Geophysics and Tectonics of the Snake River Plain, Idaho

Home , Albion Mountains, Snake River Plain

139

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 sediments. 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°

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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. (Figure 2). Here the crest of the high trends a few Sedimentary rocks with some interbedded basalt degrees more westerly than the axis of the plain and cover much of the western plain and in some areas are has one major offset. Steep gradients associated with over 1 kilometer thick. The average density of these rocks is low and their effect is to reduce the Bouguer 'Bouguer gravity anomalies include a correction for the elevation anomaly over the plain. Three drill holes (Chevron of the point of observation relative to a reference datum (usually sea level). Included is a correction for the attraction of the Highland No. I, Halbouty James No. 1, and Bostic material between the point and the reference datum. The Bouguer No. 1-A) on the western Snake River Plain have anomalies reflect all mass anomalies below the reference datum penetrated 2 to 3 kilometers of basalt, but the bulk as well as variations from the density assumed in correcting for density of this basalt has not been determined. The the material above the datum. Free-air anomalies include the samples of basalt flows correction for the elevation but not for the material above the dry bulk density of forty-nine reference datum. If a region is in isostatic equilibrium, the of the Quaternary Snake River Group on the eastern material above the reference datum will be compensated by an Snake River Plain averaged 2.55 grams per cubic equal mass deficiency at depth. The gravity effect of the two centimeter, and seven samples of basalt flows of the masses at an observation point are approximately equal and Miocene Columbia River Basalt Group north of the opposite in sign, and the free-air is near zero. However, local relief on the Earth's surface is not compensated, and in areas of plain averaged about 2.9 grams per cubic centimeter. considerable local relief, free-air anomalies are strongly depen- Two and one-half kilometers of the denser basalt dent upon the elevation of the observation point. The Faye would produce a 25-milligal anomaly, about one- anomaly includes a correction to the free-air anomaly for the fourth of the measured high. Three of the deep holes difference between the elevation of the observation point and No. 1, Bostic No. I-A, and Anschutz the average surrounding elevation. The Faye anomaly, which is (Halbouty James a special type of isostatic anomaly, is approximately equal to the Federal No. 1) found thick sequences of other free-air anomaly that would be measured at a point with an volcanic rocks that, on the average, are probably less elevation equal to the average surrounding elevation. Faye dense than the average crustal rocks. anomalies are designed to be used in areas of considerable local Residual magnetic intensity over the western Snake relief to reveal subsurface massanomalies that are not isostat- ically compensated and regional departures from isostatic equi- River Plain is generally higher than over the areas to librium. See Mabey, 1966. the north and south, but no magnetic feature coincides 142 Cenozoic Geology of Idaho

1170

0 200 KILOMETERS I

Figure 2. Bouguer gravity contour map of southern Idaho generalized from Mabey and others (1974). Heavy dashed lines indicate major divisions of the plain. Narrow dashed lines are lineaments defined by the geophysical data. Contour interval is 10 milligals. with the axis of the plain or the gravity high (Figure C. J. Strike Reservoir near the axis of the plain, and 4). The most prominent feature of the total-intensity at Mountain City south of the plain. In addition, magnetic field is a high along the south side of the underground nuclear tests on the Nevada Test Site plain with a less conspicuous low along the north were recorded. Several interpretations have been edge. Lower amplitude highs and lows, which gener- made of the data obtained, and although interpreta- ally do not correlate with surface geologic features, tions differ in detail, they agree that the total crust occur over the central part of the western plain. The under the plain is over 40 kilometers thick, which is magnetic field can be modeled in a variety of ways, about 10 kilometers thicker than in central Nevada, but the high along the south edge of the plain and the and that the upper crust is thin under the axis of the low along the north edge, along with the extent of the plain (Hill and Pakiser, 1966). associated gradients, strongly suggests that the plain Heat-flow measurements in southwestern Idaho is underlain, at a depth of a few kilometers, by a layer have defined a heat-flow high along the margins of of rock strongly magnetized in the approximate the plain and a relative heat-flow low over the plain present direction of the Earth's magnetic field. At one (Figure 5). Brott and others (1978) conclude that a location on the southwest side of the plain, an large heat source underlies the plain and that the appendage of the magnetic high extends beyond the heat-flow values measured in the near surface result plain. Underlying this appendage is the only extensive from thermal refraction in the sedimentary and body of Eocene volcanic rock exposed south of the volcanic rocks underlying the plain and from the low plain. heat generation of the rocks in the upper crust under Shot points along the seismic-refraction profile the plain. that extends south across the plain from near Boise The gravity, magnetic, and seismic data have been were at Lucky Peak Reservoir north of the plain, at combined to produce an interpreted profile across the Mabey—Geophysics and Tectonics, Snake River Plain 143

