Columbia River Flood Basalts Research Articles

Rapid eruption of the Columbia River flood basalt and correlation with the mid-Miocene climate optimum.

Flood basalts, the largest volcanic events in Earth history, are thought to drive global environmental change because they can emit large volumes of CO2 and SO2 over short geologic time scales. Eruption of the Columbia River Basalt Group (CRBG) has been linked to elevated atmospheric CO2 and global warming during the mid-Miocene climate optimum (MMCO) ~16 million years (Ma) ago. However, a causative relationship between volcanism and warming remains speculative, as the timing and tempo of CRBG eruptions is not well known. We use U-Pb geochronology on zircon-bearing volcanic ash beds intercalated within the basalt stratigraphy to build a high-resolution CRBG eruption record. Our data set shows that more than 95% of the CRBG erupted between 16.7 and 15.9 Ma, twice as fast as previous estimates. By suggesting a recalibration of the geomagnetic polarity time scale, these data indicate that the onset of flood volcanism is nearly contemporaneous with that of the MMCO.

Is the Satpura “Horst” in fact a Compressional Uplift?

ABSTRACT The >600-km-long Satpura mountain range of central India is usually considered as a horst, separated by normal faults from the Tapi and Narmada grabens. Numerous geological observations and seismicity suggest that a fundamentally different scenario is possible: the arcuate Satpura range may be a large-scale compressional uplift, riding over a north-dipping Satpura-Tapi boundary thrust, akin to the thrust-bounded regional-scale anticlines of the Columbia river flood basalt province. Erosion of the southern front of the Satpuras produces a large fault-line scarp, an illusory normal fault; close parallels exist in the Himalaya and Kachchh. The Satpura-Tapi boundary thrust is blind, buried under the post-Deccan Tapi alluvium, but periodically active, and the Tapi valley has significant seismic risk.

Western US volcanism due to intruding oceanic mantle driven by ancient Farallon slabs

The origin of late Cenozoic intraplate volcanism over the western United States is debated. One important reason is the lack of a clear understanding of the mantle dynamics during this volcanic history. Here we reconstruct the mantle thermal states beneath North America since 20 million years ago using a hybrid inverse geodynamic model with data assimilation. The model simultaneously satisfies the past subduction kinematics, present mantle tomographic image and the volcanic history. We find that volcanism in both the Yellowstone volcanic province and the Basin and Range province corresponds to a similar eastward-intruding mantle derived from beneath the Pacific Ocean and driven mostly by the sinking Farallon slab below the central-eastern United States. The hot mantle that forms the Columbia River flood basalt and subsequent Yellowstone–Newberry hotspot tracks first enters the western United States through tears within the Juan de Fuca slab. Subsequent coexistence of the westward asthenospheric flow above the retreating Juan de Fuca slab and eastward-propagating mantle beyond the back-arc region reproduces the bifurcating hotspot chains. A similar but weaker heat source intrudes below the Basin and Range around the southern edge of the slab, and can explain the diffuse basaltic volcanism in this region. According to our models, the putative Yellowstone plume contributes little to the formation of the Yellowstone volcanic province. Volcanism in the western US may result from warm oceanic mantle beneath the Pacific Ocean being drawn eastwards by mantle flow induced by the sinking of Farallon slabs, according to numerical model simulations.

Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province

Alteration, mass analysis, and magmatic compositions of the Sentinel Bluffs Member, Columbia River flood basalt province

Dissolution and surface roughening of Columbia River flood basalt at geologic carbon sequestration conditions

Carbon storage in basalt reservoirs can mitigate CO2 emissions to the atmosphere that contribute to climate change. For basalt reservoirs, CO2 injection leads to Ca2+, Fe2+, and Mg2+-rich solutions that may result in carbonate precipitation for long-term stable carbon sequestration. Examination of basalt surfaces as dissolution progresses can offer important information about which minerals are dissolving, the timing and sequence of dissolution, and the effects these processes have on surface roughness and morphology. We carried out two series of experiments using two polished Columbia River flood basalt samples in CO2-rich water at 150°C and 100bar to observe the physical and geochemical changes during dissolution. One series reacted a sample over short time increments, while the other series examined a separate sample over longer time increments. Scanning electron microscopy, 2D profilometer analysis, and 3D laser confocal microscopy were combined with ICP-MS to characterize dissolution. Dissolution resulted in pitting, dissolution along fractures and grain boundaries, and an increase in species concentrations in the bulk solution over incremented reaction times. Based on these observations, early dissolution of olivine grains contribute Mg2+ and Fe2+ to aqueous solution in initial stages (<1week at a pH<4), while slower continuous dissolution of Ca-rich pyroxene contributes Mg2+, Fe2+, and Ca2+ to the bulk solution over a longer period of time. The complete dissolution of olivine grains resulted in pits up to 200μm deep. Dissolution of plagioclase and matrix was slower and resulted in the formation of micro-sized textures (<10μm). Following 1–2months of reaction, the surface roughness parameters (mean and root mean squared) increased by factors of 42 and 28, respectively, while surface area of the flood basalt increased 20% relative to the starting polished surface. The results of this study indicate 1) pyroxene is the sustaining contributor of divalent metal cations during dissolution of basalt and 2) the limited connectivity limits of olivine and pyroxene grains the exposure of new reactive surface areas.

