Yellowstone hotspot

We report new U-Pb detrital zircon sensitive high-resolution ion microprobe (SHRIMP) age data (702 grains) from 13 samples collected from Miocene to Holocene sedimentary deposits in the western Snake River Plain region. These samples effectively show that modern stream sediments of the Snake River system reliably and repeatedly record the detrital zircon age populations that are present as sources in their drainage basins across the Cordilleran thrust belt and Basin and Range Province. We use this framework and the provenance of Neogene sedimentary rocks in the region to test the effect of the migrating Yellowstone hotspot on regional drainage patterns in southern Idaho since the middle Miocene. Our results indicate that Neogene paleodrainages were fi rst directed radially away from the tumescent Yellowstone highland, then subsequently reversed their fl ow toward the subsiding Snake River Plain basin. This occurred in east-progressing time-constrained intervals starting at 16 Ma. In northern Nevada, the drainage divide is represented by a northeast-trending, southeast-migrating crest of high topography. Specifi cally, middle to late Miocene (16‐ 10 Ma) sedimentary deposits of the western Snake River Plain and Oregon-Idaho graben contain early to middle Eocene (52‐42 Ma) detrital zircon populations sourced in Challis magmatic rocks north of the Snake River Plain. Middle Jurassic (160 Ma) and middle to late Eocene (42‐35 Ma) detrital zircons, sourced from rocks in northern Nevada, are not present. Late Eocene detrital zircons from Nevada are present in two younger than 7 Ma sedimentary units of the Idaho Group along the Oregon-Idaho border. This indicates that by the late Miocene, southeastward headward erosion of the paleo‐Owyhee River into the Owyhee Plateau had captured drainage from north-central Nevada and directed it northwestward toward the subsiding western Snake River Plain. The modern Owyhee Plateau is still a topographic high, in contrast to the modern Snake River Plain, suggesting that lowering of the regional Snake River Plain base level, rather than crustal subsidence, drove stream capture. By the late Pliocene (3 Ma), Middle Jurassic detrital zircons are recorded in the Glenns Ferry Formation and Tuana Gravel of the central Snake River Plain, suggesting that surface subsidence reversed the fl ow direction of paleo‐Salmon Falls Creek from southward into Nevada to northward toward Idaho. Miocene strata of the western Snake River Plain lack recycled Proterozoic detrital zircons that are ubiquitous in sedimentary rocks of the central and southeast Idaho thrust belts. Such detrital zircons appear on the central and western Snake River Plain in early Pliocene to Holocene (4‐0 Ma) deposits. This records capture of drainage from the eastern Snake River Plain. The Yellowstone hotspot controlled the east-migrating continental divide, in the wake of which formed the western-draining, and progressively eastward-collecting, Snake River system.

Duripans of Southwestern Idaho: Polygenesis during the Quaternary Deduced through Micromorphology

AbstractThere is uncertainty regarding the pedogenesis of duripans on the Owyhee Plateau of southwestern Idaho. Micromorphology was used to deduce the role of polygenesis in their formation. Data suggest that during the Pleistocene, duripans were cyclically subaerially exposed, then recovered by eolian dust. Two lines of evidence indicate past duripan exposure. First, sinuous laminar fabrics with lenticular voids, which occur within most duripan plates, are similar to the fabrics produced in arid regions by the action of lichens and cyanobacteria at the soil surface. Second, a laterally extensive air‐fall tephra layer in the middle of a duripan plate was deposited at one sampling location, indicating the duripan or protoduripan was exposed at the ground surface. A suggested model to explain the data is as follows: (i) during cold stages of the Quaternary, periglacial processes, possibly assisted by eolian deflation, caused the truncation of loose soil material overlying the duripan; (ii) cryptogamic organisms then colonized the exposed duripan surface and produced distinctive fabrics; (iii) in response to the increased production and delivery of eolian dust during interglacials, the duripan was recovered. Pedogenesis during interglacial times resulted in precipitation of CaCO3, thereby thickening the upward‐developing duripan and preserving micromorphological signatures of subaerial exposure. At higher elevations in this region, large‐scale removal of unconsolidated soil may have occurred during full glacial stages of the middle and earlier Pleistocene. Maximal development of duripans may require polygenesis.

In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow, Owyhee County, SW Idaho

The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography. However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices. Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features, are probably true effusive lava flows.

Morphology and maturation of melt inclusions in quartz phenocrysts from the Badlands rhyolite lava flow, southwestern Idaho

Morphology and maturation of melt inclusions in quartz phenocrysts from the Badlands rhyolite lava flow, southwestern Idaho

How voluminous rhyolite lavas mimic rheomorphic ignimbrites: Eruptive style, emplacement conditions, and formation of tuff-like textures

Silicic lavas of large volume and relatively great areal extent are not uncommon in association with the tracks of mantle plume hotspots across the continents. These units are commonly eroded and/or poorly exposed, and their interpretation as lavas has been controversial. The morphology of a 15 km-3 calc-alkalic rhyolitic unit (75 to 77 wt% SiO 2 ) on the Owyhee Plateau of southwestern Idaho shows that it is a true lava flow emplaced by effusion. The eruption tapped a zoned magma chamber with a small volume of nearly aphyric magma underlain by a much larger volume of less-evolved, crystal-rich magma. Magmatic temperatures were about 830 ° C and H 2 O contents were about 2 to 2.75 wt%. Eruptive activity first involved explosive venting of the nearly aphyric magma, followed by effusion of the crystal-rich magma. Movement of the lava over and around the tephra deposits formed large, discrete lava lobes; one flow lobe reached 9 km from the vent. Though the unit is a true lava flow, a diverse suite of fragmental textures, including rock that closely resembles welded tuff, formed by lava flow processes during the time the flow was active.

