Idaho Batholith and Its Southern Extension

WILLIAM H. TAUBENECK Department of Geology, Oregon State University, Corvallis, Oregon 97331

ABSTRACT for 28 mi in the most westerly exposures of the batholith are about S. 20° E. Southeast trends Distribution of minerals and rock types, em- also occur near South Mountain in pre-Tertiary placement structures, and postconsolidation country rocks west of the southernmost expo- history are more complex within the Idaho sures of the batholith. The southeast trends batholith than most published descriptions sug- within and outside the batholith indicate that a gest. Hornblende occurs in some areas many significant change in structural direction occurs miles within the batholith, and planar structure in southwest Idaho in the region near South also is present in some of its interior regions. Mountain. In the vicinity of the Cascade Reservoir in The locations of the south and southeast con- west-central Idaho, field relationships and mo- tacts of the Idaho batholith are uncertain, but dal data for granitic rocks are inconsistent with some inferences regarding the position of the the conclusion (Schmidt, 1964) that the bed- batholith are possible from isolated occurrences rock systematically and gradationally changes of Ordovician sedimentary rocks south of Twin from schist and gneiss to directionless granitic Falls and from exposures of pre-Tertiary sedi- rock along 35-mi traverses west to east across mentary and igneous rocks near the Idaho- the border and interior of the batholith. Nevada state line. Major fault blocks within the Idaho batholith Northward continuity of the Sierra Nevada invalidate the concept (Hamilton and Myers, batholith to the Nevada-Oregon boundary is 1966) of a resistant mass that defied internal well established. The trend of the batholith deformation during the Cenozoic evolution of bends toward the northeast before the batholith western North America. disappears under Cenozoic volcanic rocks in Gabbro and norite typically occur west of the southeast Oregon and northern Nevada. The batholith. Granitic intrusions near the batholith distribution and composition of the plutonic in western Idaho are characterized by mega- rocks near the Nevada-Oregon border suggest scopic crystals of epidote. Interstitial zeolites that the quartz diorite boundary line is about also occur in satellite masses rather than in gra- 160 mi east of the inferred location (Moore, nitic rocks of the batholith. 1959) in northern California. Granitic rocks of southwest Idaho are cor- If the Idaho and Sierra Nevada batholiths are related with the Idaho batholith primarily by connected, the Idaho batholith southeast of the location and trend of gneissic border rocks South Mountain must veer sharply west on either side of the Snake River Plain. The beneath Cenozoic volcanic rocks. Any connect- trend of S. 20° W. in the gneissic border zone ing link between the two batholiths must be north of the Snake River Plain also is present in confined to a narrow belt that extends east- rocks lying between 40 and 5 5 mi to the south- northeast for about 75 mi near the Idaho- southwest where gneissic granitic rocks reap- Nevada and Oregon-Nevada boundaries. pear in the westernmost exposures of An appreciable change in the relative posi- pre-Tertiary rocks south of the Snake River. tion of the Idaho and Sierra Nevada batholiths Gross mineralogical characteristics of granitic has occurred since Oligocene time. If the sug- rocks near the Snake River in southwest Idaho gested magnitudes of displacement from nor- closely resemble those in the west part of the mal faulting, dike intrusion, and right-lateral batholith just north of the Snake River Plain. faulting are approximately correct, the east- Southwest structural trends in granitic rocks west change in the alignment of the batholiths in southwest Idaho near the Snake River begin is as much as 50 mi. to deviate to the southeast about 25 mi due Gravity and seismic data considered in terms south of Marsing. Farther to the south, trends of surface geology and the distribution of gra-

Geological Society of America Bulletin, v. 82, p. 1899-1928, 4 figs. July 1971 1899

