We examined Grande Ronde Basalt lava flows from surface sections and boreholes throughout Washington, Oregon, and Idaho to determine chemical and physical properties that would allow the recognition and mapping of these flows on a regional scale. We estimate there are ~100 flows covering nearly 170,000 km 2 , with a total volume of ~150,400 km 3 , that were erupted over four polarity intervals (reverse 1, normal 1, reverse 2, and normal 2) in ~0.42 m.y. These flows are the largest known on Earth, with individual volumes ranging from ~100 km 3 to greater than 10,000 km 3 . Although all known Grande Ronde Basalt flows erupted in the eastern part of the Columbia River flood basalt province, the thickest and most complete sections (>3 km) occur in the central Columbia Basin. From the center of the basin, the number of flows decreases outward, resulting in a nearly complete stratigraphy in the interior and an abbreviated and variable stratigraphy along the margins. The areal extent of many flows suggests that the Chief Joseph dike swarm greatly expanded after Imnaha Basalt time, and now many dikes are buried beneath younger flows in the eastern part of the province. The Grande Ronde Basalt has a relatively uniform lithology with only a few distinctive flows. However, when compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, the Grande Ronde Basalt can be subdivided into at least 25 mappable units. Grande Ronde Basalt flows are siliceous, with typically SiO 2 >54 wt%, MgO contents ranging from ~2.5 to 6.5 wt%, and TiO 2 ranging from 1.6 to 2.8 wt%, with an enrichment in iron and incompatible elements relative to mid-ocean-ridge basalt. Although most Grande Ronde Basalt flows have homogeneous compositions, some are heterogeneous. Dikes that fed the heterogeneous flows show that the first composition erupted was not typical of the flow, but as the eruption progressed, the compositions gradually evolved to the bulk composition of flow. The average effusion rate was ~0.3 km 3 /yr, with basalt volume peaking during the R2 polarity with the eruption of the Wapshilla Ridge Member. Eruption and emplacement rates for the flows are controversial, but available data collected from the field suggest that many of the flows could have been emplaced in a few years to perhaps a decade.

The Grande Ronde Basalt lavas constitute ~63% of the Columbia River Basalt Group, a large igneous province in the NW United States. The lavas are aphyric or contain less than 5% phenocrysts of plagioclase, augite, pigeonite, and olivine (altered). Plagioclase hygrometry shows that the erupted lavas generally contained less than 0.3% dissolved H 2 O; however, the presence of rare disequilibrium An 96 plagioclase phenocrysts suggests that some magmas may have originally had 4.5 wt% dissolved H 2 O at depth, but they all degassed during ascent and eruption. The size of plagioclase phenocrysts suggests an average plagioclase phenocryst residence time in the magmas of 160 yr. Ignoring hiatuses between eruptions, we estimate that the ~110 flows of the Grande Ronde Basalt erupted over a cumulative time of 17,600 yr, with an average eruption rate of ~8.6 km 3 /yr. The average interval between eruptions is estimated to be 3658 yr. It is envisaged that a shallow intrusive dike-sill complex, rather than large kilometer-sized magma chamber(s), fed the Grande Ronde basalt lavas. We performed model simulations using the COMAGMAT software to retrace the pre-eruption histories of the Grande Ronde Basalt lavas. Based on such simulations and petrological reasoning, we find that the primary melts could have been generated from a spinel peridotite source at 1.5 GPa, perhaps under hydrous conditions. Extensive melting of lithospheric eclogite may have played an important role as well; however, this is not constrained by our simulations. All lavas were contaminated by the crust, and they were last processed (mixing, crystallization) during their short residence within shallow (6 km) intrusive rocks prior to eruption. Our petrologic conclusions lead us to present a petrotectonic model that supports the hypothesis that the Columbia River Basalt Group magma generation was greatly aided by a thinned lithosphere and H 2 O that may have come off the asthenospheric wedge.

