T~tonophysics. 164 (1989) 323-343 323 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
p. 74
Regional implications of heat flow of the Snake River Plain, he up. Northwestern United States =. AIn.
. 1968. D.D. BLACKWELL 1I beat
Ynytri. Department of Geological Sciences, Southern Methodist University, Dallas, TX 75275 (U.S.A.) 19 (2): (Received March 24, 1988; revised version accepted October 24,1988) eratioo '. , 1974. Abstract >hysi""
, 1982. Blackwell, D.O., 1989. Regional implications of heat flow of the Snake River Plain, Nonhwestern United States. In: V. sibirsk. Cermak, L. Rybach and E.R. Decker (Editors), Heat Flow and Lithosphere Structure. Tectonophysics. 164: 323-343.
The Snake River Plain is a major topographic feature of the Northwestern United States. It marks the track of an upper mantle and crustal melting event that propagated across the area from southwest to northeast at a velocity of about 3.5 cm/yr. The melting event has the same energetics as a large oceanic hotspot or plume and so the area is the continental analog of an oceanic hotspot track such as the Hawaiian Island-Emperor Seamount chain. Thus. the unique features of the area reflect the response of a continental lithosphere to a very energetic hotspot. The crust is extensively modified by basalt magma emplacement into the crust and by the resulting massive rhyolite volcanism from melted crustal material, presently occurring at Yellowstone National Park. The volcanism is associated with lillIe clust~l extension. Heat flow values are high along the margins of the Eastern and Western Snake River Plains and there is abundant evidence for low-grade geothermal resources associated with regional groundwater systems. The regional heat flow pallern in the Western Snake River Plains reflects the influence of crustal-scale thermal refraction associated with the large sedimentary basin that has formed there. Heat flow values in shallow holes in the Eastern Snake River Plains are low due to the Snake River Plains aquifer, an extensive basalt aquifer where water flow rates approach 1 2 km/yr. Below the aquifer, conductive heat flow values are about 100 mW m- • Deep holes in the region suggest a 2 2 systematic eastward increase in heat flow in the Snake River Plains from about 75-90 mW m- to 90-110 mW m- . Temperatures in the upper crust do not behave similarly because the thermal conductivity of the Plio-Pleistocene sedimentary rocks in the west is lower than thaI in the volcanic rocks characteristic of the Eastern Snake River Plains. Extremely high heat loss values (averaging 2500 mW m - 2) and upper crustal temperatures are characteristic of the Yellowstone caldera.
Introduction data are presented together with extensive infor mation for other areas of Idaho, especially the The Snake River Plain (Fig. 1) has been a focus southern part of the Idaho batholith, by Blackwell of an extensive series of heat flow studies (Brott et (1989). These data allow a detailed analysis of the aI., 1976, 1978, 1981). The purpose of this report is heat flow, the geothermal gradient distribution, to discuss new temperature gradient and heat flow and the regional geothermal potential of the vari data for the Western Snake River Plain, repre ous physiographic provines in Idaho. senting a continuation of these studies, to sum The only other published heat flow studies marize all heat flow data, to discuss new thermal dealing with the area have been local in nature data from deep holes, and to discuss the regional (Sass et aI., 1971a; Urban and Diment, 1975; implications of the thermal character. The detailed Morgan et aI., 1977; Nathenson et aI., 1980; Urban
0040-1951/89/$03.50 © 1989 Elsevier Science Publishers B.V. 324 115° 113° 111° northwe + 46° Plain. 1 46'1----1----+\ + + + + + + basemer northen Mounta which il :'::;;;::=:==T-""14 5° tary rocl NORTHERN ROCKY 1 4 + Mounta MOUNTAINS > " Central area is ( and stru + 44" alluvial 44" \ phy is Snake "R in this. fresh fal + 43° 43° Borah I Valley I and Sm of the r: Paleozo 42" + the Mes AQe part of "'ellowstone 0.6 Little Jacks 9.7 He",,. Fori< 1.3 Bruno••- Jorbrldg 11.3 N-S-trc
I'land Pork 20 Juniper MOU1toin 13.8 acterize l northwestern border of the Western Snake River the behavior of the continental lithosphere when Plain. These Miocene basalts overlie an older acted on by a massive thermal event. The setting is basement about which little is known. In the relatively unusual and still not widely appreciated northern part of Idaho the Northern Rocky as an example of massive continental volcanism of Mountains includes a large Mesozoic batholith the bimodal type which is not associated with which intrudes Precambrian belt series sedimen significant extension (in fact, the crust is thick tary rocks. A major portion of the Northern Rocky ened). Mountain province in Idaho is identified as the Central Idaho Basin and Range subprovince. This Summary of physiography and geologic history area is composed of Basin and Range topography and structure with high-relief ranges separated by Brott et al. (1978) emphasized that the three alluvial valleys. The general trend of the topogra main physiographic regions associated with the phy is NW-SE (almost at right angles to the Yellowstone-Snake River Plain feature represent Snake River Plain). The youthfulness of the ranges time snap-shots of the response of the continental in this area is clearly indicated by the numerous lithosphere to the passage of a massive thermal fresh fault scarps and the occurrence of the M 7.3 event. A suite of huge rhyolite calderas from which Borah Peak earthquake beneath the Lost River large volume ash flows (100's-1000's of cubic Valley near Mackey on October 28, 1983 (Doser kilometers) have been extruded during the past 2 and Smith, 1985; Scott et aI., 1985). The bedrock m.y. (Hildreth et aI., 1984) occurs at high eleva of the ranges consists of folded and thrust-faulted tion (approximately 2500 m) in Yellowstone Park. Paleozoic and Precambrian sedimentary rocks of To the west of this caldera system, west of the the Mesozoic Cordilleran thrust belt. The northern oldest clearly exposed caldera (in Island Park: part of the Basin and Range province consists of Christiansen, 1982), the Eastern Snake River Plain N-S-trending ranges and valleys in an area char has been described as a regional downwarp acterized by late Cenozoic extension more-or-Iess (Kirkham, 1931). Surface rocks are basalts, some at right angles to the trend of the Snake River younger than 10.000 yrs. The geologic section Plains. encountered in a 3150-m deep test well at the The Snake River Plain-Yellowstone feature Idaho National Engineering Laboratory near Arco represents a hotspot in the same energetic sense as is probably typical for this area of the Snake River Hawaii because the thermal output of the Yellow Plain. A total of 600 m of Snake River basalt is nh In. stone geothermal system and the Hawaiian encountered in the hole, followed by 50 m of 1 a~ volcanoes is similar. Using the Cl balance tech sedimentary rocks, followed in turn by 2500 m of nique, Fournier et al. (1976) measured a heat loss rhyolite ash, ashflow tuff and possible hypabyssal rate at Yellowstone of 2 X 10 9 caIjs. If a magma intrusive rocks (Doherty et aI., 1979: Morgan et temperature changes by 600 0 C (or loses 120 caIjg, aI., 1984). Although the structure of the Eastern including latent heat), a volume of 0.02 km3/yr Snake River Plain appears to be a downwarp. an. would be required assuming no losses. Thus, an along the northwestern edge the surface rocks the intrusive rate in the order of 10- 1-10- 2 km3/yr cover a large-scale normal fault or caldera rim the seems reasonable. This rate of magma emplace (Braile et aI., 1982). Based on geophysical and nd ment compares to Hawaii where, according to geologic investigations, several large calderas (Fig. Jet' Swanson et al. (1975), the magma supply rate to 1) are inferred to be buried beneath the surface 1 ~m Kilauea and Mauna Loa is in the order of 10- cover of basalts (Embree et aI., 1982: Morgan et km)/yr, and to Iceland where the eruption rate aI., 1984). 