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nitic rocks are consistent with the interpretation specimens from near the plain were collected that models of crustal structure should include for modal analyses. In northern Nevada, on the a granitic layer underlying nearly all of south- other hand, excellent exposures of granitic west Idaho. rocks imposed no restrictions on sampling. Each modal analysis (Tables 1 to 9) represents INTRODUCTION at least 2000 points for each of two to seven thin sections per rock; the number of thin sec- Purpose tions for each analysis is a function of grain size. During 1967 field studies of post-Oligocene Structural trends (Fig. 1) in some areas of dikes and dike swarms of the Basin and Range granitic rocks in Idaho vary as much as 15° structural province (Taubeneck, 1969, 1970), within 100 ft or less. Such variations are most gneissic granitic rocks similar to the distinctive common in exposures in southwest Idaho more border zone rocks of the west part of the Idaho than 17 mi south-southwest of the Snake River. batholith were observed south of the Snake Wherever possible, trends shown in Figure 1 River Plain in southwest Idaho. Moreover, the represent the average of 20 to 40 observations structural trends in these rocks were found to in an area of at least 1 sq mi. be on strike with trends in the gneissic border rocks on the north side of the plain. These dis- THE IDAHO BATHOLITH coveries in southwest Idaho focused my atten- The Idaho batholith is the least known of the tion on the possible continuity of Mesozoic large batholiths of the western United States. granitic rocks between the Idaho batholith and Although hundreds of papers discuss various the Sierra Nevada batholith. aspects of the batholith, almost no detailed This paper (1) presents new data on the petrographic studies are available (Ross, 1963, Idaho batholith, (2) recognizes a southward ex- p. 45). tension of the batholith that includes nearly all General knowledge of the batholith was sum- granitic rocks of southwest Idaho, (3) discusses marized most recently by Ross (1963), near the a possible connection between the Sierra end of a lifetime devoted largely to the geology Nevada batholith and the Idaho batholith, and of Idaho. His conclusions were based partly on (4) concludes that models of crustal structure a reconnaissance of much of the batholith dur- should show a layer of low-velocity ("gra- ing the summer of 1962. Ross (1963, p. 52) nitic") continental crust underlying nearly all correctly reported that "both the border zone of southwest Idaho. and the interior mass are more complex in detail than would be supposed from published Methods descriptions." Field studies concentrated on relationships of The batholith traditionally has been de- granitic rocks in Idaho and Nevada included 19 scribed in terms of a gneissic shell of quartz days in northern Nevada, 17 days in southwest diorite that encloses quartz monzonite and Idaho, and 34 days along the west border of the granodiorite (for example, Ross, 1936). Some Idaho batholith in west-central Idaho. In addi- modern generalizations are rather misleading tion, observations of granitic rocks in central in referring to the rocks within the gneissic and south-central Idaho were made during 23 shell as "continuous massive granodiorite and days of reconnaissance studies of Cenozoic quartz monzonite" (Hamilton, 1962, p. 513). dikes and dike swarms within the Idaho bath- Planar structure, in some areas many miles olith. Knowledge of pre-Tertiary rocks in west- within the batholith, will permit detailed struc- ern Idaho, several miles or more west of the tural studies that ultimately will provide consid- batholith and between the Snake River Plain erable information regarding its emplacement and the Clearwater River some 150 mi to the history. north, was acquired mostly in 36 days, during Another misconception regarding the Idaho which Cenozoic dikes were studied. batholith is that hornblende occurs only in bor- Except in glaciated alpine areas, rocks of the der rocks. Actually, hornblende is present in Idaho batholith commonly are deeply weath- some interior parts of the batholith, especially ered (Russell, 1902, p. 40; Larsen and Schmidt, south and southwest of Atlanta (Fig. 1). Near 1958, p. 3), especially near the Snake River Trinity Peak, about 21 mi southwest of Atlanta, Plain where fresh specimens generally are diffi- granitic rocks contain several percent of horn- cult or impossible to obtain. Therefore, few blende. The distribution of hornblende is of