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-20 0 +20 +40

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Figure 3. Faye gravity anomaly map of southern Idaho. Contour interval is 20 milligals. Heavy dashed lines indicate major divisions of the plain. Narrow dashed lines are lineaments defined by the geophysical data and approximate extent of the Idaho batholith. The Faye anomaly is the free-air anomaly corrected for the difference between the elevation of the gravity station and the average elevation surrounding the station. For this map the elevations were averaged over an area 64 kilometers in radius. western Snake River Plain. The extent of the flanks strongly magnetized rock, the magnetic data can be of the gravity high beyond the plain require that a used with the gravity data to infer the depth and part of the positive mass either extends across the extent of this unit. The near-surface anomalies can be borders of the plain or lies at depths greater than 10 attributed to variations in the thickness and density kilometers below the surface of the plain. Thus, the of overlying low-density rocks and the remaining anomaly is not produced entirely by dense rock gravity anomaly attributed to the thinning in the underlying the plain at depths of less than 10 upper crust. Such a model is shown in Figure 6. Large kilometers. Very likely, the part of the anomaly with variations in this model are possible (for example, see a deep source relates to the thinning of the upper Mabey, 1976), but good evidence exists for the crust as indicated by the refraction data. The steepest approximate position and size of the major units. gravity gradients associated with the high, however, Southwest of Boise, the crest of the gravity high require a source above 10 kilometers. Some of these along the axis of the plain is offset about 30 steep gradients can be attributed to variations in the kilometers in a right-lateral sense(dashed line, Figure thickness of the low-density sedimentary and silicic 2). This offset may reflect displacement upon a north- volcanic rocks, but a high-density mass above 10 trending strike-slip fault. Such an offset is consistent kilometers is also indicated. If the magnetic anomaly with the magnetic data and the configuration of the over the plain is assumed to reflect a layer of dense, southwest margin of the plain. North of the plain, the 144 Cenozoic Geology of Idaho trend of the offset projects along the linear western to describe the western Snake River Plain, a qualify- margin of the exposed part of the Idaho batholith. If ing statement should be included noting that pre- the two highs reflect a geologic unit that has been Cenozoic rocks similar to those exposed north and offset, the offset probably occurred after most of the south of the plain are not known to be downfaulted mass anomalies underlying the plain were emplaced under the plain. but before part of the relative subsidence of the plain The western Snake River Plain has been described that produced the present topography occurred. by Warner (1975) as a lateral rift, and by several All of the geological and geophysical evidence is others as a rift. Warner cites several lines of evidence consistent with major segments of the western Snake to prove that 80 kilometers of left-lateral displace- River Plain being bounded by normal faults, although ment has occurred along the Snake River Plain, these faults may not have been the fundamental mostly since early Pleistocene time. He also infers structure that formed the plain. A thick sequence of some separation across the western Snake River volcanic and sedimentary rock underlies the plain, Plain. Yet, Warner's evidence for lateral movement but what, if any, pre-Cenozoic basement underlies the does not appear to have been generally accepted by plain is not known. The large Bouguer gravity high investigators of the plain. The trend of the western and the approximate isostatic equilibrium of the Snake River Plain is approximately parallel to the plain relative to the adjoining highlands is in marked San Andreas and related faults southwest of the contrast to the basin and range structures to the plain, but no evidence of the right-lateral movement south. With the basin and range structures, gravity of this system along the plain has been reported. highs do not occur over the basins; isostatic balance Although the geophysical data do not establish that does not exist between the basins and the ranges; and lateral movement has not occurred along the western the bounding faults do not appear to extend deep into Snake River Plain, lateral movement does not appear the crust (Mabey, 1966). In contrast, the structure of to be a likely cause of the major geophysical anoma- the western Snake River Plain extends deep into and lies associated with the plain. perhaps through the crust. If the term graben is used Several investigators have inferred a relationship

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Figure 4. Residual total intensity magnetic map of southern Idaho at 3.8 kilometers above sea level. Heavy dashed lines indicate major divisions of the plain. Narrow dashed lines are lineaments defined by the geophysical data. Contour interval is 100 gammas. Map from Bhattacharyya and Mabey (1980). Mabey—Geophysics and Tectonics, Snake River Plain 145

rocks that were not strongly magnetized is not consistent with the measured magnetic anomaly, which requires strongly magnetized rock at depth. relatively thin slide block, a WAS INGT N Perhaps, the granite is a floored intrusive unit, or a narrow sliver. The possibil- 1,5 A ity that a thick sequence of pre-Miocene volcanic NTANA rock may underlie the western Snake River Plain has not been evaluated. IDAHO The western Snake River Plain appears to be a rift. OREGON However, the western Snake River Plain differs significantly from typical modern continental rifts such as the Rio Grande rift in Colorado and New Mexico and the Dead Sea and east African rifts. One

/iii//222/ CALI ORNIA NEVADA UTAH

/11 111 2.5

0 500 KILOMETERS

Figure 5. Contour map of heat flow in Idaho and adjacent areas. Map modified from Lachenbruch and Sass (1980). Values are heat-flow units.