The influence of volcanic rocks on the characterization of Rosebank Field – new insights from ocean-bottom seismic data and geological analogues integrated through interpretation and modelling

Abstract During Late Paleocene–Early Eocene times, the modern Rosebank structure was located at the juxtaposition of the easterly advancing Flett volcanic system and the northerly prograding Flett delta. As a result, the Rosebank reservoir sandstones are interstratified with volcanic and volcaniclastic rocks, offering challenges for reservoir imaging, depth prediction and reservoir characterization. These challenges have driven the application of Ocean Bottom Node (OBN) seismic technology. OBN data have yielded improved velocity models for depth conversion, better reservoir definition and key insights to aid the modelling of sand distribution from seismic attributes. Spectral decomposition of the OBN seismic data has facilitated the extraction of distinct volcanic subunits, whilst spectral enhancement has enabled visualization of complex stacking patterns within individual igneous layers. To complement the seismic analysis, detailed geological analogue studies have been undertaken in volcanic provinces such as the Palaeogene volcanic district of SE Greenland and the Columbia River Flood Basalt Province, USA. No single outcrop provides a definitive analogy to Rosebank, but each offers insights that provide an important link to understanding and managing the main subsurface uncertainties associated with field development. Integration of these multiple workflows have improved the reservoir characterization and provided the foundation for the optimization of the field development plan.

A new late Hemingfordian vertebrate fauna from Hawk Rim, Oregon, with implications for biostratigraphy and geochronology

ABSTRACTThe Hemingfordian North American Land Mammal Age is not well sampled, especially in the Pacific Northwest. Here we present both a description of a new fauna and two radiometric dates, 16.26 Ma and 16.44 Ma, constraining the Hawk Rim locality of central Oregon. Hawk Rim represents the first diverse late Hemingfordian fauna in the Northwest and is one of the stratigraphically lowest fossiliferous outcrops of the Mascall Formation. Much of Oregon was blanketed by Columbia River Flood Basalts during late Hemingfordian time, limiting not only outcrops but places for organisms to have survived. The site yields a taxonomically rich fauna sharing strong faunal similarity with the type locality of the Mascall but also containing taxa new to the formation and region. We describe occurrences of five genera of Artiodactyla, four genera of Perissodactyla, three genera of Rodentia, and six genera of Carnivora, with all but three new occurrences for the Hemingfordian of the Pacific Northwest. In particular, the carnivore fauna extends the geographic and temporal range of several carnivorans, and we describe a new hypercarnivorous mustelid, Watay tabutsigwii.http://zoobank.org/urn:lsid:zoobank.org:pub:B415069F-8BE6-4629-A872-5A640A644210SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVPCitation for this article: McLaughlin, W. N. F., S. S. B. Hopkins, and M. D. Schmitz. 2016. A new late Hemingfordian vertebrate fauna from Hawk Rim, Oregon, with implications for biostratigraphy and geochronology. Journal of Vertebrate Paleontology. DOI: 10.1080/02724634.2016.1201095.