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117 116°

117° 116° Figure 1. Map showing inferred contacts of Idaho bered localities discussed in text. batholith, structural trends, and locations (dots) of num- almost immediate importance, for example, in trate that the petrography and the distribution facilitating the radiometric investigations of of rock types within the batholith are more geochronologists. complex that implied by published statements. Modal data (Tables 1 and 2) for granitic This part of the batholith was studied by rocks in the area between the Cascade Reser- semireconnaissance methods that involved ob- voir (Fig. 1) and the Snake River Plain illus- servations restricted mostly to localities not

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Specimen Potassium Quartz Plagio- Biotite Muscovite Horn- Accessories number feldspar clase blende Opaque Nonopaque

122 0.1 25.5 50.8 19.7 2.0 0.0 0.4 1.5 123 0.0 26.1 54.7 14.5 0.8 2.6 0.5 0.8 124 0.1 25.9 59.4 13.6 0.4 0.2 0.0 0.4 125 0.0 27.6 57.5 13.6 0.7 0.2 0.0 0.4 126 0.0 26.7 58.2 14.2 0.4 0.1 0.0 0.4 127 0.1 29.2 55.9 13.8 0.4 0.1 0.1 0.4 128 2.4 30.8 53.4 11.9 0.9 0.0 0.3 0.3 129 0.1 28.7 55.9 14.1 1.0 0.0 0.0 0.2 130 3.5 31.8 54.0 9.8 0.8 0.0 0.0 0.1 131 10.6 32.4 45.9 10.0 0.7 0.0 0.3 0.1 132 4.7 32.2 51.7 11.3 0.0 0.0 0.0 0.1 177 0.0 22.1 59.7 10.6 0.1 5.8 1.0 0.7 251 0.2 27.8 53.3 16.1 1.7 0.3 0.2 0.4 252 0.1 32.1 55.5 10.1 1.7 0.0 0.2 0.3 253 0.0 32.8 57.2 9.0 0.1 0.0 0.0 0.9+

+Each modal analysis is the average of two thin sections. Includes 0 .7 percent garnet.

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Specimen Potassium Quartz Plagio- Biotite Muscovite Home- Accessories number feldspar clase blende Opaque Nonopaque

178 11.2 26.9 51.8 8.5 0.6 0.0 0.3 0.7 287 8.2 28.9 50.8 10.9 0.4 0.0 0.2 0.6 176 0.0 5.9 69.2 2.6 0.0 20.2 1.0 1.1 179 4.1 23.5 55.2 13.7 0.0 1.9 0.2 1.4 180 8.5 28.6 48.3 12.4 0.0 0.2 0.1 1.9 242 6.7 26.1 55.5 10.1 0.3 0.0 0.3 1.0 243 10.1 26.4 50.2 11.4 0.1 0.4 0.2 1.2 244 3.6 28.8 57.2 8.7 0.1 0.0 0.1 1.5 236 21.9 29.0 42.5 5.6 0.6 0.0 0.2 0.2

237 llv£ 27.6 51.9 8.0 0.6 0.0 0.1 0.2 240 10.4 36.8 47.3 3.5 1.9 0.0 0.0 0.1 241 8.5 32.1 54.3 4.1 1.0 0.0 0.0 0.0 Data for specimens 178, 287, and 237 are averages of five thin sections each; specimens 236, 240, and 241 are averages of seven thin sections each; other specimens are averages of two thin sections each.