2 3 ro- (over a 400-km long zone) is 5 X 10- km /yr. The Western Snake River Plain consists of a au However, whether this hotspot is mechanistically deep sedimentary basin with a thickness of .he similar to those operating beneath the ocean plates Pleistocene-Pliocene rocks in excess of 2-2.5 km. t~. is still the subject of discussion. Study of the Interbedded with and beneath the sedimentary he Snake River Plain allows detailed investigation of rocks are a series of basalt flows. The main sedi 326 mentary basin (the Western Snake River Basin) YELLOWSTONE CALDERA (Newton and Corcoran, 1963) appears to be a NW to-----ot SE 4 fault-bounded graben and the internal structure of AVERA o,~•••~.~.". I.I.I!,.!!!!I!.... SL ...... , ...... 5.1".,,, ~ .. [LEI/AT the basin is now becoming clear (Wood et aI., .~ .~ :t:~.=.r-;~•."!' 1<"'" 1980; Wood and Anderson, 1981; Wood, 1984). 20 :':;":"~""7~':'~: The locus of silicic volcanic activity 10-20 m.y. ~b(·:·~~. .' I.. ago was at the extreme western edge of Idaho south of the sedimentary basin. Most of the calderas that were the sources of the ashflows N:-CIBR----ESRP-I----BR--~ during this period appear to be located beneath 3 basalt flows and silicic volcanics in the Owyhee Plateau south of the Western Snake River Plain sedimentary basin (see Malde, 1989) for a discus sion, and Fig. 1). Evidence of the divergence of the trend is the fact that deep wells in the Western 40 Snake River Plain (e.g., Ore-Ida 1 (3100 m) and o J.N. James 1 (4267 m), just west of Boise» do not encounter significant thicknesses of rhyolitic rocks. fig 3. ~ These seemingly disparate physiographic re gions are related to each another by their unusual crustal structure. The crustal section, based on seismic refraction experiments (Hill and Pakiser, by L~ 1965; Braile et aI., 1982) consists of an unusually ward thick lower crust and an unusually thin upper basalt] volcaIl crust. Three cross sections are shown in Fig. 2 (the 100 200 300 locations are shown in Fig. 1). Because of the OISTANCE. km in the inherent ambiguities, earlier gravity studies were Fig. 2. Crustal cross sections for the Snake River Plain- 'lei· diagra often in!prpreted as indicating a thinner than nor lowstone region. The Western Snake River Plain (WSRF) lavas cross section is from Hill and Pakiser (1965) and the Eastern mal crust (Hill, 1963; Mabey, 1976). A unique progre Snake River Plain (ESRP) and Yellowstone cross sectiOn> are the Sn feature of the crust which underlies the Eastern from Braile et al. (1982). Locations of the cross sections are (1~ Snake River Plain, Western Snake River Plain and shown in Fig. 1. Patterns identify velocity layering (km s - 1) al. the Owhyee Plateau, and which may be evolving and not necessarily lithology. CIBR -Central Idaho Basin and that 0' under the Yellowstone region, is that it is unusu Range; BR -Basin and Range province. a ridg ally thick, even though the Snake River Plain has crease been interpreted as an extensional feature (rift the crust (and underlying mantle) cooled following therm valley). The crust is thicker because during pas the passage of the hotspot, thermal contraction is and S, sage of the continent over the hotspot a large responsible for as much as 2-2.5 km of subsi· Lee intrusive body (probably of gabbroic composition) dence. The actual observed topographic surface mater has been emplaced in the mid-lower crust. This west of Yellowstone is approximately proportional the IT mafic intrusive body has differentiated and has to the square root of time (Brott et aI., 1978, 1981) lower also partially melted the lower and upper crust. As and decreases about 1.5 km (Fig. 3). although based a result, extensive silicic ashflow sequences and simple analysis is complicated in the west by the that c calderas have formed in the upper crust (Brott et formation of the sedimentary basin. (Fig. : aI., 1978, 1981; Leeman, 1982a). Significant den The propagating volcanic episode was de km \\ sity changes were associated with loss of the light scribed by Christiansen and Lipman (1972) and magrn granitic component of the crust due to erosion and the sequence of volcanism illustrated by a gener· solely ashflow expulsion (Brott et aI., 1981). There may aliz.ed stratigraphic section was presented by figure also have been a small amount of extension. As Armstrong et al. (1975). This section, as modified ductic I IIII: 6010h .::::: Rhyolil. Fig. 3. Stratigraphic chart of the Snake River Plains-Yellowstone region (Leeman, 1982a) and mean elevation-distance curve from ELKO Brott et aI. (1978). SRG-Snake River Group volcanics; CRB-Columbia River basalt. _5 ~~s... ~Ay~'R': : ,'.~.,:: -~~: by Leeman (1982a), is shown in Fig. 3. The east Thus, each point along the Yellowstone-Snake ward younging of the silicic volcanic rocks, the River Plain system marks a point in the evolution basaltic volcanic activity that follows the silicic ary sequence following passage of the hotspot volcanism at each location, and the sedimentation beneath the continental lithosphere. Rhyolite 30C in the Western Snake River Basin are shown. The volcanism and granite emplacement at shallow lain- Yd· diagram is based on radiometric dating of exposed depths above a large mafic batholith forming in (H'SRPI lavas and tuffs. Also shown in the figure is the the lower crust predominate over the top of the e Eastern progressive westward decrease in the elevation of hotspot. Subsequently, as the hotspot moves east ~tions arc the Snake River Plain. As pointed out by Brott et ward, cooling, contraction and subsidence begin. :tions arc (km s - I) aI. (1978), this change in elevation is similar to At some point, basalt magma is able to penetrate Basin and that observed as ocean lithosphere ages away from to the surface and is extruded over the subsiding a ridge crest. In the case of the oceans the de surface of rhyolitic ashflows. With the passage of crease in elevation is associated with cooling and time, cooling continues and additional subsidence )Bowing thermal contraction of the lithosphere (Parsons occurs. If the area is topographically