Giant plagioclase basalts: Continental flood basalt–induced remobilization of anorthositic mushes in a deep crustal sill complex

Giant plagioclase basalts, rich in plagioclase megacrysts commonly 5 cm long, form widespread lava flows and dikes in the Deccan Traps continental flood basalt province. They also occur in the Scottish Hebrides, Columbia River, and Emeishan continental flood basalt provinces, and closely similar rocks are known from Archean and Proterozoic terrains and oceanic settings. The giant plagioclase basalts have usually been interpreted as end products of magmatic cycles operating in high-level chambers, and taken as the boundaries of stratigraphic formations. A new model is proposed in which the plagioclase megacrysts grow over long (15 k.y.) time periods in a deep crustal (possibly Moho-level) sill complex, which forms tens to hundreds of thousands of years before the continental flood basalt eruptions. Megacryst flotation produces anorthositic mushes containing ferrodioritic interstitial liquids. Giant plagioclase basalts form when these components (and any melt from the crust heated by the sill complex) are remobilized by a newly injected tholeiitic magma batch, and brought to the surface or high crustal levels by dikes. Giant plagioclase basalts are thus mixed, heterogeneous rocks from the deep continental crust. The new model explains a range of geological, petrological, mineralogical, and geochemical data on the giant plagioclase basalts. Formation and remobilization of anorthositic mushes appear to be fundamental processes producing highly feldspar-phyric basalts over a wide range of tectonic settings. An important corollary of the new model is that anorthosites have continued to form well after the Precambrian, on a smaller scale, during Phanerozoic continental flood basalt events. Therefore, continental flood basalt provinces should be recognized as sites of anorthosite formation in the deep continental crust.

Geology of the Mid-Miocene Rooster Comb Caldera and Lake Owyhee Volcanic Field, eastern Oregon: Silicic volcanism associated with Grande Ronde flood basalt

The Lake Owyhee Volcanic Field (LOVF) of eastern Oregon consists of rhyolitic caldera centers and lava fields contemporaneous with and spatially related to Mid-Miocene Columbia River flood basalt volcanism. Previous studies delineated two calderas in the southeastern part of LOVF near Owyhee Reservoir, the result of eruptions of two ignimbrites, the Tuff of Leslie Gulch and the Tuff of Spring Creek. Our new interpretation is that these two map units are differentially altered parts of a single ignimbrite produced in a major phreatomagmatic eruption at ~15.8Ma. Areas previously mapped as Tuff of Spring Creek are locations where the ignimbrite contains abundant clinoptilolite±mordenite, which made it susceptible to erosion. The resistant intracaldera Tuff of Leslie Gulch has an alteration assemblage of albite±quartz, indicative of low-temperature hydrothermal alteration. Our new mapping of caldera lake sediments and pre- and post-caldera rhyolitic lavas and intrusions that are chemically similar to intracaldera Tuff of Leslie Gulch point to a single ~20×25km caldera, which we name the Rooster Comb Caldera. Erosion of the resurgently uplifted southern half of the caldera created dramatic exposures of intracaldera Tuff of Leslie Gulch cut by post-caldera rhyolite dikes and intrusions that are the deeper-level equivalents of lava domes and flows that erupted into the caldera lake preserved in exposures to the northeast.The Rooster Comb Caldera has features in common with more southerly Mid-Miocene calderas of the McDermitt Volcanic Field and High Rock Caldera Complex, including formation in a basinal setting shortly after flood basalt eruptions ceased in the region, and forming on eruption of peralkaline ignimbrite. The volcanism at Rooster Comb Caldera postdates the main activity at McDermitt and High Rock, but, like it, begins ~300ky after flood basalt volcanism begins in the area, and while flood basalts don't erupt through the silicic focus, are contemporaneous with the latest stages of eruptions nearby. High Rock and McDermitt rhyolites are associated with propagation of Steens Basalt dikes to the south, and LOVF rhyolites with later propagation of Grande Ronde Basalt dikes to the north and north-northwest.

Crustal seismic structure beneath the source area of the Columbia River flood basalt: Bifurcation of the Moho driven by lithosphere delamination

AbstractThe source area of the ~16 Ma Columbia River flood basalt eruptions originated beneath the Wallowa Mountains in northeast Oregon, with a distinct circular pattern of topographic uplift. Teleseismic receiver functions reveal two layers between latitude 45.5° and 46.5° beneath north of the Wallowas, one at 25 km depth and the other one at 45 km depth. A new full‐wave ambient noise tomography model shows a circular anomaly, which is seismically fast in the upper crust and slow from lower crust to uppermost mantle in comparison with the surroundings, coincident with the circular pattern of the Wallowas. The seismic structures suggest that delamination of the Farallon lithosphere initiated the basalt eruptions and, consequently, modified the lowermost crust, forming a new shallow Moho. The Farallon slab is probably detached directly beneath the Wallowas while being maintained at the northern edge of the Wallowas, corresponding to the deeper interface.