DURIPANS OF THE OWYHEE PLATEAU REGION OF IDAHO

Mineralogical and micromorphic techniques were used to investigate secondary minerals that occur in duripans (Durargids and Durixeralfs) on the Owyhee Plateau region of southwestern Idaho. Opaline silica and sepiolite were the principal secondary minerals found. Opaline silica exhibited three morphogenetic forms. Clearopal, consisting of opal-A with included calcite and unknown crystallites, occurred throughout the duripan, principally as pseudomorphic replacements after calcite pendants. Dirty-opal, a gray, semitran-sparent and nearly x-ray amorphous material, was found in highly altered cores of complex pedogenic concretions. Some opaline silica occurred in a web-like arrangement intimately mixed with calcite and was located in the matrix among loess agglomerates, in the laminar cap and in loess pedotubules. The opal in this microgranular calcite-silica was opal-A with included quartz and appeared to form, in part, by silica supplied in situ from alteration of primary aluminosilicate minerals. Transmission electron microscopic examination of colloidal silica revealed the presence of two types of coatings. Gray-gel was visually homogeneous and semielectron opaque whereas mottled-gel was -less electron opaque and contained many, minute, more electron opaque spheroids. The reorganization of these gels seems to control the formation of opaline silica spheres. In addition, most opaline silica spheres exhibited bubble-like substructures heretofore unrecognized. The presence of poorly crystalline sepiolite was indicated by x-ray diffraction. Imaged with the transmission electron microscope, the sepiolite exhibited a hierarchical structure. At the highest scale of magnification, laths with a crosssectional area of 13 by 23 nm were evident. The laths were organized in bundles. The most visually distinctive bundles were 950 by 1200 nm in length and lacked gel-like coatings. Other bundles were less distinct, more curvilinear, and exhibited less parallel orientation or mesh-like arrangements. These laths and fibers were from 250 to 375 nm in length, coated wtih structurally amorphous material resembling the grey-gel, and surficially sprinkled with opaline silica spheres and spherical units of mottled-gel. In the duripans studied, sepiolite was largely limited to pedogenic concretions in which it occurred as acicular crystals that radiated outward from concretionary laminae of opaline silica. This finding and the association of opal spheres with sepiolite laths suggest that some sepiolite forms via the addition of Mg to opal.

Extension of the Yellowstone plateau, eastern Snake River Plain, and Owyhee plateau

Formation of the late Cenozoic volcanic province comprising the Owyhee plateau, eastern Snake River Plain, and Yellowstone plateau has been accompanied by east-northeast-directed crustal extension. A new vector of 45 mm/yr, N56{degree}E for the migration of silicic volcanism across the volcanic province is calculated. If migration of volcanism reflects west-southwest continental drift over a mantle plume, a zone of crustal extension must separate the volcanic province from the more slowly moving North American craton. Space-time relations of basin fill in the adjacent Basin and Range province provide evidence for a zone of extension, about 125 km wide, coincident with and east of coeval silicic volcanism. Since 16 Ma, the zone of extension has migrated along with silicic volcanism, maintaining its position between the province and the unextended craton.

Causes of Juniper Invasion in Southwestern Idaho

Invasion of western juniper into vegetation dominated by mountain big sagebrush and perennial bunchgrass on the Owyhee Plateau of southwest Idaho appears to be directly related to cessation of periodic fires. Evidence from adjacent climax juniper stands indicates that fires were frequent for at least several hundred years preceding white settlement. Fires have been much less frequent during the past century due to active fire control, development of roads and other fire barriers, and reduced fuel because of heavy grazing and a shift towards decreased precipitation. Physical and biotic factors affecting the establishment of juniper, seed dispersal mechanisms, and the fire history of the study area were investigated. Results indicated that range condition as such had a negligible effect on juniper establishment. Juniper seedlings became established most readily on areas supporting well—developed herbaceous and shrubby vegetation. Seed dispersal was primarily localized, and accomplished by gravity and disturbance by animal trampling. Abundant evidence of fire in the form of charred stumps and fire scars on living trees was found throughout the study area. Old juniper stands are confined to rocky ridges where understory vegetation is sparse and fires less intense. Juniper was apparently kept out of the denser vegetation of deeper soils by more intense fires. Most herbaceous and shrubby species survived this treatment due to greater tolerance to fire, or rapid reproduction from seed.

GLASS-BEAD SILICA DETERMINATION FOR A SUITE OF VOLCANIC ROCKS FROM THE OWYHEE PLATEAU, OREGON

Silica content of fine-grained volcanic rock may be estimated by means of a curve that correlates index of refraction of artificial glass beads made from the rock with chemically determined silica. Such a correlation curve for a suite of volcanic rocks from the Owyhee Plateau, Oregon, is presented. It is essentially indistinguishable from a curve based upon several other volcanic suites from western North America.