closed. sedi lction is and Sclater, 1977). ment will be deposited in the basin. and if the f subsi- Leeman (1982a) has calculated the volumes of orientation is suitable, a partially extensional rift surface material added to the crust and the energetics of basin may form. Filling of the Western Snake ortional the modification of the crust. He postulated a River Plain Basin continued until the western end 8.1981) lower crustal thickening rate of about 1 km/m.y. of the Snake River was captured by the Columbia lthough based on the presumed initial crustal section and River drainage about 4 m.y. ago (Wheeler and : by the that observed in the Western Snake River Plain Cook, 1954; see discussion in Malde, 1989). (Fig. 2). With the active zone 600 km long and 100 vas de krn wide he calculated an addition of basaltic Thermal character n) and magma to the crust of about 0.035 km3/yr. Based 1 gener solely on the energetics of Yellowstone, a similar The provinces north of the Snake River Plain lted by figure can be deduced as described in the intro have a similar thermal character (Blackwell, 1978, lOdified duction. 1989). The average heat flow is 75 ± 5 mW m - 2 328 and the heat flow-heat generation relationship is likely to sample the predominant lithology in the similar to that proposed for the Basin and Range area. Also, because there is a large number or province by Roy et a1. (1968). This zone of higher measurements in many areas, gradient averages than normal heat flow continues north into Canada can be used to increase the reliability of a heat (Davis and Lewis, 1984). The heat flow in the flow value determined for a single well that is Part northern part of the Basin and Range province of a larger group. There are over twenty holes in south of the Owyhee Plateau-Snake River Plain is the depth range 300-500 m. The heat flow in the not well known. Lachenbruch and Sass (1977) deep and shallow holes is generally consistent. suggest that the area is part of the Battle Moun tain Heat Flow High where average heat flow Western Snake River Plain 2 values are in the order of 100 mW m- . Blackwell (1983) also discusses the nature of the heat flow The thermal character of the Western Snake distribution in northern Nevada. In fact, there are River Plain was first studied in detail by Brott et few data in the adjacent part of the Basin and a1. (1976, 1978). Subsequent studies were de Range province and the heat flow there is not well scribed by Smith (1980, 1981) and by Mitchell established. (1981). Detailed aspects of subareas have also been discussed; e.g., the extensive low-temperature Techniques of heat flow measurement geothermal systems along the northern and south ern margins, the Weiser, Boise, and the The heat flow dataset is quite voluminous and Bruneau-Grand View-Oreana areas have been is described in detail by Blackwell (1989); thus, the object of several studies (e.g., Young and only summary results are given in this paper. The Lewis, 1982). New data and reinterpretation of field and laboratory equipment and techniques some of the data contained in the papers by Brott have already been described (Blackwell and Spaf et a1. (1976, 1978) and by Smith (1980,1981) are ford, 1987). Most measurements were made in included in the report by Blackwell (1989) and are holes drilled by rotary techniques and thermal shown in Fig. 4. Included on the map are heat 100 1020: L! ! ! conductivity measurements were made on cuttings flow values in the Idaho batholith close to the KILOMEl as described by Sass et a1. (1971 b). Terrain correc Snake River Plain and in the Owyhee Plate3u tions were made where necessary. The special along the southern margin of the Snake Rl\er difficulties of thermal studies in the Eastern Snake Plain. The contours near the Oregon border arl' Fig. 4. Del River Plain due to the extensive aquifer there have based on data from the Western Snake River Plain wells discu been described in detail by Brott et a1. (1981). in Oregon (Blackwell et aI., 1978). In spite of the Almost all of the holes in the Snake River Plain extensive database, some details of heat flow and for which data have been collected were drilled for the geothermal gradient pattern remain uncertain the purpose of water development and no core or because of the complexity. The broad outlines of 3°e/kn cuttings were saved for thermal conductivity mea the distribution are quite clear at this point. how variatior surements. Thus, only a single value of thermal ever. The various areas of contrasting heat flo .... average conductivity measured on a sample collected from and geothermal gradient are shown in Fig. 4 and ology in the surface cuttings piles may be available. In a identified by name for ease of reference in thi~ with the few cases there are multiple samples available discussion. were dri from the same hole. Therefore, in general, it has The data within the Western Snake River Plain are distr been necessary to estimate mean conductivity val fall into two general categories. These categorie~ ern and ues for holes or sections of holes based on lith correspond to areas of relatively high gradient and (Boise : ology from cuttings piles and well logs. This pro heat flow (in the order of 100 ° C/km and 120-150 Bruneau cedure is relatively unreliable and may miss sig mW m -2), and areas of moderate gradients (about anomali, nificant depth variations in thermal conductivity. 40°C/km) and average heat flow values (60-80 heat flO' Thus. the deeper holes are likely to yield more mW m - 2). Most of the gradients range between River PI reliable heat flow estimates because they are more 45 and 85 ° C/krn, with an average of 69 ± (WSRPl 329 il the + + + + ;:r or rages heat ; part les in Heot Flow, mWm-' .n the 0-20 • • 80- 90 It. + 20-40 • 90 -100 C~\O.J;.rd" Volley o 40-60 A 100-'20 '\~£) o 60-70 • 120-I~O 6 + 6 70-80 > f~O Snake * :Oll et e de itchell ;: also Tature south ~o.,,\.,.1 j the 43 • ~ been ,?Vi'f:' g and Sf:'0...• ion of A ~Brott 31) are • ~AO• md are ~ .. 6 , a:J :e heat • * '00 '02030 40 50 " 0 •• ! ! ! ! ! I • A to the KILOMETERS 0-:; Plateau • A" 42· • • • ~ Ri\·er 117° 116" II:S° 114" 113· der are Fig. 4. Detailed heat flow map of Southwestern Idaho. Heat flow contours are shown at 20 mW m -, intervab. Locations l)f d~(r :r Plain wells discussed in the text are shown with symbols. The line of cross section AA' in Fig. 5 is shown. WSRPL- Western Snake River : of the Plain Low. ow and lcertain lines of 3 0 C/km (Table 1). Heat flow values show more TABLE 1 t, how variation, ranging from 50-150 mW m- 2 with an Average geothermal gradient and heat flow values at flow average of 99 ± 4 mW m - 2 (Table 1). The lith Province Geother- Heal Nun1· .4 and ology in most of the holes is lacustrine sediment, mal flow her in this with the exception of a few of the holes which gradient (mWm- ') were drilled in basalt. The areas of high heat flow (oC; :r Plain are distributed in two bands along the northwest km) :egories ern and southern margins of the Snake River Plain Southern Idaho Batholith ;:nt and (Boise and Camas Prairie in the north, and (excluding the Snake River Plain margin and 20-150 Bruneau-Grand View and Western Snake River geothennal systems) 27±3 77 ± 4 12 .