Extrinsic forcing of plant ecosystems in a large igneous province: The Columbia River flood basalt province, Washington State, USA

Volcanism associated with large igneous provinces (LIPs) has been implicated in both global climate and environmental change. To determine the impact of LIP volcanism on plant ecology we have examined plant community succession in sedimentary interbeds of the Columbia River flood basalt province (CRBP; northwest United States). Interbasaltic vegetation is characterized by primary succession communities that inhabit fresh lava surfaces until terminated by the next eruptive event, and it is assumed that longer volcanic hiatuses should lead to more mature plant communities. This expected succession trajectory is contradicted by palynological data that show that seral succession declines during the phase of waning CRPB volcanism and prolonged interbed intervals. Frequent volcanic activity and increased deposition of Snake River Plain hotspot ashes during this phase resulted in ecological disturbance of intralava field vegetation. Together with geochemical proxies from interbed sediments, this suggests that CRBP flora was largely driven by extrinsic forcing, and implies that LIP volcanism of similar scale and magnitude to that of the CRBP had a limited environmental impact. This study supports the theory that past biotic extinctions were triggered by numerous factors rather than a single geological event.

Isotopically diverse rhyolites coeval with the Columbia River Flood Basalts: evidence for mantle plume interaction with the continental crust

AbstractThe Columbia River Flood Basalts (CRB) of the northwestern USA are coeval with eruptions of several thousand km3 of rhyolite. A broad survey of major phenocryst oxygen isotopes and of O and Hf isotopes in zircons from these rhyolites reveals significant diversity in inferred δ18Omelt values, ranging from +1.9 to +10.5‰ (SMOW), and in zircon Hf isotope compositions, which range from εHf = −39 to +9. This newly identified isotopic diversity shows that the syn‐CRB rhyolites were derived from high‐percentage melting of the crust. Low‐δ18O rhyolites, which fingerprint the melting of hydrothermally altered crust, are concentrated at the edge of the North American craton. This suggests that the conditions of crustal heating, faulting, and hydrothermal alteration required for the production of these rhyolites were concentrated there by the contrasts in crustal thickness and rheology associated with the boundary between the North American craton and younger accreted terranes.

S.P. Reidel, V.E Camp, M.E. Ross, J.A. Wolff, B.S. Martin, T.L. Tolan, and R.E. Wells (eds): The Columbia River flood basalt province

S.P. Reidel, V.E Camp, M.E. Ross, J.A. Wolff, B.S. Martin, T.L. Tolan, and R.E. Wells (eds): The Columbia River flood basalt province

Intracanyon basalt lavas of the Debed River (northern Armenia), part of a Pliocene–Pleistocene continental flood basalt province in the South Caucasus

Late Pliocene to Early Pleistocene (~3.25–2.05Ma), 200–400m thick basalt lavas outcrop in the South Caucasus region, including the Kars–Erzurum Plateau (northeastern Turkey), the Javakheti Plateau (Georgia–Armenia), and the Lori Plateau (northern Armenia). These fissure-fed, rapidly erupted fluid lavas filled pre-existing river valleys over many tens of kilometres. The basalts exposed in the Debed River canyon, northern Armenia, are ~200m thick and of three morphological types: (1) basal pillow basalts and hyaloclastites, overlain by (2) columnar-jointed pahoehoe sheet flows, in turn overlain by (3) slabby pahoehoe and rubbly pahoehoe flows. The lower and middle lavas show evidence for damming of river drainage, like many lavas of the Columbia River flood basalt province, Scotland, Ireland, and Iceland. There is also evidence for syn-volcanic faulting of the early lavas. Related basalts also outcrop in the Gegham Uplands and the Hrazdan River basin in Armenia. This 3.25–2.05Ma South Caucasus basalt province, covering parts of Turkey, Georgia and Armenia, has an estimated areal extent of ~15,000km2 and volume of ~2250km3. Because its main geological features are remarkably like those of many continental flood basalt (CFB) provinces, we consider it a true, albeit small, CFB province. It is the smallest and youngest CFB in the world. An analogue closely similar in major features is the Late Miocene Altos de Jalisco CFB province in the western Trans-Mexican Volcanic Belt. Both provinces formed during lithospheric pull-apart and transtensional faulting. Their broader significance is in showing flood basalt size distribution to be a continuum without natural breaks, with implications for geodynamic models.