A composite section of eight Grande Ronde Basalt flows delineates the margin of the Columbia River Basalt Group on this portion of the eastern flank of the Cascade Range. The Grande Ronde Basalt flows belong to the following units (in descending stratigraphic order): Sentinel Bluffs Member (Basalt of Museum 2 and Museum 1; Basalt of Stember Creek; and upper and lower flows of the Basalt of McCoy Canyon), Ortley member (informal), Grouse Creek member (informal “Meeks Table” flow), and Wapshilla Ridge Member. All these Grande Ronde Basalt flows display similar intraflow structures (cooling joint patterns) and lithology, but they are separable by chemical compositions (i.e., TiO 2 , MgO, P 2 O 5 , Cr, Ba, and Zr). Individual Grande Ronde Basalt flows can range in thickness from 8 to 180 m, with the maximum total thickness of the Grande Ronde Basalt section being 555 m. As the Grande Ronde Basalt flows advanced into the map area, they covered plains, filled stream-cut valleys and canyons up to 160 m deep, and surrounded extinct volcanoes 750 m tall. In post–Grande Ronde Basalt time, the Grande Ronde Basalt flows were deformed into a series of ENE-striking anticlines, synclines, and associated faults that define this portion of the Yakima Fold Belt. During this same time, transpressional deformation activity increased folding and thrust faulting in the Cle Elum–Wallula Lineament, a structural segment of the Olympic-Wallowa Lineament. In addition, series of NNW-striking, dextral strike-slip and normal faults were developed with displacements up to 4.5 km on the strike-slip faults and 1 km on the normal faults. The N-striking Goat Creek and NW-striking Indian Flat and Devils Slide faults merge with the White River fault to the west. These faults, along with the E-NE–striking Bethel Ridge anticline and NNW-striking Cleman Mountain anticline, form the major structures in this area.

Grande Ronde Basalt (GRB) flows from 135 surface stratigraphic sections and 34 boreholes throughout Washington, Oregon, and Idaho, were examined to determine which chemical and physical properties would allow the recognition and mapping of GRB on a regional scale. At least 120 major GRB flows, with individual volumes ranging from 90 km 3 to more than 2,500 km 3 , produced a total volume of 148,600 km 3 , which erupted between 17.0 and 15.6 Ma. Although all known GRB feeder dikes and vents occur in the eastern and southeastern part of the Columbia Plateau, the thickest and most complete basalt sections (>3.2 km) occur in the Pasco Basin. The number of flows and section thickness decrease outward from the central Columbia Plateau so that a consistent stratigraphy exists in the interior, but an incomplete and variable stratigraphy exists along the margins. The distribution of some flows suggests that their vents lie buried in the northern part of the Columbia Plateau, far north of the known vent area. The GRB has a narrow range of chemical compositions and a relatively uniform lithology. Many flows have similar chemical compositions, and few flows have distinctive lithologies that can be mapped with confidence across the Columbia Plateau. When chemical compositions are combined with paleomagnetic polarity, lithology, and stratigraphic position, we are able to subdivide the 4 GRB magnetostratigraphic units into 17 informal units that are mapped and recognizable across the Columbia Plateau. These new informal units incorporate and expand on previously defined units using proven techniques for identifying Columbia River basalt flows. The informal stratigraphy proposed here provides a framework for correlation and resolution of local stratigraphies across the Columbia Plateau.

Stratigraphy of individual basalt flows in the N 2 magnetostratigraphic unit of the Grande Ronde Basalt (GRB) within the central Columbia Plateau has been developed using data from seven surface sections and fifteen boreholes. Twenty-one individual flows have been identified and grouped into eight flow packages. The flow correlations were developed based on chemical composition, paleomagnetic vector direction, stratigraphic position, and thickness of individual flows. A multivariate statistical procedure, discriminant analysis, was used to test the validity of using chemical composition alone to define the flow packages. Results of the test show that samples can be correctly classified within one adjacent flow package in 94 percent of the cases. Application of discriminant analysis to chemical composition data indicates that within the Pasco Basin the upper two-thirds of the N 2 GRB contains 17 individual flows, of which only 9 to 15 may be present at any one location. Seven of these flows are present throughout the portion of the Pasco Basin studied. Correlation of flows between boreholes or surface sections means that units with the same stratigraphic position and diagnostic characteristics have been identified. In most cases this means that a single flow formed from one eruption. However, some flows may not be continuous, or some correlated flows may represent eruptions separated by as much as several thousand years but which gave rise to flows with identical stratigraphic positions and similar characteristics. The greatest number of flows occurs in the southeastern part of the basin, but the thickest total section occurs in the western and northwestern parts of the basin. The detailed flow correlations provide evidence of deformation during emplacement of the N 2 GRB. Variations of the thickness of individual flows and packages of flows disclose that subsidence was greatest in the western portion of the basin and that growth of the Yakima Ridges began at least by late GRB time. This timing of deformation of the Yakima Ridges is consistent with previous interpretations. We conclude that discriminant analysis applied to chemical composition of GRB flows provides a means of quantitatively defining correlation of flow packages. The method yields estimates of uncertainty in correlations but must be applied in the context of an appropriate stratigraphic framework. Discriminant analysis should be useful in a variety of volcanic terranes where subtle but consistent differences in composition occur between individual flows or flow packages. Successful application of the technique requires that variations in the chemistry of individual flows or packages of flows be generally less than chemical differences among flow packages and that a relatively large number of samples be available for classification.