40 80 120 between the western Snake River Plain and a better KILOMETERS exposed but narrower rift of parallel trend in northern Nevada (Christiansen and McKee, 1978; Zoback and SNAKE RIVER PLAIN 0 -0255 Cc Thompson, 1978; Mabey, 1976). The rift in Nevada, •0 3g cc which is Miocene in age, consists of a swarm of basalt a Y +03 dikes overlain by a graben, filled with basalt flows 20 and sediments. Christiansen and McKee suggest that the rift in Nevada and the western Snake River Plain Figure 6. Interpreted gravity profile A-A', across the western Snake River Plain, Idaho. Location of profile shown in Figure 1. developed as a continuous rift about 17 million years ago. They postulate that the two structures were offset in a right-lateral sense by a transform fault prominent difference is that the other rifts coincide coincident with the eastern Snake River Plain and the with gravity lows produced by low-density sediments Brothers fault zone and related fault zones in Oregon. within the rift. Although the sediments underlying the If a rift is defined as an elongate rupture of the western Snake River Plain produce a gravity low, the entire thickness of the lithosphere under tension, such low is much smaller than the larger gravity highs with an origin of the western Snake River Plain appears approximately the same extent produced by deeper consistent with most of the geological and geophysical mass anomalies. The gravity anomaly is similar to evidence. The upper crust apparently was thinned or that in the Salton Trough or the mid-continent pulled apart as major extension began about 17 gravity high. million years ago, and the resulting depression was Two primary structures can explain the heat-flow filled with upwelling basalt and debris from the pattern in the western Snake River Plain. Brott and adjoining highlands. The only major inconsistencies others (1978) propose the emplacement of a large are the cuttings of granitic rock reported from the heat source under the plain about 20 million years bottom of the deep drill hole 70 kilometers south of ago and the development of an overlying crust with Boise (McIntyre, 1979) and the suggestion given by low thermal conductivity and low heat-generating the magnetic high over Eocene volcanic rocks south capacity. The resulting model would account for the of the plain that the magnetic high under the plain measured high heat flow along the margins of the may relate to Eocene volcanic rocks. The presence of plain and the low values on the plain. An alternate granitic basement at the bottom of the drill hole explanation would involve the continued extension overlain by a section of volcanic and sedimentary concentrated along the margins of the plain and 146 Cenozoic Geology of Idaho continued subsidence as the plain seeks isostatic the north edge of the western Snake River Plain balance. Either thermal model is consistent with the generally parallel its margin, the magnetic low is formation of the western Snake River Plain in interrupted where this margin of the plain begins the Miocene time as a tension rift. eastward arc. Patterns in both the gravity and the magnetic data can be interpreted to indicate a structure across the plain on a projection of the linear CENTRAL SNAKE RIVER PLAIN northeast border of the western plain (Figures 2, 3, and 4). A northwest grain in the magnetic data is apparent to the east of this projection. The boundary between the western and central The amplitude of the gravity high over the central Snake River Plain is here taken as an east-northeast segment of the plain is about 75 milligals, contrasted line extending across the Snake River Plain from the with 100 milligals over the western plain. The crest of south end of the Owyhee Mountains at Poison Creek the gravity high lies near the north edge of the plain, to the southern flank of the Mount Bennett Hills and in two places to the south (Figure 2), closed (Figure 1). Southeast of this line the plain widens as gravity highs trend parallel to this main gravity crest. the southwest margin of the plain loses topographic Although the amplitude of the gravity high over the expression and bends to the south and as the central plain is lower than over the western plain, the northeast margin curves to the east and also becomes plain and the total gravity high are broader and the a less prominent topographic feature. The prominent cross-sectional area of the positive mass underlying magnetic high that lies along the southwest margin of the central plain is as large or larger than that under the western plain bends to the south and continues to the western plain. near the Idaho-Nevada border (Figure 4). Here, the The magnetic pattern over the central Snake River high makes a bend of about 90 degrees to the east, Plain is similar to that over the western plain but the and generally corresponds to the margin of the amplitude of the high along the south edge is greater. Bruneau-Jarbidge eruptive center (Bonnichsen, 1982 A northwest grain to the magnetic anomalies is also this volume). A less prominent gravity gradient is more prominent, particularly in the eastern part. coincident with the magnetic anomaly. The margin of No deep seismic-refraction data are available across the Snake River Plain in this segment has no well- the central Snake River Plain, but the gravity anomaly defined topographic or surface geologic expression suggests a thinning of the upper crust. The thinning, and is here assumed to coincide with the south edge distributed over a greater width than under the of the subsurface mass indicated by the magnetic high western plain or shallower positive mass anomalies, and the gravity gradient. may cause the greater width of the gravity high. As in Silicic volcanic rocks extend southwest of the plain the western plain, a layer of dense, strongly magne- into northern Nevada and southeast Oregon. South tized material appears to be overlain by less dense of the plain silicic volcanic rocks lap onto a highland and less strongly magnetized rock. in northern Nevada with a core of pre-Tertiary The heat-flow pattern over the central plain is sedimentary and intrusive igneous rocks and onto the similar to the west except that high heat flow on the Albion Mountains with a core of Precambrian gneiss southwest is continuous with the large area of high domes. On the north are the Tertiary silicic volcanic heat flow in northern Nevada (Figure 5). The model rocks of the Mount Bennett Hills and further north, of Brott and others (1981) for the western plain also beyond the west-trending Camas Prairie, is the Idaho applies to the western part of the central plain, but to batholith. the east an area of low heat flow related to the The Tertiary sedimentary rocks that cover much of horizontal movement of ground water in volcanic the surface of the western Snake River Plain extend aquifers extends into the central plain. into the northwest part of the central plain, but most The similarity between the geophysical anomalies of the central plain is covered by volcanic rocks. In and the geology in the western and central Snake the west and south, the volcanic rocks are Miocene River Plain suggests that these two parts of the plain and Pliocene rhyolite overlain by Pliocene and Quater- have a similar origin. One explanation is that they nary basalt and, in the east, Quaternary basalt. Faults both developed during extension in response to trending north to northwest are common in the older regional tension approximately normal to the trend volcanic rocks. No data from deep holes are available of the present western Snake River Plain. Southwest from this part of the plain, but Bonnichsen (1982 this of the edge of the Idaho batholith, the extension and volume) has suggested that calderas are beneath this the resulting thinning of the upper crust and subsi- area, buried within the Bruneau-Jarbidge eruptive dence of the surface may have been concentrated in a center. narrow zone, thus producing the structure of the Although the gravity and magnetic features along western Snake River Plain. Southeast of the batholith, Mabey—Geophysics and Tectonics, Snake River Plain 147

