Origin and Evolution of the Western Snake River Plain: Implications From Stratigraphy, Faulting, and the Geochemistry of Basalts Near Mountain Home, Idaho John W. Shervais,1 Gaurav Shroff,2 Scott K. Vetter,3 Scott Matthews,3 Barry B. Hanan,4 and James J. McGee2 ABSTRACT SRP basalts. The second episode is represented by sur- face flows of Pleistocene age that overlie lacustrine and Most models for the origin and evolution of the Snake deltaic sediments of Lake Idaho. These basalts are asso- River Plain (SRP) volcanic province focus on the central ciated with young faults that reflect Basin and Range role of the Yellowstone hot spot and its effects on the extension. lithosphere of North America in response to plate mo- We propose that the western SRP graben represents tions relative to this hot spot. Movement of the North an aulocogenlike structure formed in response to ther- American plate over the Yellowstone hot spot has resulted mal tumescence above the Yellowstone plume head as it in a linear track of volcanism that parallels this plate rose under eastern Oregon and Washington. The plume motion, represented today by volcanic rocks of the east- head was deflected northwards either by subduction of ern SRP. the Farallon plate (Geist and Richards, 1993) or by im- The western SRP is a structural graben oriented at a pingement of North American plate lithosphere (Camp, high angle to the trace of the Yellowstone plume and to 1995). Basaltic volcanism in the western SRP may be the axis of the eastern SRP. It is filled largely with lacus- related to the flow of depleted plume source mantle along trine sediments related to Pliocene Lake Idaho, a large, a sublithospheric conduit beneath the western SRP gra- long-lived lake system that formed first at the northwest- ben from the Columbia River Plateau. The basalts would ern end of the graben (near Oregon) and extended to the form by pressure-release melting of this previously de- southeast along with the structural graben. High-tempera- pleted material, along with the overlying mantle lithos- ture rhyolite lavas that mark the onset of extension also phere. The younger volcanic episode apparently formed become younger to the southeast. in response to Basin and Range extension, in a fashion Basaltic volcanism in the western SRP occurred in analogous to young basaltic volcanism in the eastern SRP. two distinct episodes. The first episode, represented by The source of these basalts is uncertain, but may be plume- samples from a deep drill core near Mountain Home and modified subcontinental lithosphere. by older surface outcrops that sit directly on rhyolite, is Key words: continental volcanism, hotspots, basalt, characterized by ferrobasalts that are distinct from other Snake River Plain Editors’ note: The manuscript was submitted in June 1998 and has been revised at the authors’ discretion. INTRODUCTION 1Department of Geology, Utah State University, Logan, UT 84322 2Department of Geological Sciences, University of South Carolina, The Neogene Snake River Plain (SRP) comprises two Columbia, SC 29208 distinct tectonic terranes with different crustal structure, 3Department of Geology, Centenary College, Shreveport, LA 71134 4 Department of Geological Sciences, San Diego State University, stratigraphy, and volcanic history (Figure 1). One is the San Diego, CA 92182 northeast-trending eastern SRP, a downwarped structural Shervais, J.W., Gaurav Shroff, S.K. Vetter, Scott Matthews, B.B. Hanan, and J.J. McGee, 2002, Origin and evolution of the western Snake River Plain: Implications from stratigraphy, faulting, and the geochemistry of basalts near Mountain Home, Idaho, in Bill Bonnichsen, C.M. White, and Michael McCurry, eds., Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province: Idaho Geological Survey Bulletin 30, p. 343-361. 344 Tectonic and Magmatic Evolution of the Snake River Plain Volcanic Province depression with only minor fault control along its mar- forming a carapace over sediments of the Idaho Group gins (Mabey, 1978, 1982; Malde, 1991; Zentner, 1989; and documenting an episode of volcanism that occurred Pierce and Morgan, 1992). This depression becomes pro- some 8 million years after the Yellowstone plume passed gressively elevated towards the east, culminating in the this longitude. This area is critical to understanding Yellowstone Plateau, the current locus of volcanic activ- intracontinental plume tracks because it lies near the in- ity (Christiansen, 1982; Malde, 1991). The eastern SRP tersection of the Yellowstone plume track and the west- trend is thought to mark the track of the Yellowstone ern SRP graben—a tectonic enigma that does not con- plume from its mid-Miocene location beneath the Owyhee form to current models of plume-lithosphere interaction Plateau to its current place under Yellowstone (e.g., Pierce (Figure 1). We begin by reviewing first the stratigraphy and Morgan, 1992; Malde, 1991; Smith and Braile, 1994), of volcanic and sedimentary units, their structural