Joint width and mineral infillings are examined in approximately 3,200 cooling joints in basalt drill core. The core is from 4 Grande Ronde Basalt flows in the central Columbia Plateau of Washington. Statistical analysis shows that basalt flows, intraflow structural units, and core holes cannot be differentiated consistently on the basis of joint width. The joints appear to be members of only one, log normally distributed, width population with a median width of 0.14 mm. Clay is the predominant infilling mineral in the basalt joints, followed by silica and zeolite. Only 0.6 percent of the joints have observable void space. Basalt flows and intraflow structural units generally cannot be differentiated on the basis of joint infilling minerals. The one exception is the Umtanum entablature. A relationship can be observed between joint width and predominant infilling minerals. As joint width increases, fewer of the joints are observed to be filled predominantly with clay and zeolite, and a greater percentage with silica. The narrowest joints are filled almost exclusively with clay. A conceptual model of the mineral infilling sequence indicates that clay precipitated first in the joints and dominated throughout the entire infilling period. Silica precipitation was less in early stages of infilling and increased in later stages. Zeolite precipitation was also less in early stages, remained fairly constant throughout the middle stages, and dropped off dramatically in the latest stages.

The Grande Ronde Basalt (GRB) is the most voluminous formation of the Columbia River Basalt Group of the northwestern U.S. The trace-element geochemistry of a portion of the GRB (26 flows) from a single section in southeastern Washington was evaluated to determine the relative contributions of source and assimilant chemical effects. High-Mg, low-Mg and transitional compositions occur within the section. Stratigraphic compositional variations within the section are not regular. No progressive variation is seen: the various chemical types recur throughout the section. Although the stratigraphic variations resemble reversals produced during replenishment, multiple evolution lines seen in major- and trace-element diagrams indicate multiple magma batches. These probably erupted contemporaneously through independent(?) plumbing systems and interlayered to form a composite section. The high-Mg and transitional compositions are similar in terms of their respective (La/Sm) n values. The more evolved transitional lavas, however, tend to possess lower values of (La/Yb) n and (Tb/Yb) n and higher incompatible-element (e.g., La, Th, etc.) content than the less evolved high-Mg varieties. This characteristic is inconsistent with the two types being derived from a common mantle source or, if the difference in major elements—particularly MgO and CaO—is considered, with their being related through assimilation processes. Rather, the variations are thought to reflect differences in the mantle source regions. Consistent with this are the preliminary isotopic data, which suggest that the high-Mg lavas have higher 87 Sr/ 86 Sr than the transitional flows. Although the nature of the mantle source cannot be well constrained with the present data, the high rare-earth-element/high-field-strength-element ratios [(La) n /Ta = 50–85] and low Ta/Ta* values (0.27 to 0.41) suggest that enriched subcontinental lithosphere was involved in the production of the basalts. The low-Mg compositions trend away from the transitional compositions in all diagrams and show a marked decoupling of the high-field-strength element (e.g., Hf) and large-ion-lithophile element (e.g., Th and La). The low-Mg compositions define linear arrays in variation diagrams that have trajectories similar to that predicted for combined assimilation-fractional crystallization involving parental transitional compositions and silicic crustal rocks. Assimilation of approximately 10 to 20 percent by weight of such crust can accommodate the variation observed in the low-Mg compositions. Preliminary Sr ratios indicate variations from 0.70454 in the least evolved low-Mg lavas to >0.707 for the more evolved flows. Trace-element variations indicate that, similar to the high-Mg and transitional groups, the low-Mg compositions evolved along more than a single evolution path—more than one parental magma was involved.