(about anomalies in the south). The low gradients and Western Snake River Plain 69±3 99± 4 80 (60-80 heat flow are found along the axis of the Snake Owyhee Plateau 51 ±4 98± 7 23 ~etween River Plain between Caldwell and Mountain Home Eastern Snake River Plain • 69 ± (WSRPL in Fig. 4). Northern margin 55±9 93± 13 23 Southern margin 71 ± 7 113±11 80 Above Snake River aquifer 18 ± 2 27± 4 125 . Brott et aL (1981). HEAT FLOW CROSS SECTION - WS R P 11!l , I:> A' 1'1 ( i:.. A \. NE ,J I:>j \ SW 1!l0 /. \1:> BOISE FRONT i ! I ',BRUNEAU - GRAND VIEW ANOMALY/! I TANOMALY "IEI2!l • E I i I:> I"\ \ . •gloo I II. J i / .t-/·~-- ... C( III • :z: 1!l • • "Cf-I \/, I !l0 .0 \j 2!l oL...... -'----4o...... -'--.L...-"""-~!lO...... '--'--""---...... ,.,!10"'0...... -'-...... """"::I!I~0~-'----I.--'-:2:d0b;o~"""""-I DISTANCE. km Fig. 5. Heat flow cross section of the Western Snake River Plain. The line of the section is shown in Fig. 4. Points on the cross section are shown by circles and dots. Generalized heat flow east and west of the line of the section is shown diagrammatically and the geothermal areas are identified. See text for further explanation. A heat flow cross section is shown in Fig. 5. bly representing part of the shallow regional flo\\, The line of the section is shown in Fig. 4. The pattern occurs south of the Bruneau-Grand Vie\\, origin of the main features of the observed pattern area (Fig. 4). was discussed in detail by Brott et al. (1978) on Thus, hydrologic boundaries at the edge of the the basis of a substantially smaller amount of Snake River Plains cause upflow which gives rise data. With additional data, the origin of some to the geothermal systems at the various locations. parts of the pattern are now clearer. Deep drilling The effects on the heat flow are generally modest. in the Boise front area and in the Bruneau-Grand however. The average heat flow values observed View region had demonstrated that the high heat are in the order of 50-100% above the regional flow values there are related to intermediate tem background values. Outside the areas of most perature (40 0 _80 0 C) geothermal systems and rel active fluid flow, temperature-depth curves are atively local geothermal anomalies. The shallow linear to depths of at least 400-500 m, and in the heat flow pattern is indicated by the dash-dot line case of well Bostic I-A (Arney et al., 1982; Arney, in Fig. 5. Holes in these systems show isothermal 1982) to a depth of 2500 m (see below). or low-gradient sections starting between 80 and Lower, but still high, gradients and heat flow 280 m with maximum temperatures in the depth values are found in holes drilled in granitic roch range 200-500 m varying from 40 0 to 80 0 C. This on both margins of the Snake River Plain (Urban pattern of heat flow and gradient is probably due and Diment, 1975; Brott et al., 1978; Blackwell. Fig. 6. ( to systematic regional flow of groundwater toward 1989). The high heat flow in these rocks (presuma the margins of the Snake River Plain from higher bly not major participants in the regional regions to the north and south (Young and Lewis, groundwater flow systems) is related to the large holes l 1982). The flow is driven by elevation differences scale nature of crustal disruption associated with the pa on the water table. Very low heat flow also possi the Snake River Plain (Brott et al., 1978). These dashe( 331 140 GEOTHERMA l GRADIENT (. CI KM) o 100 o o o , o o o o 5 HEAT FLOW (HFU) o o o o o o o o o o ~ CENOZOIC VOLCANIC r--l.c·... CENOZOIC SEDIMENTARY :tion L:..:....::..J ROCKS L--l ROC KS I t~ NORTH SOUTH UPPER 10 -- 250·C_ CR-- UST --- . the ---- rise ...- /--- -" ons. ...- - --.-. ~20 VI lest. t:t: - W LOWER rved ~ CRUST ..,.,------ W ~ :mal o .--...... ---- g 30~---.;: -- nosl are :I: ~ Q. the W MANTLE o ney. 40 ------""'- " ------ MO H 0 - --- 1000"C_ low >cks ban o 50 I I KILOMETERS veil. Fig. 6. Calculated and observed geothennal gradient, and heat flow and associated generalized geologic model of the Western Snake ma River Plain (Broll et aI., 1978). mal rge holes are shown by a special symbol in Fig. 5 and is based on data from the deepest holes and from with the pattern of regional heat flow is shown by the holes drilled in granite along the margins. The dashed and solid curves in the figure. This pattern regional heat flow is about 100 mW m - 2 south 332 and about 75 mW m- 2 north of the Snake River the young Red Rock Valley-Centennial Moun. Plain. In the center of the Snake River Plain, the tains at the northeastern edge of the Eastern Snake J Inside heat flow is about 60-75 mW m- , while on the River Plain (Fig. 1) may be an analogous feature. Aquifer margins it is 25-50% higher than in the provinces The geothermal features in the Camas Prairie have to the north or south. been discussed by Mitchell (1976) and Mitchell et The two-dimensional thermal model of the al (1980). The existence of high geothermal gradi. Western Snake River Plain presented by Brott et ents was pointed out by Walton (1962) based On al. (1978, fig. 3) is shown in Fig. 6. The major the increase in flowing temperature with well de • features of this model are a low-thermal conduc pth in the artesian wells in the valley. He calcu. tivity basin about 6 km thick and 60 km wide lated an average gradient of 92 0 Cjkm in the filled with basalt and sedimentary rocks. The major low-conductivity clays of the valley fill. Heat no .... heat source was thermally modeled as an instanta values in granite rocks adjacent to the Cama~ *1 neous intrusion 28 kIn thick and 116 km wide at a Prairie, included in Fig. 4, are also anomaloush depth of 10 km below the surface, with an initial high compared to the Northern Rocky Mountai~ temperature of 1350 0 C. The size of the intrusion background. Furthermore, silicic volcanic centers is based on seismic evidence on the size of the younger than would be expected are present in the anomalous lower crust beneath the Snake River area, based on the hotspot model (Fig. 3) (sec Plain. Solutions for various times were obtained Leeman, 1982b). Thus, the Camas Prairie area i~ and shown by Brott et al. (1978, fig. 4). The clearly part of the Snake River Plain regional 12.5-Ma solution is shown in Fig. 6, as this time is thermal anomaly. appropriate for this area (see Figs. 1 and 3). The theoretical pattern predicted by the solu Thennal regime in the Eastern Snake River Plain tion is that the average heat flow is lower in the center of the Snake River Plain (and the gradient Geothermal data in the Eastern Snake Riwr 10 0 102 higher) than in the surrounding terrain. At the Plain and Island Park area are shown in Fig. 7. UKtLoi edge of the basin, higher heat flow values in the This map includes data that postdate those pre basement rocks are predicted because of the re sented by Brott et a1. (1981). Many of the hob fractiol1 effert. The exact type of anomaly found have temperatures that are controlled by ground at the edge depends on the geometry of the con water flow in the Snake River Plain aquifer (thl" FI~. 