the extension may have been distributed over a wider basalt in the central part of the eastern plain (Kuntz zone producing the structure of the central Snake and Dalrymple, 1979; Spear and King, 1982 this River Plain. If the crest of the gravity high reflects the volume). Rhyolite ash flows are common around the zone of maximum thinning of the upper crust, the margins of the plain, and rhyolite has been found in eastward arcing of the crest of the gravity high all of the deeper drill holes on the plain. Cenozoic suggests a greater south component of movement of sediments are abundant in the surface and shallow the block south of the Idaho batholith. Perhaps the subsurface near the margins of the plain. Although block south of the plain was rotated clockwise. cracks and fissures with trends generally normal to or parallel to the axis of the plain are common,evidence of displacement along them is rare. EASTERN SNAKE RIVER PLAIN The structure of segments of the margins of the eastern Snake River Plain appear to differ. The linearity and steep gravity gradient parallel to the The magnetic and gravity patterns change abruptly margin of the plain east of Arco suggest a normal on the east at a zone that trends northwest across the fault near the plain margin. Along the south edge of plain from the northeast end of the Albion Mountains the plain west of Pocatello, the ranges appear to dip (dashed line, Figure 1). Southwest of this zone, a under the margins of the plain but the gravity data pronounced single or compound gravity high trends suggest they are terminated in the subsurface a few subparallel to the plain. Northeast of the zone, the kilometers to the north. East of Idaho Falls, a Bouguer gravity anomaly is generally higher than segment of the margin of the plain is coincident with over adjoining areas but consists of a complex the edge of the Rexburg caldera complex (Embree pattern of highs and lows with the largest closed and others, 1982 this volume). gravity high elongated approximately normal to the Although the eastern Snake River Plain extends axis of the plain. No continuous magnetic high lies into the Intermountain seismic belt, there are few along the south edge of the eastern plain. Over the earthquakes under the plain. Apparently, whatever plain instead is a series of magnetic highs and lows deformation is occurring does not involve large-scale with high amplitude and a grain normal to the plain. brittle failure of rocks underlying the plain. Thus, no evidence exists of a dense, strongly magne- Northeast of the eastern Snake River Plain lie the tized layer underlying the eastern Snake River Plain. large Island Park and Yellowstone caldera complexes. The land surface along most of the axis of the eastern Another large caldera complex, the Rexburg caldera plain is higher than the margins, indicating greater complex, has been identified on the eastern Snake recent subsidence of the margins. River Plain, and the existence of several others has Southeast of the eastern plain are basin and range been inferred (Embree and others, 1982 this volume). structures trending approximately normal to the A northeast decrease in age of the silicic volcanic plain. The ranges are primarily folded pre-Tertiary rocks in and adjacent to the plain has been noted sedimentary rock, and the valleys commonly are from southwestern Idaho to Yellowstone National underlain by thick accumulations of Cenozoic sedi- Park (Armstrong and others, 1975). This large-scale ments. Pliocene and Quaternary volcanic rock occur progression has been interpreted as indicating that in several areas. Northwest of the eastern plain is a the central and eastern Snake River Plain coincides zone of basin and range structures flanked on the with the track of a northeastward-moving center of southwest by the Pioneer and White Knob Mountains silicic volcanism (Christiansen and McKee, 1978). and on the northeast by the east-trending Centennial Only one deep hole has been drilled on the eastern Mountains. Folded pre-Tertiary sedimentary rocks Snake River Plain. This hole, which is on the Idaho are the most common rocks in the ranges north of the National Engineering Laboratory about 30 kilometers eastern plain, but Eocene extrusive and intrusive east of Arco, was 3,155 meters deep and was bottomed igneous rocks are abundant. Geologic mapping north in a rhyodacite porphyry. The rock in the lower part and south of the plain has not revealed any evidence of the hole, which has been dated at 11.2 million years of lateral movement parallel to the axis of the plain (McBroome, 1981), may be an ash-flow tuff or