Volatile concentrations in olivine-hosted melt inclusions from the Columbia River flood basalts and associated lavas of the Oregon Plateau: Implications for magma genesis

The volatile budgets and elemental compositions of primary magmas and their sources can be estimated through study of olivine-hosted melt inclusions in basaltic lavas, thereby providing insights about melting processes in the mantle. We report data on melt inclusions in 58 host-olivine crystals from seven basalt samples from the Columbia River Basalt (CRB) group in eastern Oregon and Washington, which have been analyzed for their major-oxide, trace-element and volatile (H2O, S, F, and Cl) concentrations to constrain the origin of this large igneous province. A wide range in H2O concentrations has been discovered, with the highest amount of 4.2wt.% found in olivine grains from a lava flow in the Malheur Gorge area, which erupted several million years after the hypothesized initial impingement of the Yellowstone Hotspot against the lithosphere of the North American Plate. This H2O concentration and others in our Columbia River–Oregon Plateau sample suite are significantly higher than those observed in lavas from hotspots such as Hawaii (~0.8–0.9wt.%) or in Mid-Ocean Ridge Basalts (MORBs) (~0.2wt.%), and are comparable to those measured for the lavas in the Snake River Plain–Yellowstone volcanic corridor and in some island and continental arcs.Lavas considered to be of plume origin from the Picture Gorge basalts in the main-eruptive-stage of the CRB show H2O concentrations of up to 2.4 wt.%, also significantly higher than values published previously for any plume-related volcanism. The highest H2O concentrations have been found most often in the least differentiated melt inclusions, indicating that these volatiles are not the result of differentiation at shallow levels in the crust. The enrichment of Ba relative to Th in many of the melt inclusions may indicate that volatiles in the CRB-OPB large igneous province have a subduction zone origin, which underscores the importance of flux melting in the generation of at least some of Earth’s large igneous provinces.

Lava-Sediment Interaction and Drainage-System Development In A Large Igneous Province: Columbia River Flood Basalt Province, Washington State, U.S.A

Abstract Large Igneous Provinces (LIPs) have been implicated in some of the largest environmental perturbations to have affected the Earth through geological time. Yet the impact of LIP development on drainage and ecosystem development in the immediate vicinity of these provinces are still poorly constrained to date. Based on a detailed, integrative facies scheme we characterize the interaction between volcanism and fluvial, lacustrine, and wetland environments in the Miocene Columbia River Flood Basalt Province (CRBP) LIP exposed in central Washington State, USA. The facies scheme proposed here comprises a detailed description and interpretation of siliciclastic, bioclastic (diatomite), volcaniclastic, and paleosol facies and subfacies intercalated with lavas of the Columbia River Basalt Group (CRBG). Facies and facies associations of individual interbeds are further correlated to reconstruct changes in sedimentary settings and topography of lava fields during deposition. Field observations and facies analysis help to explain the effects of flood-basalt volcanism on drainage-system development. We propose a generic model of lava–drainage interplay and distribution of sedimentary settings in flood-basalt provinces, which will contribute to our understanding of sedimentological, environmental, and volcanic processes in the CRBP. Hydrocarbon exploration in volcanic terrains requires detailed information on the distribution and development of sedimentary settings. This model will help to better predict the character and distribution of sedimentary bodies and potential hydrocarbon reservoirs in volcanic terrains.

The Strawberry Volcanics: generation of ‘orogenic’ andesites from tholeiite within an intra-continental volcanic suite centred on the Columbia River flood basalt province, USA

Abstract The widely distributed, mid-Miocene lavas of the Strawberry Volcanics of NE Oregon are compositionally diverse, ranging from basalt to rhyolite. They are composed mainly of calc-alkaline and mildly tholeiitic basaltic andesite and andesite. Ar–Ar dating and stratigraphic relationships indicate that the volcanic field was active from &gt;16 Ma to c. 12 Ma ago and thus is coeval for the first 1–2 Ma with strongly tholeiitic flood basalts of the Columbia River Province that encircle the Strawberry Volcanics. Tholeiitic and calc-alkaline compositions develop subtle but noticeable differences towards higher silica contents. At silica contents of &lt;55 wt% SiO 2 , calc-alkaline and tholeiitic lavas are essentially indistinguishable. Trace element constraints among Strawberry Volcanics and crustal rocks indicate that open-system processes, such as assimilation or magma mixing, are responsible for evolution along a calc-alkaline trend leading to ‘orogenic’ andesites from tholeiite. Exclusively tholeiitic basalts carrying evidence for a metasomatized mantle source erupted during the mid-Miocene of eastern Oregon. Consequently, tholeiite imparted the ‘subduction signals’ and crustal processing generated the calc-alkaline character to end up with compositions of typical ‘orogenic’ andesites at the Strawberry Volcanics. No primitive calc-alkaline basalt from the mantle is needed as parental magma here and possibly at other similar intra-continental calc-alkaline suites.