7. He ~hown in tact (see Lachenbruch and Marshall (1966) and aquifer has been described by Mundorff et al. Blackwell (1983», but the heat flow will be en (1964) and Lindholm (1985». The data from hole, hanced by a factor of 2-3. This enhancement is in the Snake River Plain aquifer do not represent not sufficient to explain some of the heat flow heat flow measurements in the conventional sense This dl values in the rhyolites, which must be due to fluid and details of their interpretation are discussed hy and gn circulation. However, the high heat flow values in Brott et al. (1981). The data shown in Fig. 7 Se\'eral the granites around the margin of the Snake River include average heat flow values above the Snake include Plain are caused by the refraction effect. Similarly. River Plain aquifer (small symbols) and com'en The lower (relative) heat flow values in the basin are tional corrected heat flow values for holes outside. outside consistent with the model predictions. or that penetrate below, the aquifer (for locations (weldec where two or more wells are too close to he Cenozo Camas Prairie resolved, a representative value is shown). Most of teredo ~ the wells in the study were drilled for water. but rhyolit~ The Camas Prairie is an E-W fault-bounded eight of them, averaging 100 m in depth, were The pre basin, and thus has a strike at odds with the drilled specifically for heat flow, and four, over aquifer generally high-angle intersection of most Basin 500 m in depth, were drilled in the Snake Plain As \ and Range-type structures to the Snake River aquifer for geothermal studies. In addition. a Plain. Plain. Although this trend is anomalous, the cur dataset from a geothermal exploration project was values I rently active Hebgen Lake earthquake zone and made available by Oxy Geothermal Incorporated. and 1m 333 + + Moun_ + + Snake In..de Outside Heot Flow eature. Aquifer Aquifer mWm-1 )( 0-20 ie have + 20-40 :hell et 40-60 gradi °o 60-10 sed On A 10-80 • 80-90 'ell de • 90-100 calcu_ /00-120 • 120-1~0 in the • + 14 >I~O 11 flow • Camas * Hot SprinQ llously * ,untain ::enters in the 3) (see * area is :gional ...... At r Plain @ '\ Twin ~~-'-.J * Fall~ River ~.* * \0 0 10 20 30 40 50 * \ Fig. 7. ) I I ! ! ! ! * KILOMETERS °14 * *. ;c prl" •• 'i* • * ... 14 ** , hule' ,-----a.---*--+=-- ...... _+- lL---j------'+42°* 0 0 0 0 round· 115 114 113 112 IW Fig, 7. Heat flow map of Southeastern Idaho. The heat flow values are coded as shown. The Snake River Plain aquifer outline is ~r (the .hown in generalized form. Small heat flow symbols are plotted for heat flow values above the aquifer. Large symbols are heal flow et al. values outside or below the aquifer. Deep wells are identified by abbreviations. I holc~ )resent I sen~e This data includes holes drilled for geothermal m - 2) in the Snake River Plain aquifer. Although a sed b~ and gradient studies to depths of up to 300 m. qualitatively similar heat flow distribution is ob Fig. 7 Several holes wi thin the Island Park caldera are served in the Western Snake River Plain. i.e.. low Snakl" included in this dataset. heat flow in the Western Snake River Plain and )n\'en The lithology encountered in most of the wells high heat flow on the margins. the causes of the utside. outside the aquifer consists of rhyolite ashflows pattern there are somewhat different. The low :ations (welded and unwelded). In some holes, basalt and heat flow in the Western Snake River Plain results to he Cenozoic sedimentary rocks were also encoun from large-scale refraction of heat due to crustal lost of tered. Typical thermal conductivity values of the thermal conductivity contrasts. with regional I I :r. but rhyolites are approximately 1.9-2.4 W m- K- . aquifer motion affecting only the margins. In con , were The predominant lithology of the holes within the trast. the low heat flow in the central part of the " ovcr aquifer is basalt. Eastern Snake River Plain is caused by regional Plain As was the case for the Western Snake River cold groundwater circulation in the major aquifer Ion. a Plain. the data show generally high heat flow system. The thermal refraction effect in the East 2 ct wa~ \'alues (many over 100 mW m- ) on the margins ern Snake River Plain is minor because there is no lrated. and low values (mostly in the range of 0-40 mW large. deep sedimentary basin. 334 The statistics of the heat flow values are shown anomalously high on the margins and anoma. in Table 1. The division of the northern and lously low in the Snake River Plain aquifer. southern margins of the Eastern Snake River Plain into eastern and western parts is along a line Temperature in deep wells in the Snake Rher which passes approximately through Arco and Plain Pocatello. The values east of this line are associ ated with silicic volcanics which are 5 m.y. old or Most of the thermal data discussed in previou1> younger. The low heat flow values on the northern sections have been obtained from shallow welb margin near Arco are due to lateral movement of drilled for water or for heat flow and geothermal groundwater into the Snake River Plain aquifer, gradient exploration. Typical hole depths are and these values are not included in the averages. 150-300 m. In this depth range the results that The average values for the northern margins are have been discussed have clearly shown that mUch poorly constrained due to the paucity of accessible of this thermal data is affected by groundwater wells. Most of the wells along the northern margin, flow. Temperature and heat flow data from deep eight out of eleven, were drilled specifically for holes are obviously necessary to evaluate the deep heat flow. The large variation in values within thermal conditions. In this section, data from each of the areas suggests that geothermal systems several wells over 500 m deep are discussed. have a major effect on the distribution of surface The locations of holes over 300 m deep are heat flow along the margins of the Snake River shown in Fig. 8. The holes deeper than 1 km are Plain aquifer. In spite of these complexities, the listed in Table 2. Accurate equilibrium tempera average surface heat flow values are clearly ture-depth information is available for the Ore-Ida + ++ + + Hole Depth, m 0300- 500 0500-1000 co • 1000-2000 55 105 • >2000 o Heat Flow, ntNm-2 o 116 116 o 79 o Fig. 8. Location of intermediate depth and deep wells (300-3400 m) in the Snake River Plain. The holes are coded by depth. Well> discussed in the text and listed in Table 2 are S (Stunn 1), MC (Madison County), G (lNEL-GT 1), AC (Anderson Camp). B (Bostic I-A), MH (Mountain Home Air Force Base). F (Anschutz Federal 60-13 1), 1 (James 1). and 0 (Ore-Ida 1). Heat Ocm values for the holes are shown (mW m 2 ). The James well (1) is not listed in Table 2 because only a bottom hole temperature" available. The lines of sections in Fig. 9 are shown. 