139 Geophysics and Tectonics of the Snake River Plain, Idaho by Don R. Mabeyl ABSTRACT different from the adjoining regions. The evolution of the eastern Snake River Plain appears to have consisted of a northeastward-moving center of volcan- Although the regional surface geology of the ism that is now at Yellowstone National Park and of Snake River Plain in Idaho is well known and many an increase in the density of the crust by a correspond- shallow and a few deep holes have been drilled on the ing thinning of the upper crust and the resulting plain, major uncertainties remain concerning the subsidence. Current deformation of the eastern plain structure of the plain. Seismic refraction profiles have is a southwest extension, parallel to the plain. revealed that under the plain a thin upper crust overlies a lower crust which is thicker than that under the Basin and Range province to the south. The large Bouguer gravity high over the plain reflects this thin INTRODUCTION upper crust, but in the central and western plain it also indicates a dense layer at intermediate depth. The Snake River Plain, which extends in a 600- Superimposed on the gravity high is a low produced kilometer arc across southern Idaho, is the most by low-density sedimentary and volcanic rocks that prominent Cenozoic feature in the partly fill the depression of the plain, which is in state (Figure 1). approximate isostatic equilibrium with the adjoining The plain is distinguished from the surrounding terrain by highlands. The regional magnetic anomaly over the lower elevation and lower surface relief and central and western plain reflects a layer of strongly by a complete cover of Cenozoic sedimentary and magnetized rock that is probably volcanic and, in the volcanic rocks. Well-defined topographic features form the boundary of the Snake River east, an upper crust made up of a complex of Plain through- out most of magnetized bodies. Near-surface heat flow is lower its length except in the southern part of the arc of the plain, where the over the axis of the plain than along the margins, but moderately dissected this may or may not reflect the deep heat flow. A surface of the plain rises to merge with the mountains large negative P-wave delay anomaly lies along the to the south. This segment of the boundary has northwest edge of the eastern plain. commonly been drawn as an arc connecting the well- The western and central segments of the Snake defined segments to the northwest and east. However, the gravity and River Plain appear to be a fault-bounded depression magnetic data suggest that the subsurface structure of the plain extends farther containing a layer of dense, strongly magnetized rock that may be Miocene basalt overlain by silicic vol- south, and the boundary suggested by the geophysical data will be used in this report. canic rocks and interbedded basalt flows and sedi- ments. Rocks typical of the upper crust of continents The regional surface geology of the Snake River may not occur under the axis of this part of the plain. Plain has been known for many years, and in recent The eastern Snake River Plain is more likely a years, much of the surface geology has been studied downwarp containing a complex of calderas with a in moderate detail. Added to the surface information great thickness of silicic volcanic rocks overlain by are data from thousands of water wells, several deep interbedded basalt flows and sediments. Fissures, oil and geothermal exploration wells, and a wide along which the younger basalts have been erupted, variety of geophysical surveys. Yet the Snake River parallel trends of the basin and range structures to the Plain remains one of the least understood geologic north and south of the eastern plain and may be structures in the United States. The data on the plain extensions of these structures. Although typical upper have been interpreted in very divergent ways by crustal rocks may underlie the eastern plain, the different investigators. The Snake River Plain has geologic structure in these rocks appears to be been described as a depression, downwarp, graben, rift, and lateral rift. In addition, several volcanic or 1 U. S. Geological Survey, Salt Lake City, Utah 84101. thermal events have been used to explain the forma- Mabey, D. R., 1982, Geophysics and tectonics of the Snake River Plain, Idaho, in Bill Bon nichsen and R. M. Breckenridge, editors, Cenozoic Geology of Idaho: Idaho Bureau of Mines and Geology Bulletin 26, p. 139-153. 140 Cenozoic Geology of Idaho 117° 111° 45° YELLOWSTONE CentennialMts NATIONAL PARK +++++. "e •,,„4. • ear+++ r + r r + + + + + + + + r + + + + + •• • + + + + + •• +++++ + + + ++ • + + + + + + + + + + + +++++++++++ <CY.+40+4++ •• ++++++41-rr++ + + + +• + + • + + + + + + + + + + + + 4 + + + + + + + ♦ • + + + + + + + + + + + + + + + + r + r + • ++++++++ • + + + + + < CENTRAL SNAKE RIVER PLAIN: • :4 boa N+ • + + + + 4-1 •"I Twin Falls ^ 7, 7 „ ',.-,•' ,':`,"11 '%;','' +‘ ' 'A ',,,,,• ,, • v ,..‘ ,....e.41:7>+ ,-1 „,1 .. 7, A, ;I lit , A • s. r- 4 ". 4 , %, •.‘•••‘"frainlmieten ..L .a...: '-.'' 