We present a reappraisal of Columbia River basalt petrogenesis based on an internally consistent X-ray fluorescence and inductively coupled plasma–mass spectrometry data set for major and trace elements plus new and existing isotopic analyses of the Imnaha, Steens, Picture Gorge, and Grande Ronde Basalts. Source materials for the main-phase Columbia River Basalt Group are upwelling ocean-island basalt source–like mantle, depleted mantle variably fluxed by slab-derived fluids, Phanerozoic arc crust, and ancient North American cratonic crust. The mantle upwelling may be a deep-seated plume or material displaced and mobilized by fragmented sinking slabs. We endorse the conclusions of earlier workers that the Imnaha, Steens, and Picture Gorge Basalts represent different mixtures of upwelling mantle, depleted mantle, slab-derived fluids, and crust. Cratonic crust of the Idaho batholith has a minor role as a contaminant of Imnaha basaltic magma and a major role in the petrogenesis of the Grande Ronde basaltic andesites, which we model as contaminated Imnaha basalt. We emphasize the geochemical continuity of the Imnaha and Grande Ronde Basalts and propose that they derive from a single central crustal magma system (or chamber) that lasted from ca. 16.7 Ma to 16.0 Ma. The Imnaha–Grande Ronde magma system was centered beneath the location where the western Snake River Plain, Oregon-Idaho graben, and Chief Joseph dike swarm converge, and probably transgressed the cratonic boundary to the east. Magma from this system was injected into dikes and traveled hundreds of kilometers northward to erupt and feed the gigantic Grande Ronde lava flows. In contrast to previous studies, we question the idea that the dike swarm provides a geographic map of the magma source regions.

ABSTRACT Emplacement models for voluminous sheet flows of the Columbia River flood basalts vary significantly in style and duration, with the latter ranging from as little as one week to decades and even centuries. Testing the efficacy of such models requires detailed field studies and close examination of each stratigraphic unit. The Steens Basalt, the oldest formation of the Columbia River flood basalts, differs from the later formations in that it is composed of stacked successions of thin, commonly inflated flow lobes combined into thicker compound flows, or flow fields. These flow lobes are of limited geographic extent, with relatively high emplacement rates, but they are otherwise similar to modern examples. Evidence for flow inflation in the much larger sheet flows of the Grande Ronde Basalt, Wanapum Basalt, and Saddle Mountains Basalt is also apparent, but with more variable rates of emplacement. For example, the Asotin and Umatilla Members (Saddle Mountains Basalt) and Sentinel Bluffs Member flows (Grande Ronde Basalt) erupted distinct compositions along their linear vent systems, but over 200 km west of their vents, these flows are no longer distinct. Instead, they exist as compositional zones of a single, moderately mixed lava flow. Such flows must have been emplaced rapidly, in perhaps weeks to months, while others have been shown to erupt over much longer time periods. We conclude that emplacement rates may be quite variable throughout the Columbia River flood basalt province, with thin flow units of Steens Basalt erupting continuously and rapidly, and larger inflated sheet flows erupting over variable time spans, some from a few weeks to months, and others over a duration of years.

The Columbia River flood basalt province, United States, is likely the most well-studied, radiometrically well-dated large igneous province on Earth. Compared with older, more-altered basalt in flood basalt provinces elsewhere, the Columbia River Basalt Group presents an opportunity for precise, accurate ages, and the opportunity to study relationships of volcanism with climatic excursions. We critically assess the available 40 Ar/ 39 Ar data for the Columbia River Basalt Group, along with K-Ar data, to establish an up-to-date picture of the timing of emplacement of the major formations that compose the lava stratigraphy. Combining robust Ar-Ar data with field constraints and paleomagnetic information leads to the following recommendations for the age of emplacement of the constituent formations: Steens Basalt, ca. 16.9 to ca. 16.6 Ma; Imnaha Basalt, ca. 16.7 to ca. 16 Ma; Grande Ronde Basalt, ca. 16 Ma to ca. 15.6 Ma; Wanapum Basalt, ca. 15.6 to ca. 15 Ma; and Saddle Mountains Basalt from ca. 15 Ma to ca. 6 Ma. The results underline the previously held observation that Columbia River Basalt activity was dominated by a brief, voluminous pulse of lava production during Grande Ronde Basalt emplacement. Under scrutiny, the data highlight areas of complexity and uncertainty in the timing of eruption phases, and demonstrate that even here in the youngest large igneous province, argon dating cannot resolve intervals and durations of eruptions.