a high- (S. S. Oriel, oral communication, 1980). level intrusive rock. Doherty and others(1979) suggest Most of the eastern Snake River Plain is covered that this hole was drilled into a caldera. Drilling and by Quaternary basalt flows. The younger flows resistivity soundings (Zohdy and Bisdorf, 1980) sug- appear to have erupted along fissures normal to the gest that Quaternary basalt is generally less than 1 plain approximately parallel to and in some places kilometer thick on the eastern plain, although in part along extensions of basin and range structures to the of the area it may be thicker. north and south. Rhyolite domes ranging in age from Extensive seismic-refraction profiling has been 0.3 to 1.5 million years rise above the surface of the done in the eastern Snake River Plain and Yellow- 148 Cenozoic Geology of Idaho stone National Park. Preliminary reports on the than adjoining areas. interpretation of these data suggest that the velocity Deep conductivity data are available in the eastern structure of the crust under the eastern Snake River Snake River Plain from magnetotelluric(Stanley and Plain is similar to that of the western plain with a thin others, 1977) and magnetic variometer soundings upper crust and a thick lower crust and suggests that (Fitterman, 1979; Wier, 1979). Published interpreta- the total crust thickens northeastward toward Yellow- tions of the magnetotelluric data indicate a conductive stone National Park (Braile and Smith, 1979). layer in the crust at depths of from 12 to 22 The regional gravity high centered over the eastern kilometers, but neither technique reveals a deep Snake River Plain extends well beyond the plain conductive anomaly coincident with the plain. (Figure 7). This high apparently reflects the thinning A delay of up to 1.5 seconds in teleseismie P-wave of the upper crust toward the center of the plain arrivals suggests that a low-velocity zone extends to indicated by the refraction data, and the thinning depths of over 200 kilometers under the northwest appears to extend well beyond the plain. Super- edge of the eastern Snake River Plain (Evans, 1981). imposed on this high are two gravity lows produced This anomaly is a westward continuation of the P- by the Cenozoic sedimentary and volcanic rocks wave delay anomaly underlying Yellowstone National underlying the margins of the plain. The maximum Park. Thus, the Snake River Plain appears to be thickness of these Cenozoic rocks has not been offset from the low-velocity zone in the mantle. Shear determined, but it is likely to be several kilometers. wave velocities of the crust and upper mantle are The source of most of the more extensive magnetic lower under the eastern Snake River Plain than in the highs on the eastern Snake River Plain is uncertain. Basin and Range province to the south (Greensfelder The basalt flows in the near surface of the plain are and Kovach, 1980). Both low-velocity anomalies strongly magnetized with both normal and reversely might reflect high-temperature zones. magnetized flows, but the more extensive magnetic Although the surface relief across the eastern highs have been interpreted as having a deeper source Snake River Plain is about equal to that of the plain (Bhattacharyya and Mabey, 1980). Some of the highs farther west, the gravity and magnetic anomalies on the plain near the south edge are over uplifted suggest a somewhat different crustal anomaly. Rather blocks of Cenozoic rock and can be inferred to relate than an abrupt thinning or perhaps complete parting to underlying Cenozoic intrusive rocks. Eaton and of the upper crust, as appears to have occurred in the others (1975) and Mabey and others (1978) have western plain with an outpouring of large volumes of pointed out that the eastern Snake River Plain is part basalt, the thinning of the crust under the eastern of a northeast-trending magnetic zone: the Humboldt plain is a broader feature; no evidence of large early zone, that extends through Yellowstone National eruption of basalt is apparent. Instead, a complex of Park. To the northeast of Yellowstone, this zone is silicic volcanic rocks and related intrusive bodies reflected in Precambrian rocks. Although the source appear to underlie most of the eastern plain. of the deeper magnetic anomalies over the eastern The heat-flow pattern over the eastern plain is also Snake River Plain is not known, the crust under the similar to that in the west. Over most of the eastern plain is composed of more strongly magnetized units plain, the near-surface heat flow is disturbed by the southwestward movement of ground water down the