0 200 KILOMETERS EXPLANATION Quatemary sediments Tertiary sedimentary rocks +• + + Quaternary basalt Pre-Tertiary rocks + +• • Quaternary rhyolite and -I - Cretaceous intrusive rocks Tertiary volcanic rocks Figure 1. Geologic map of southern Idaho showing the Snake River Plain and location of profiles. Heavy dashed lines indicate major divisions of the plain. Narrow dashed line is lineament defined by the geophysical data. Geology generalized from geologic map of the United States (King and Beikman, 1974). tion of the plain, and the plain has been tied to several meters wide, trends northwest, and is generally con- plate tectonic models. Although the Snake River sidered to be a fault-bounded depression with normal Plain is a continuous arc across southern Idaho, faults forming major segments of both edges of the major differences exist between the surface geology plain (Malde, 1965). Much of the northwestern part and the geophysical anomalies in the western, central, of the western Snake River Plain is covered by and eastern parts of the plain. It is, therefore, conven- Quaternary alluvium, To the southeast, the surface ient to discuss the plain as three units: the northwest- rocks are primarily Tertiary and Quaternary lacus- trending western Snake River Plain, the northeast- trine sediments and basalt flows, and the land surface trending eastern Snake River Plain, and the central and the rocks generally dip gently toward the axis of Snake River Plain between them (Figure 1). the plain. Southwest of the plain are the Owyhee Mountains with a core of metamorphic rocks and Cretaceous granitic rock overlain by Tertiary volcanic WESTERN SNAKE RIVER PLAIN rock. To the northeast are the Cretaceous granitic rocks of the Idaho batholith locally overlain by Tertiary volcanic rock. The western Snake River Plain is 50 to 70 kilo- Several oil and geothermal test wells have been Mabey--Geophysics and Tectonics, Snake River Plain 141 drilled on the western Snake River Plain to depths as the gravity high show that at least part of the great as 4,300 meters. These wells have revealed anomaly is produced by a positive mass anomaly lacustrine sediments several hundred meters thick within the upper crust, but part has a deeper source. underlain by 1,000 to 2,000 meters of basalt flows The Bouguer gravity high is approximately coincident with some interbedded sediments. The two deepest with the topographic low. Because the isostatic holes, Chevron Highland No. 1 and Halbouty James compensation for the plain is unusually local, the No. 1, had reportedly bottomed in interbedded silicic value of isostatic anomalies computed for gravity volcanic rocks, lacustrine sediments, and basalt. To stations on or near the plain are highly dependent on the southeast, one deep drill hole, 22 kilometers the model assumed in computing the anomaly. The southeast of Mountain Home, Bostic No. 1-A, pene- free-air and Faye anomalies over southwestern Idaho trated about 2,300 meters of interbedded basalt and average slightly positive, and those over the axis of sediment and bottomed in silicic volcanic rock (Arney the gravity high on the plain are more positive than and others, 1980). A drill hole about 70 kilometers the average for southwestern Idaho (Figure 3). south of Boise, Anschutz Federal No. I, penetrated Although the plain in the area of the crest of the basalt and sediment to a depth of 800 meters and then gravity high may not be in complete isostatic equilib- predominately silicic volcanic rock to 3,400 meters, rium, the western plain as a unit appears to be in where granite was found (McIntyre, 1979). approximate isostatic equilibrium with the adjoining The major sets of geophysical data available in the regions, and the relative depression of the plain western and central Snake River Plain are regional probably reflects an isostatic response to the large gravity and magnetic surveys (Mabey and others, positive mass anomaly underlying the plain. 1974; Zietz and others, 1979), a seismic refraction The gravity anomaly over the western Snake River profile (Hill and Pakiser, 1966), and numerous Plain can be interpreted in several ways, but most thermal-gradient and heat-flow measurements (Brott interpretations involve a combination of dense basalt and others, 1976). More detailed geophysical studies and a thinning of the upper crust that underlies the have been made in local areas, particularly near some plain. At least three mass anomalies contribute in a of the deep drill holes. major way to the gravity anomaly over the western The Bouguer' gravity high over the Snake River Snake River Plain: a thin upper crust, low-density Plain, which was first reported by Bonini and Lavin sediment with interbedded basalt, and massive dense (1957), has been studied by several investigators. The basalt flows. The thinning of the upper crust, indi- maximum amplitude of the high, which is about 100 cated by the refraction data, is a large positive mass milligals, is over the western Snake River Plain anomaly and could produce most of the gravity high.