Newly generated and previously published strontium isotope signatures of plagioclase phenocrysts in Columbia River Basalt Group lavas exhibit heterogeneity largely imposed by mantle-derived magmas assimilating variable crustal rocks. Steens basalts assimilated accreted terrane crust with 87 Sr/ 86 Sr ratios of <0.7040. In contrast, Imnaha, Grande Ronde, Wanapum, and Saddle Mountains basalts likely assimilated crust with more radiogenic Sr (>0.7040) including a cratonic component, perhaps as a result of residence in magma chambers partly located east of the 87 Sr/ 86 Sr = 0.7060 line. Strontium isotope ratios in plagioclase phenocrysts from early- erupted Imnaha basalts anticipate whole-rock signatures of later-erupted Grande Ronde basalts consistent with a geochemical continuum between the two formations, which is also seen in whole-rock trace element abundances, undermining the notion of an abrupt change in the magma sources generating Imnaha and Grande Ronde basalts.

The Lewiston Structure is located in southeastern Washington and west-central Idaho and is a generally east-west–trending (~075°), asymmetric, noncylindrical anticline in the Columbia River Basalt Group that transfers displacement into the Limekiln fault system to the southeast and the Silcott fault system to the southwest. A serial cross-section analysis and three-dimensional (3-D) construction of this structure show how the fold varies along its trend and shed light on the deformational history of the Lewiston Basin. Construction of the fold’s 3-D form shows that the fold’s wavelength increases and amplitude decreases near its eastern and western boundaries. Balanced cross sections show ~5% shortening across the structure, which is consistent with the Yakima Fold Belt. An angular unconformity below the Grande Ronde Basalt N1 magnetostratigraphic unit, in addition to a variation of N1 unit thickness across the structure, suggests that the fold was forming before N1 time. Analysis of structural data using the Gauss method for heterogeneous fault-slip data indicates north-south (~350°) shortening prior to and after N1 emplacement. The presence of a reverse fault on the southern limb of the Lewiston Structure is controversial. This fault crops out to the east of the field area where Grande Ronde Basalt magnetostratigraphic unit R2 is thrust over Pliocene(?) gravels. However, better control on unit thicknesses and map contacts rules out an exposed reverse fault on the southern limb of the fold west of the Washington-Idaho border, suggesting the fault either dies out or becomes blind.

The K-Ar data for various sections of the Columbia River basalt (Imnaha, Picture Gorge, and Grande Ronde Basalt formations) have been reevaluated, utilizing the atmospheric argon contents of the rocks to identify the least altered samples. These ages, together with the results of paleomagnetic studies on these same sections, were then fitted into the known magnetostratigraphy of the Columbia River basalt. Finally, these findings are integrated with the geomagnetic polarity time-scale for mid-Miocene times. The Imnaha Basalt was formed ~17.2 Ma, and the Picture Gorge Basalt ~ 16.0 Ma. The Grande Ronde Basalt was extruded between ~16.9 and 15.6 Ma, with >50 percent of the total volume (magnetostratigraphic units R 2 –N 2 ) formed within ~300,000 yr, around 15.8 Ma.

Nearly twenty flows of the Columbia River Basalt Group (CRBG) can be paleomagnetically and chemically correlated westward as far as 500 km from the Columbia Plateau in Washington, through the Columbia Gorge, to the Coast Range of Oregon and Washington. In the Coast Range near Cathlamet, Washington, the CRBG flow stratigraphy includes 10 flows of Grande Ronde Basalt (1 low-MgO R 2 flow, 6 low-MgO N 2 flows, 3 high-MgO N 2 flows), 2 flows of Wanapum Basalt (both flows of Sand Hollow from the Frenchman Springs Member), and the Pomona Member of the Saddle Mountains Basalt. Elsewhere in the Coast Range, additional Grande Ronde Basalt flows, including flows of Winterwater or Umtanum, and additional Wanapum flows, including the flows of Ginkgo, have been reported. Thus at least 18 to 20 CRBG flows reached the coast region. Several of these distal flows have distinctive chemical and magnetic characteristics that are shared by nearby isolated intrusions in Coast Range sedimentary rocks, thus strongly supporting recent suggestions that these intrusions are invasive bodies fed by CRBG flows. Magnetization directions from several flows indicate 16 to 30° of clockwise rotation of the coast with respect to the plateau since middle Miocene time.