NW SE plain in the volcanic aquifer that underlies most of 8 s00- the eastern plain. Brott and others (1981) believe that this movement of water is the cause of the low heat 80 flow measured over the interior of the plain and that the heat flow under the aquifer is very high. However, so the thermal data from the three holes drilled through 00 the aquifer in the INEL area of the plain do not 40 confirm that the thermal gradients below the aquifer 8OUGUER GRAVITY ANOMALY are higher than those along the margins of the plain. 20 The free-air gravity anomaly indicates that the eastern part of the criake River Plain, like its central 0 o 50 100 150 200 250 and western segments, is in approximate isostatic KILOMETERS equilibrium with the adjoining highlands. The thin-

SNAKE RIVER PLAIN ning of the upper crust under the plain represents a positive mass anomaly that is the compensation for .03g q, 20— the low surface elevation of the plain. The relative Figure 7. Interpreted gravity profile B-130, across the eastern Snake thinning of the upper crust could result from exten- River Plain, Idaho. Location of profile shown in Figure I. sion normal to the plain, from lateral mass transfer Mabey—Geophysics and Tectonics, Snake River Plain 149 related to igneous activity, or a combination of the basalt is erupted through them suggests that they two. Extension normal to the plain would account for extend to great depth. The model of Brott and others the thinned upper crust, and the concentration of the (1981) suggests that cooling and related contraction thinning near the margins could explain the apparent at depth could account for the opening of the rifts subsidence and high heat flow there. However, the normal to the axis of the plain. However, these rifts evidence of northwest regional extension in this part may reflect a more regional extension suggested by of the western United States in late Tertiary and the basin and range structures to the north and south. Quaternary time is unconvincing. The average density The most prominent volcanic rift on the eastern of the crust can also be increased by the ejection of Snake River Plain is the Great Rift along which the lower density material in volcanic eruptions; however, Craters of the Moon lava field occurs. On the north, there is no evidence of the very large eruptions that this rift is coincident with an aeromagnetic anomaly would be required to remove the volume of upper that extends across the edge of the plain and for 100 crustal material necessary to produce the density kilometers north of the plain. To the north this decrease indicated by the gravity anomaly. anomaly is coincident with a zone which contains In its simplest form, the thermal model suggests a several Cenozoic plutons. On the plain, the rift arcs to heat source propagating northeast relative to the the south to parallel basin and range structures south surface from southwest Idaho to a current position of the plain. Thus, the rift appears to be related to under Yellowstone National Park. Brott and others structures that extend beyond the plain. (1981) propose a quantitative model with a 120- The apparent relationship between the rifts on the kilometer-wide heat source plane normal to the eastern Snake River Plain and the basin and range Snake River Plain moved through a 42-kilometer- faults to the north and south suggests they represent thick slab of lithosphere at a velocity of 35 millimeters different responses to extension of the crust approxi- a year. The model combines the heating and cooling mately parallel to the axis of the plain. Normal faults in this thermal event with a density increase under the south of the plain, and presumably those north of the plain, which they believe could be caused by extension, plain, appear to be controlled by old structures, many dispersion of granitic material, extrusion of ash related to overthrusting; many of the normal faults flows, or erosion. In this model, the major subsidence flatten at depth to merge with major thrust faults. of the plain is caused by isostatic adjustment of the Movement along these faults results in a thinning of a density increase. In their model, the continued subsi- surface layer that appears to generally correspond dence is caused by contraction due to cooling, as is to the allochthonous unit involved in the thrusting. the development of rifts and the elevation decrease of The underlying layer appears to extend by ductile the plain toward the southwest. Morgan (1972) had flow without the development of major normal faults. earlier suggested that the Snake River Plain is the Although the belt of overthrusting probably initially track of a mantle plume over which the continent extended across the area of the Snake River Plain, moved in Cenozoic time, whereas Smith and Christian- subsequent igneous activity along the plain and sen (1980) suggest a mantle hot spot. Blackwell(1978) perhaps extension normal to the plain have disrupted has suggested that the heat source is related to the thrust faults. Current extension of the upper layer magma developed over a subducted slab. Others have on the plain is not accommodated along preexisting suggested that the Snake River Plain is a zone of thrust faults but rather occurs by the opening of extension or a propagating fracture (Hamilton and vertical fractures to the depth at which extension is by Myers, 1966; Lachenbruch and Sass, 1978; Smith, ductile flow. The greater subsidence of the margins of 1977, 1978; Furlong, 1979). In these models, a the plain could occur under tension parallel to the mechanical process is the primary cause of the margins, if the ductile layer is thicker there, or it may structure and the thermal events. Numerous combina- reflect the development of more local isostatic balance. tions of thermal and mechanical processes are pos- Although the entire Snake River Plain appears to sible. A further complication is that the low-velocity be in approximate isostatic equilibrium with the area zone in the mantel indicated by the P-wave delay to the north and south, a more regional isostatic anomaly, which is probably related to a deep heat anomaly is apparent on the Faye or average free-air source, lies north of the plain. anomaly map of southern Idaho (Figure 3). Over Numerous fractures extend inward from the mar- most of southern Idaho, the average free-air anomaly gins of the eastern Snake River Plain. From segments values are positive, as contrasted with near-zero of these fractures, large volumes of basalt have values in Nevada(Mabey, 1966). However, the south- erupted; other are open fissures in basalt flows(Kuntz eastern Idaho values are even more positive with and others, 1982 this volume; King, 1982 this volume). most of the area higher than +20 milligals. The These fractures must be tensional features related to isostatic equilibrium of the plain relative to adjacent extension parallel to the axis of the plain; the fact that areas superimposed on a more regional isostatic 150 Cenozoic Geology of Idaho