335 10ma_ TABLE 2 Thermal data from deep wells in the Snake River Plain Well Latitude Longitude Depth Average Average Average Temperature range gradient thermal heat log (m) (0 Cjkm) conductivity now I (W m- K -I) (mW m- 2) 0 evious Sturm 1 44° 5.55' 111 25.73' 0-1210 (48.0) 2.26 103 NonequiJ wells Madison County 43°48.65' 111 °47.15' 0-1495 11.3 (1.5) 17 Nonequil. lerma} INEL-GT 1 • 43°37.41' 113°3.95' 1000-3100 39.5 2.76 109 Equil. Anderson Camp 42°39.90' 114°17.30' 65- 645 63.0 (1.45) 91 Equil. IS are Bostic I-A 43°2.45' 115 ° 27.55' 0-2712 65.9 1.38 96 Equil. s that Mountain Home AFB 43° 3.26' 115 ° 50.35' 823-1207 67.2 1.23 83 Nonequil. mUch Federal 60-13 1 42°59.06' 116 °17.20' 0-3390 (39.8) 1.91 76 Nonequil. 0 jwater are-Ida 1 44 001.93' 116 57.22' 0-3100 54.8 1.50 82 Equil. 1 deep Ore-Ida 1 0-2700 62.0 1.42 88 Art.? e deep Values in parenthesis are estimated. from • Data from Brott et al. (1981). ep are ~m are 1, Bostic I-A, INEL-GT 1 and the Anderson ern Snake River Plain from the Federal 60-13 1 npera Camp wells. Logs of unknown quality measured well through a recently drilled well at Mountain he-Ida shortly following completion of drilling are availa Home Air Force Base (Lewis and Stone, 1988) to ble for the Sturm 1, Madison County 1, Federal the Bostic I-A well. This section shows a thicker 60-13 1 and Mountain Home Air Force Base sedimentary package on both margins with the wells. Only bottom hole temperature information thickest basaltic section in the middle. and the was available for the James 1 well so this well is second thickest basaltic section on the east side. not shown in Table 2. The lithologies encountered This type of relationship is not likely to extend to in the wells are volcanic rocks of rhyolitic and all parts of the Snake River Plain. but the types of basaltic composition and lacustrine sedimentary lateral variations that might occur are illustrated. rocks. An exception is the Federal 60-13 1 well The westernmost wells, James 1 and Ore-Ida 1 which bottoms in the Idaho batholith granite. wells cut a predominantly sedimentary and basaltic Figure 9A shows a diagrammatic longitudinal section with few, if any, rhyolitic volcanic rocks section along the Snake River Plain based on well present. log and lithologic information from the Sturm 1, Temperature-depth curves for the wells are INEL-GT I, Bostic I-A and Ore-Ida 1 wells. The shown in Fig. 10. Equilibrium and nonequilibrium well section in Fig. 9A illustrates the eastward logs are identified. The temperature-depth curves transgressive sequence of continental lacustrine for most of these holes are quite complicated and rocks onlapping basalt, in turn onlapping rhyolite require some interpretation. The temperature mea along the Snake River Plain (this is also shown surements for the Sturm 1 well, at the edge of. but diagrammatically in Fig. 3). Whitehead (1986) has outside the Island Park caldera, are not equi presented a detailed set of geophysical maps, in librium measurements and have the characteristic cluding numerous cross sections based on well pattern of thermal drilling disturbance. i.e.. high data, of the Snake River Plain. The sections pre temperatures at shallow depths and a hook at the sented in Fig. 9 are more generalized and extend bottom as the temperature-depth curve ap to greater depths than those of Whitehead (1986). proaches equilibrium. This well was drilled en h. Wei" A cross section across the Snake River Plain at tirely in silicic volcanic rocks even though it is on amp). B eat fill" any location may show significant variation from the flanks of, and not inside, the Island Park rature l' this highly generalized longitudional section. Fig caldera. Based on data from a shallow-gradient 0 ure 9B shows a transverse section across the West test well nearby, the high temperatures (30 _ IN IN 0 on 200 400 600 ~" .. ) ..c, 800 .' .. I.~ .. .. ~ ~ c ~ .. '200 YNP '400 TO.-'210'" SEDIMENTS 2200 o [] BASALT 2600 ' .. '"." SILICIC VOLCANICS 0,t, .. . -' HORIZONTAL SCALE: ~ 50 100'" ORE-IDA FENCE DIAGRAM OF GROSS LITHOLOGIES TD.- 3050m INEL-I (ALL DEPTHS ARE RELATIVE TO SURFACE ELEVATIONS) 1.0.·3160"" A DIAGRAMMATIC LONGITUDINAL SECTION Fig. 9. A. Diagrammatic longitudinal geologic section of the Snake River Plain hased on the Sturm 1, INEL-GT I, Bostic I-A and Ore-Ida I wells. The caret pattern depicts granite (}'NP), Yellowstone National Park. This section and the one in Fig. 9B illustrate the general geologic section to about 3 km. B. East-west transverse section across the Western Snake River Plain hased on the Bostic I-A, Mountain Home Air Force Base and Anschutz Federal 60-13 I wells. 337 3S 0 C) at shallow depth may be real rather than o.;.B:..-or- -r_.--_-r-, an effect of the drilling. This high temperature is 200 consistent with the presence of a geothermal heat input as most groundwater temperatures within 4CO and adjacent to the Island Park caldera are barely in excess of the mean annual temperature of So_ 7 0 C (Whitehead, 1978). The temperatures are anomously low at the site of the Madison County 1 well (Kunze and Marlor, 1982). The drilling history of this hole was com plicated. Lost circulation and caving were exten sive and well logs were not obtained for much of the well. Thus, the temperature data quality is poor. Nevertheless, temperatures in this well ap pear to be extremely low at depth as is characteris tic of shallower holes in this vicinity (Brott et aI., 1976). In the Eastern Snake River Plain, the highest quality data are from the INEL-GT 1 well (Brott et aI., 1981). The upper part of the INEL-GT 1 well has very low gradients and near-isothermal conditions characteristic of the Snake Plain aquifer. The temperature gradient below ap proximately 200 m averages 40 0 C/km with varia o 50 FEDERAL 60-13 tions associated with local water movement. T.O. ~ 3385 m There is a large distance between the INEL-GT Fig. 9 (continued). 1 and Madison County wells and the next deep TEMPERATURE, DEG C 150 200 I EQUILIBRIUM + INEl-en ~~ 7/25/83 x OREIllo'.-' 7/20/81 • ANOCt.lPWw 3/27/86 sot • eoSTIC1A 8/15/74 j (f) 1000, n:: w r f- l- W ~ ~ I 1500 L I l- f- NON EQUILIBRIUM 0... W ,...~ ~ STURU-l 0 8/29/79 2OO0r- (!) ROH-"'CG1 6/02/8' ....HAFBl 7/29/86 $ F!D 6013 2500r 8/ 19/74 [ I J 3000' Fig. 10. Temperature-depth curves for deep wells in the Snake River Plain. 