Osmium isotope variations accompanying the eruption of a single lava flow field in the Columbia River Flood Basalt Province

Geochemical interpretations of continental flood basalts usually assume that individual lava flows represent compositionally homogenous and rapidly erupted products of large well-mixed magma reservoirs. However, inflated pāhoehoe lavas may develop over considerable periods of time and preserve chemical variations that can be temporally linked through flow formation to eruption sequence thus providing an understanding of magma evolution over the timescale of a single eruption. This study presents comprehensive major, trace element and Re–Os isotope data for a single eruption that formed the 2660km3 Sand Hollow flow field in the Columbia River Basalt Province, USA. Major and trace element variations accompanying flow emplacement (e.g. MgO 3.09–4.55wt%, Ni 17.5–25.6ppm) are consistent with fractional crystallisation, but other petrogenetic processes or variable sources cannot be distinguished. However, there is a systematic shift in the initial 187Os/188Os isotope composition of the magma (age corrected to 15.27Ma), from 0.174 (lava core) to 1.444 (lava crust) within a single 35m thick sheet lobe. Lava crust values are more radiogenic than any known mantle source, consistent with previous data indicating that neither an enriched reservoir nor the sub-continental lithospheric mantle are likely to have sourced these basalts. Rather, these data indicate that lavas emplaced during the earliest stages of eruption have higher degrees of crustal contamination. These results highlight the limitations of applying chemostratigraphic correlation across continental flood basalt provinces, the use of single data points to define melt sources and magmatic processes, and the dangers of using conventional isochron techniques in such basalt sequences for absolute chronology.

Investigating the origin of candidate lava channels on Mercury with MESSENGER data: Theory and observations

AbstractVolcanic plains identified on Mercury are morphologically similar to lunar mare plains but lack constructional and erosional features that are prevalent on other terrestrial planetary bodies. We analyzed images acquired by the MESSENGER spacecraft to identify features on Mercury that may have formed by lava erosion. We used analytical models to estimate eruption flux, erosion rate, and eruption duration to characterize the formation of candidate erosional features, and we compared results with analyses of similar features observed on Earth, the Moon, and Mars. Results suggest that lava erupting at high effusion rates similar to those required to form the Teepee Butte Member of the Columbia River flood basalts (0.1–1.2 × 106 m3 s–1) would have been necessary to form wide valleys (&gt;15 km wide) observed in Mercury's northern hemisphere, first by mechanical erosion to remove an upper regolith layer, then by thermal erosion once a lower rigid layer was encountered. Alternatively, results suggest that lava erupting at lower effusion rates similar to those predicted to have formed Rima Prinz on the Moon (4400 m3 s–1) would have been required to form, via thermal erosion, narrower channels (&lt;7 km wide) observed on Mercury. Although these results indicate how erosion might have occurred on Mercury, the observed features may have formed by other processes, including lava flooding terrain sculpted during the formation of the Caloris basin in the case of the wide valleys, or impact melt carving channels into impact ejecta in the case of the narrower channels.

Upper mantle seismic structure beneath the Pacific Northwest: A plume-triggered delamination origin for the Columbia River flood basalt eruptions

We invert teleseismic P and S body waves constrained by an ambient-noise surface wave model and Moho depth inferred from receiver function analysis (Gao et al., 2011) to image mantle structures continuously from the surface to the base of the upper mantle. The major structures coincide with prominent geological features. We focus on a NE-Oregon structure, termed here the Wallowa anomaly, which coincides with the source area for the ∼16Ma Columbia River flood basalt eruptions and a circular area of topographic relief created during and after these eruptions. Resolution tests indicate that the curtain-like structure previously interpreted as Farallon lithosphere connects with the Wallowa anomaly above 150km along the northeast margin of the Wallowa anomaly. This connection, along with the pre-flood basalt magmatic and tectonic history of the Pacific Northwest, lead us to conclude that arrival of the Yellowstone plume to south-central Oregon initiated delamination of remnant Farallon lithosphere from the base of NE Oregon, exposing ocean crust to Yellowstone asthenosphere. This hypothesis accounts for the propagation of flood basalt volcanism far north of the Yellowstone hotspot track, and for the high-silica composition of most of the flood basalt magmas.