disequilibrium suggests that the compensation of the crust. Active volcanism, open fissures, and evidence plain occurs at relatively shallow depths but that a of continued subsidence show that the plain is an much larger region apparently centered northeast of active structure that continues to develop. A sugges- the Snake River Plain is undercompensated. Perhaps, tion exists in the magnetic data that the eastern Snake deep dynamic forces account for a part of the high River Plain may be part of a northeast trend reflected elevation of this large region. in Precambrian rocks to the northeast, but no evidence has been reported to suggest that the plain existed before Miocene time. Several lines of evidence do suggest that the development of the plain may CONCLUSIONS coincide with the large-scale extension of the Basin and Range province to the south, which began about 17 million years ago. Geophysical data and deep drilling have been used The western and central Snake River Plain appears to obtain information on several aspects of the deep to be, at least in part, a down-faulted block. It may be structure of the Snake River Plain. Major problems termed a graben or a rift but differs from typical addressed are: (1) What regional anomalies in the continental grabens and rifts. It probably developed crust and upper mantle are associated with the plain? in Miocene time in response to west-southwest re- (2) What is the nature of the stress fields and thermal gional extension, and perhaps clockwise rotation of events that have formed the plain?(3) What is the age the block south of the plain, which greatly thinned or of the plain? (4) What is the primary structure of the completely parted the upper crust. The gravity and major segments of the plain?(5) What rocks underlie magnetic data suggest that a thick layer of dense the plain at depths greater than the deeper drill holes? strongly magnetized rocks formed within the resulting (6) Do structural patterns adjacent to the plain depression. The plain has continued to subside per- extend onto the plain? haps because of cooling, continued tension, loading The entire Snake River Plain appears to be of sediment, and isostatic adjustment due to the large underlain by a thicker crust than the Basin and Range positive mass under the plain. province to the south. The Snake River Plain crust The eastern Snake River Plain appears to have a consists of a thin upper crust and thick lower crust. more complex history than the western plain. The P-wave delays and shear-wave velocities indicate eastern plain is basically a downwarp containing that the mantle underlying the region of at least the numerous calderas. Extension normal to the plain is a eastern Snake River Plain is also anomalous. The possible explanation for the thinned upper crust velocity structure of the eastern Snake River Plain under the plain and the high thermal flux indicated has been studied intensely in recent years, and an by the intense volcanic activity, Alternately, the plain analysis of the data obtained will provide a velocity may represent the track of a thermal anomaly now model of this part of the plain. More refraction data centered under Yellowstone National Park that moved are needed to define the velocity structure of the northeastward across southern Idaho producing a central and western plains. The anomalous crust and complex of calderas that are now largely obscured by upper mantle very likely are directly related to the younger basalt flows on the Snake River Plain. Such formation of the plain and probably preceded or a thermal event can be used to explain the formation accompanied the formation of the plain. The thin of the eastern Snake River Plain only if some upper crust can be explained by extension, but the mechanism for the development of a large positive cause of thick lower crust is not obvious. Brott and mass anomaly under the plain is also involved. The others (1981) postulate that magma above a subduct- current tectonics of the eastern plain appear to be ing slab coalesced under the Snake River Plain in the extension parallel to the axis of the plain. manner that salt flows laterally into an initial pertur- The gross structure of the central Snake River bation. A mantle hot spot or plume may have Plain combines elements of both the western plain thickened the lower crust as it moved under the plain. and the eastern plain. There is evidence of a down- The Snake River Plain-Yellowstone crust and upper faulted block filled with dense, strongly magnetized mantle appear to be a unique intraplate anomaly, and rock, but the current deformation appears to be the events in its development are not yet understood. downwarping. This segment of the plain probably Regardless of how the crustal and upper mantle formed