338 hole to the west. One hole in this gap was drilled western end of the Snake River Plain. This hole near the southern margin (Anderson Camp) and penetrated an extraordinarily thick sedimentary detailed temperature data are available as shown package; almost the total depth of the hole is in Fig. 10. These data indicate typical gradients of composed of sedimentary rocks, and only the bot 60 0 C/kID or more for this section of the Snake tom few hundred meters has interbedded basalts, River Plain. The basalt thickness at the site of the sedimentary units and tuffs. The measured bottom hole is about 240 m. hole temperature at a depth of 3.2 kID is ap Lithologic and alteration data from the 2.9-km proximately 195 0 C. When measured, the hole was deep Bostic I-A well have been discussed in detail flowing artesian water (2-4 Ijmin) from perfora by Arney et al. (1982) and Arney (1982). Arney tions in the casing at a depth of 1800 m; thus, the (1982) published the temperature-depth curve upper part of the curve is affected by this artesian :x: ..... shown in Fig. 10. This curve is from a commercial flow (observed temperatures are about 10 0 C above Q. LL well log, but it shows the characteristics that might in-situ temperatures due to the flow). Neverthe C be expected of a good temperature-depth log with less, the gradient averages approximately 90 0 C/ higher gradients at shallow depths associated with km between the surface and 1 km and 55 0 C/km the lower thermal conductivity sedimentary rocks between 1 and 3 km. and lower gradients at depth associated with the Thus, all the holes, with the exception of the higher thermal conductivity basaltic and silicic Bostic I-A well, show possible evidence of in volcanic rocks. There is no evidence of fluid flow traborehole fluid flow disturbance. In spite of the effects on the temperatures. Temperatures are also evidence for some fluid flow, except for the depth shown from a nonequilibrium log measured a few interval 0-250 m in INEL-GT 1, 0-1400 m in days after drilling was completed in a well at Federal 60-13 1, in Madison County 1 and in Fig. 11. 1 Mountain Home Air Force Base. The tempera Sturm I, the gradients are dominated by thermal in the So tures in the well are very close to those in the conduction. Even in the very irregular log of the hole upper part of the Bostic I-A well about 20 km to Federal 60-13 1, made 2 days following comple keyed to and Fedf the southwest. tion of drilling. gradients below 1400 m can be correctiol Te;nperature-depth data from the Federal 60 correlated with thermal conductivity. 13 1 well also show characteristics of a recently Thermal conductivity measurements are availa drilled well (the log was made 2 days following ble on cuttings (Ore-Ida 1 and Federal 60-13 1) completion of the well) with high shallow temper and in cores (INEL-GT 1 and MHAFB 1) from younge atures and near-isothermal conditions. The gradi four holes. The measurements available for three demon: ent between 1500 and 3390 m averages over of the holes are shown in Fig. 11. Because the in the' 32 0 C/km. The bottom hole temperature (non stratigraphic section encountered in all the holes is heat fl( equilibrium) is 145 0 C at 3390 m. As is the case similar, and because many additional measure The with the Sturm 1 well, the upper part of this hole ments are available for samples from shallow holes. mation may in fact not be as disturbed as it appears. it is possible to establish reasonable estimates of temper Drillholes in the depth range 300-500 m in the thermal conductivity versus depth for all the holes. are sh( immediate vicinity are part of the Bruneau-Grand In addition to estimating values based on lith this pr View geothermal anomaly and temperatures of ology alone, a correlation was constructed be servati 40 0 _60 0 C occur at depths of 200-400 m. This tween sonic travel times as measured by well logs. except system appears to be quite shallow as deeper and thermal conductivity for the basalts and the Ml temperatures are not anomalously high for the rhyolites (Williams, 1981). This correlation was with v. Snake River Plain. Even in this irregular log, used to assist the determination of thermal con the we gradients below 1400 m can be correlated with ductivity values for several of the wells. three , thermal conductivity variations associated with The temperature-depth curves show the curi in the different lithologies (McIntyre, 1979). ous fact that the temperatures are higher in the could I The highest reliably documented temperature is west (Ore-Ida 1, Bostic I-A and MHAFB 1) and real d observed in the Ore-Ida 1 well at the extreme lowest in the east where the silicic volcanism is differe 4 is hOle 00 Discussion lentary 0 ORE-IDA The objective of this paper is to summarize the lole is SEDIMENT le bot 0.5 FEDERAL regional geology, thermal conditions and geologic 4- BASALT )asalts, history of the Snake River Plain. The heat flow in INEL - GTI )ottorn • RHYOLITE the Yellowstone area has been discussed by Four IS ap_ 1.0 nier et al. (1976), Morgan et al. (1977) and )Ie Was Blackwell et al. (1989). The geologic, geophysical erfora_ and thermal data presented here are consistent E 1.5 with the origin of the feature as a propagating US, the .>0'. rtesian :x: hotspot beneath the North American lithosphere. I- , : above Cl. f The area is characterized by a sequence of events ~ 2.0 Q associated with a massive thermal event including verthe , X)°C/ ~ , thermal expansion and uplift, crustal melting of C/km }> , silicic rocks by a large mafic intrusive mass gener 2.5 ating silicic volcanism, and caldera formation. Very of the f high heat flow and massive geothermal systems of in are associated with this period. As the hotspot 30 of the t moved eastward and cooling began in the crust ~ depth and lithosphere. contraction was dominant. At I this time some mafic magma was able to penetrate m in 3.5L....1.. ..L... .l...o. ..1 and in FIg. II. Thennal conductivity versus depth for three deep wells the upper crust and basaltic volcanism became hermal in the Snake River Plain. The shape of the symbol is keyed to prominent. The present-day example of this part log of the hole name and the condition (open, filled or crossed) is of the system is of course the Eastern Snake River ample ke~'ed to the lithology of the sample. Samples from Ore-Ida I Plain. A third part of the evolutionarv pattern and Fedaa' 60- I3 J are cUllings measurements with porosily :an he may be the formation of a large sedimentary basin. corrections applied. The values have not been corTlxted for the Formation of a basin, however, requires a trap in-situ temperatures. availa ping mechanism. The Western Snake River basin )-13 1) may owe its origin to the fact that the Snake River ) from youngest. However, the data illustrated in Fig. 11 drained westward at a higher grade level prior to r three demonstrate that the average thermal conductivity about 4 m.y. ago (see Malde. 1989). At that time. Ise the in the west tends to be lower, so that the relative it was captured by the Columbia River system and IOles is heat flow values are not obvious. could then rapidly cut lower into the basin instead ~asure The heat flow values were determined by esti of allowing the basin to continue to fill. Alterna , holes. mation of the mean thermal resistivity and mean tively. the orientation and depth of fill of the Hes of temperature gradient for each hole. These values basin may be controlled by modest extension along , holes. are shown in Table 2. The errors resulting from directions sympathetic with Basin and Range n lith this process are difficult to estimate, but a con trends. Evidence for the extension model may be ~d be servative value is ± 10%. The resulting values, the different response of the Owyhee Plateau and II logs. except for the very low value of 17 mW m - 2 in the Western Snake River Plain. both of which sand the Madison County well, show the expected trend probably participated in a similar thermal dis n was with values higher in the east and lower values in turbance. I con the west. There is variation in the heat flow for the Temperatures at a depth of about 3 km vary by three wells making up the transverse cross section 50 0 C from place to place and may be higher in : cun in the Western Snake River Plain. The difference the Western Snake River Plain than in the Eastern in the could easily be attributed to error in measurement, Snake River Plain. This effect is related to the :) and real differences due to regional effects, or real very low thermal conductivity of the sedimentary Ism IS differences due to structure (Fig. 6). rocks in the western holes. 