defor- although this hypothesis has been challenged (Humphreys mation, and the geochemistry of the basalts in the Moun- and others, 2000; Christiansen and others, 2002). Recent tain Home area. We then discuss the implications of these tomographic data suggest that the plume may actually data for the Neogene tectonic evolution of the western rise to the west and be tilted eastward by counterflow in SRP and the relationship of the western SRP to the the mantle that results from the sinking Farallon slab Yellowstone plume system. (Smith and others, 2002; Steinberger and O’Connell, 2002). The other terrane is the western SRP, a northwest- GEOLOGIC SETTING trending structural graben bounded by en echelon nor- mal faults and filled with sediment up to 1.7 km thick Mountain Home is located on the northeast margin (Malde, 1959, 1991; Malde and others, 1963; Mabey, of the western SRP, approximately 40 miles southeast of 1976, 1982; Leeman, 1982). Sedimentary deposits in the Boise and 70 miles northwest of Twin Falls (Figure 2). western SRP range in age from Miocene through Quater- The area immediately surrounding Mountain Home is a nary and are known collectively as the Idaho Group volcanic plateau, which is underlain by basalt flows and (Malde and Powers, 1962, 1972; Malde and others, 1963). lacustrine sediments of the Idaho Group, at an average These deposits are dominantly lacustrine, with subordi- nate fluviatile and phreatomagmatic deposits (Kimmel, 1982; Smith and others, 1982; Jenks and Bonnichsen, 1989; Malde 1991; Godchaux and others, 1992; Jenks and others, 1993). Sedimentation during this time was controlled by the presence of a large lake, Lake Idaho, which expanded or shrank in response to variations in climate and tectonics (Kimmel, 1982; Jenks and Bonnichsen, 1989). Volcanic activity in the western SRP began about 11 million years ago with the eruption of rhyolite lavas from fissures that paralleled the range-front faults coeval with extension and graben formation (Ekren and others, 1982, 1984; Bonnichsen, 1982). These rhyolites form the local basement upon which subsequent sediments and basalts were deposited. Basaltic activity in the western SRP be- gan about 9 million years ago, forming lavas that under- lie sedimentary deposits of Lake Idaho and, later, local basalt horizons intercalated with these sediments (Malde and Powers, 1962, 1972; Amini and others, 1984; Malde, 1991; Jenks and Bonnichsen, 1989; Jenks and others, Figure 1. Overview of the northwestern U.S. showing the location of 1993). the Snake River Plain and adjacent physiographic regions including the western SRP (horizontal bars), the eastern SRP (dark gray), Colum- In this paper we summarize the Neogene volcanic bia River basalt province, the Oregon Plateau basalt province, the Or- history of the western SRP in the area around Mountain egon Coast Ranges, the Owyhee Plateau, the Basin and Range Prov- Home, with particular emphasis on the basaltic volcan- ince (pale gray), the Idaho batholith (ticks), the Colorado Plateau, ism and volcanic stratigraphy. Volcanic rocks less than Yellowstone caldera, and the Sr “0.704” and “0.706” lines, which mark 2.0 Ma are exposed as surface flows near Mountain Home, the western extent of Proterozoic and Archean crust. Shervais and Others—Origin and Evolution of the Western Snake River Plain 345 Figure 2. Index map showing southern Idaho and the western Snake River Plain region. Also shown are adjacent parts of the eastern SRP province and the Idaho batholith, and the location of cross sections A-A' (through Mountain Home) and B-B' (through Caldwell; see Figure 6). Unpatterned areas include both rhyolite volcanics and Neogene sediments. Location of the area mapped in Figure 3 is shown by black box. elevation about 3,200 feet above sea level. The southern (43º0'N. to 43º15'N.), encompassing about 450 square limit of this plateau is an escarpment overlooking the miles (1,170 square km). Rhyolite lavas of the Idavada current drainage of the Snake River, just upstream from Formation crop out in the northeast corner of the map its confluence with the Bruneau River; the elevation at area; lacustrine and fluvial-deltaic sediments of the Idaho river level is about 2,800 feet. The plateau is truncated Group crop out in embayments along the southern mar- along its northern margin by the Danskin Mountains (to gin. The area between the rhyolitic Mount Bennett Hills the north) and the Mount Bennett Hills (to the northeast). and the southern escarpment is underlain largely by ba- These mountains consist of rhyolite lava flows that have salt flows covered by soil, windblown dust, and alluvial been uplifted along major down-to-the-southwest normal sediments (Figure 3). faults, creating a rugged topography with summit eleva- There are fifteen identifiable basaltic vents in this area: tions from 5,500 to 7,000 feet. two maarlike ring structures, three cinder cones, and nine Eight 7.5-minute quadrangles were mapped for this shield volcanoes (Figure 3).