with the western plain but has an overprint of anomaly under the Snake River Plain formed, the the structure of the eastern plain. structure of the upper crust underlying the plain What rocks and structures underlie the Snake appears to have developed in a regional stress pattern River Plain at great depth remains unknown. One of that is much more extensive than the plain. the deep drill holes on the western Snake River Plain The Snake River Plain is a major feature in the apparently bottomed in granite, which could be part Mabey—Geophysics and Tectonics, Snake River Plain 151 of an extensive Cretaceous mass of granite that Bhattacharyya, B. K. and D. R. Mabey, 1980, Inter- makes up the Idaho batholith north of the plain and pretation of magnetic anomalies over southern is present south of the plain in less extensive outcrops Idaho using generalized multibody models: U. S. in southwestern Idaho and northern Nevada. How- Geological Survey Open-File Report 80-475, 48 p., ever, the very large gravity and magnetic anomalies 9 figs. over the western plain suggest that the crust under the Blackwell, D. D., 1978, Heat flow and energy loss in plain is drastically different from adjoining areas. the western United States, in R. B. Smith and Thus, it appears unlikely that the pre-Tertiary rocks G. P. Eaton, editors, Cenozoic Tectonics and north and south of the western plain make up the Regional Geophysics of the Western Cordillera: bulk of the crust under the plain. The magnetic Geological Society of America Memoir 152, p. anomaly and particularly the gravity anomaly over 175-208. the eastern plain are not as profound as over the Bonini, W. E. and P. M. Lavin, 1975, Gravity anoma- western plain. The upper crust under the eastern plain lies in southern Idaho and southwestern Montana: is composed of more magnetic units than are adjoin- Geological Society of America Bulletin, v. 68, p. ing areas, indicating a greater abundance of igneous 1702. rocks in the crust under the plain. The response of the Bonnichsen, Bill, 1982, The Bruenau-Jarbidge erup- eastern plain to the regional stress is very different tive center, southwestern Idaho, in Bill Bonnichsen from areas to the north and south. This suggests that and R. M. Breckenridge, editors, Cenozoic Geology the structure and perhaps the lithology are different. of Idaho: Idaho Bureau of Mines and Geology The average density of the upper crust under the Bulletin 26. eastern plain is probably not greatly different from Braile, L. W. and R. B. Smith, 1979, The structure of the adjoining areas, but intense igneous activity in the crust in the Yellowstone-Snake River Plain Cenozoic time has probably produced major changes area and adjacent provinces and the implications in the lithology and destroyed most of the older for crustal evolution: EOS, American Geophysical structures. The basin and range structures north and Union, v. 60, no. 46, p. 941. south of the eastern Snake River Plain either termi- Brott, C. nate at the margin of the plain or extend only a few A., D. D. Blackwell, and J. C. Mitchell, 1976, kilometers onto the plain. However, the pattern of Geothermal investigations in Idaho, Part 8, Heat flow volcanic rifts on the plain suggests that they may be study of the Snake River Plain, Idaho: related to the basin and range structures. Idaho Department of Water Resources Water Concealed beneath a complete cover of Cenozoic Information Bulletin 30, 195 p. sediments and volcanic rocks, several fundamental , 1978, Tectonic implications of the heat aspects of the deep structure of the Snake River Plain flow of the western Snake River Plain: Geological remain uncertain despite the acquisition of consider- Society of America Bulletin, v. 89, p. 1697-1707. able regional geophysical data and the drilling of Brott, C. A., D. D. Blackwell, and J. P. Ziago, 1981, several holes to depths of over 3 kilometers. More Thermal and tectonic implications of heat flow in geophysical data and creatively combined interpreta- the eastern Snake River Plain, Idaho: Journal of tions of all of the available data will continue to add Geophysical Research, v. 86, p. 11709-11734. to our understanding of the area, but many critical Christiansen, R. L. and E. H. McKee, 1978, Late data can be obtained only by drilling very deep holes Cenozoic volcanic and tectonic evolution of the and taking continuous core samples. Great Basin and Columbia Intermontane regions, in R. B. Smith and G. P. Eaton, editors, Cenozoic Tectonics and Regional Geophysics in the Western Cordillera: Geological Society of America Memoir 152, p. 283-312. REFERENCES Doherty, D. J., L. A. McBroome, and M. A. Kuntz, 1979, Preliminary geological interpretation and Armstrong, R. L., W. P. Leeman, and H. E. 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