340 115· 113· III· about plain. 46' model 46 + + + + + + + source clear I A 0 A with 1 ~ A A A A 50 krr "",,0 A A A • A A ""A 45' Basin SA A A", tf>AA 45 A A AAA ,& AA A AtnAA ~ AA dividf AA • A A A A t. 0 occur Ill> "",A A AAA ity of A A AfiJe " A A A ~AAAA A AA be de A )A~::A:AA ThiSt ~ 44' 44" AAA~A A " A AA bJI". ".'. AA et aI. Atr A A A A aseiSJ --;;,," ",,,,,e~(:' 0' the shoul dena + 43° + 200 Plain A of tl "" A manl A A A tapO o 0 A+ 42° 42· A A A A A Aclu ~ tf t':,A A EPI CE NTER MAP about 60 krn wide in the Eastern Snake River from alteration minerals, Bostic 1-A well, Mountain Home. Plain. Brott et ai. (1978) used a 116-km wide Idaho. Geotherm. Resour. Counc. Trans., 6: 3-6. Arney, B.H., Goff, F. and Harding Lawson Associates, 1982. model for the west and Brott et ai. (1981) used a 46' Evaluation of the hot dry rock geothermal potential of an source width of 120 krn for the east. In the east, area near Mountain Home, Idaho. Los Alamos Natl. Lab. clear evidence for thermal contraction associated Rep. LA-9365-HDR, 65 pp. with recovery following hotspot passage extends Blackwell, D.D., 1978. Heat now and energy loss in the west 50 km north of the Snake River Plain where three ern United States. In: R.B. Smith and G.P. Eaton (Editors). Basin and Range valleys show internal drainage Cenewic Tectonics and Regional Geophysics of the West ern Cordillera. Geol. Soc. Am. Mem. 152: 175-208. divides (Ruppel, 1967). A similar phenomenon Blackwell, D.D., 1983. Heat now in the northern Basin and occurs on the south side of the area. The seismic Range Province. In: The Role of Heat in the Development ity of the Intermountain Seismic Belt appears to of Energy and Mineral Resources in the Northern Basin be deflected around the Eastern Snake River Plain. and Range Province. Geotherm. Resour. Counc. Spec. Rep. This deflection is illustrated in Fig. 12 (from Smith 13. pp. 81-92. Blackwell, D.D.• 1989. Heat flow of Idaho. Idaho Dep. Water et aI., 1985). Smith et ai. (1985) referred to the Resour. Water Supply Pap., SUbmitted. aseismic area between the Snake River Plain and Blackwell, D.D. and Spafford. R.E.. 1987. Experimental meth the seismically active area as the .. thermal ods in continental heat now. In: CG. Sammis and T.L. shoulder". Thus, the seismic and topographic evi Henyey (Editors). Experimental Methods in Physics Geo dence suggest thermal effects over a region physics, Part B, Field Measurements. Academic Press. 3° 200-250 krn wide in the Eastern Snake River Orlando. pp. 189-226. Blackwell. D.D.. Hull. D.A., Bowen. R.G. and Steele. J.L.. Plain. This width may be a more realistic estimate 1978. Heat now of Oregon. Oreg. Dep. Geol. Min. Ind. of the width of the disturbance in the upper Spec. Pap. 4. 42 pp. mantle and lower crust than is the width of the Blackwell. D.D., Spafford. R.E., Steele, J.L., Frohme. M. and topographic feature. Morgan. P., 1989. Heat now in Yellowstonc Lake. Eos. 2° Trans. Am. Geophys. Union, submitted. Braile. L.W .. Smith. R.B.. Ansorge. J.. Baker. .\l.R.. Sparlin. Acknowledgements M.A.. Prodehl. C. SchiJly. M.M.. Heal\. J.H. Mueller. S. and Olsen. K.H.. 1982. The Yellowstone Snake Ri"cr Plain Collection and analysis of data from the deep seismic profiling experiment: Crustal structure of the Eaq wells in the Snake River Plain was supported by em Snake River Plain. J. Geophys. Res., 87: 2597-2610 funds from the National Science Foundation Brott, CA., Blackwell. D.D. and Mitchell, J.C. 1976. Heat through NSF grant No. EAR-8213156 to the now study of the Snake River Plain region. Idaho. Idaho Dep. Water Resour. Water Inf. Bull., 30 (8): 195 pp. Southern Methodist University. Malcolm Moss Brott, CA., Blackwell, D.D. and Mitchell. J.C, 1978. Tectonic of man, of the Anchutz Corporation made samples implications of the heat now of the Western Snake River from well Federal 60-13 1 available for thermal Plain. Idaho. Bull. Geol. Soc. Am.. 89: 1697-1707 conductivity measurement. Leah Street logged the Brott, CA., Blackwell. D.D. and Ziagos. J.P.. 1981. Thermal Anderson Camp well and made the log available. and tectonic implications of heat now in the Eastern Snake River Plain, Idaho. J. Geophys. Res.. 86: 11709-11734. :r Roger Jenson logged the Mountain Home Air Christiansen, R.L.. 1982. Late Cenozoic volcanism of the Is .Ie Force Base well and Robert Lewis gave permis land Park area. In: B. Bonnichsen and R.M. Breckenridge n sion to use the data and make thermal conductiv (Editors), Cenozoic Geology of Idaho. Idaho Bur. Mines o ity measurements on several core samples. Without Geol. Bull., 26: 345-368. er the cooperation of these people this paper would Christiansen, R.L. and Lipman, P.W., 1972. Cenozoic volcanism :p not have been possible. and plate tectonic evolution of the western United States. II. Late Cenozoic. Philos. Trans. R. Soc. London. Ser. A.. 271: 249-234. References Davis, E.E. and Lewis, T.J., 1984. Heat now in a back-arc environment: Intermountain and Ominica crystalline belts. Armstrong. R.L.. Leeman, W.P. and Malde. H.E., 1975. southern Canadian Cordillera. Can. J. Earth Sci .. 21: Ie Quaternary and Neogene volcanic rocks. of the Snake River 715-726. Plain, Idaho. Am. J. Sci., 275: 225-251. :e Doherty. D.J., McBroome. L.A. and Kuntz. M.A .. 1979. Pre Arney, B.H.. 1982. Evidence of former higher temperatures liminary geological interpretation and lithologic log of the 342 ellploratory geothermal test well (lNEL-l). Idaho National Mabey. D.R., 1976. Interpretation of a gravity profile across Engineering Laboratory, Eastern Snake River Plain, Idaho. the western Snake River Plain, Idaho. Geology. 4: 53-55. U.S. Geol. Surv. Open-File Rep. 79-1248, 9 pp. Mabey, D.R., 1978. Regional gravity and magnetic anomalies Doser, D.I. and Smith, RB., ]985. Source parameters of the 28 in the eastern Snake River Plain, Idaho. J. Res. U.S. Geol. October 1983 Borah Peak, Idaho earthquake from body Surv., 6: 553-562. wave analysis. SeismoJ. Soc. Am. Bull., 75: 1041-1051. Malde, H.E., 1989. Quaternary geology and structural histon Embree, G.F., McBroome, L.A. and Doherty, D.J., 1982. Pre· of the Snake River Plain, Idaho and Oregon. In: R.B Iiminary stratigraphic framework of the Pliocene and Morrison (Editor). Quaternary Nonglacial Geology: Con Miocene rhyolite, eastern Snake River Plain, Idaho. In: B. terminous U.S. Geol. Soc. Am., Boulder, Colo.. in press. Bonnichsen and R.M. Brechenridge (Editors), Cenozoic McIntyre, D.H., 1979. 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