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more than 0.5 mi from roads. The batholith in mafic rocks is more felsic (Table 2) than the this region is not amenable to easy investigation porphyritic granodiorite that extends south- because of much timber and brush, as well as a southwest from Cascade Reservoir to the Snake heavy coating of lichens on most rocks in the River Plain. more open country at the lower elevations near In summary, modal data in Tables 1 and 2 the Snake River Plain. The following discus- support the conclusion that the petrography sion of rocks to the east of the inferred contact and over-all distribution of rocks within the (Fig. 1) of the batholith includes no mention of Idaho batholith are more complex than is ap- relationships between different rock types be- parent from published descriptions. cause the critical relationships are unknown. Schmidt (1964, p. 8) concluded from recon- Rocks of the foliated border zone within 25 naissance petrographic studies in Adams and mi of the Snake River Plain are mostly quartz Valley Counties in west-central Idaho that the diorite that is characterized by a comparatively "bedrock systematically and gradationally large amount of biotite, minor muscovite, and changes from schist and gneiss to directionless little or no hornblende. Rocks more than 25 mi granitic rock" in 35-mi west to east traverses north of the plain commonly contain garnet. across the border and interior of the batholith. The general absence of phenocrysts of potas- Schists and metasedimentary gneisses are pre- sium feldspar is a distinctive feature of the fo- sent, primarily to the west, but most rocks in the liated rocks. Modal data for typical border area described by Schmidt (1964) are part of rocks are given in Table 1. Except for a higher igneous plutons which commonly have at least percent of biotite, the rocks are more closely some well-defined intrusive contacts. Figure 2 related mineralogically to trondhjemite is a reconnaissance map of the most pertinent (Goldschmidt, 1916, p. 77), than to quartz di- part of the area considered by Schmidt (1964). orite. The rocks are discussed from west to east. A porphyritic granodiorite with phenocrysts The quartz diorites of Council Mountain and of potassium feldspar occurs east of the gneissic Deserette are satellites of the batholith and are shell of the batholith. To the north, this rock the most westerly pre-Tertiary rocks in the re- was called the "granodiorite near Cascade" by gion. Both masses are of igneous origin. The Larsen and Schmidt (1958, p. 6). A modal anal- Council Mountain pluton exhibits intrusive re- ysis given by Larsen and Schmidt (1958, Table lationships to adjacent country rocks. Although 3, column 8) for a specimen of this granodiorite contacts of the Deserette pluton with bordering collected about 30 mi north of the Snake River country rocks are concealed beneath Columbia Plain compares very closely with modal anal- River Basalt, xenoliths within the pluton show yses of two porphyritic granodiorites (Table 2, evidence of transportation and intrusion. Most specimens 178, 287) from near the plain. exposures of the plutons are good to excellent, Within 30 mi of the Snake River Plain, any but poor exposures within 100 ft of their west to east traverse through porphyritic mutual contact prevented a determination of granodiorite will pass within about 5 mi, or less, their relative ages. Both quartz diorites have into a northerly trending belt of more mafic gneissoid to gneissic borders that grade inward rocks that apparently are mostly hornblende- to rocks with a more nearly directionless fabric. bearing nonporphyritic granodiorite and Although the quartz diorite of Council Moun- quartz diorite with minor diorite. This elon- tain is exposed in two areas (Fig. 2), all expo- gated belt of relatively mafic granitic rocks ex- sures may be part of one large pluton concealed tends northward to within about 20 mi of mostly by Columbia River Basalt. Likewise, a Cascade Reservoir; the width of the belt is continuous mass of the quartz diorite of De- probably not more than 7 mi. Table 2 contains serette may underlie the Columbia River Basalt modes for six specimens (176, 179, 180, 242, in the area between the main exposures of the 243, 244). rock and the small outcrops about 1 mi to the Porphyritic granodiorite without horn- north (Fig. 2). blende occurs east of the zone of relatively Modal analyses of the quartz diorites of mafic rocks. Specimens 236 and 237 (Table 2) Council Mountain and Deserette are given in were collected several miles to the east, Tables 3 and 4. As one of two specimens of the whereas leucocratic specimens 240 and 241 are quartz diorite of Deserette from the restricted from typical exposures about 7 mi to the east. northern exposure (Fig. 2) has the composition The porphyritic granodiorite east of the zone of of a granodiorite, part of the concealed rock

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II6°00'

EXPLANATION

44=45'

Si Hi t^i Quartz dionte of Quartz dionte Quartz diorite Council Mountain of Deserette of Donnelly i a'' i i—i Schist, gneiss, and Gneiss, migmatite, pegmatite, minor pegmatite and granitic rocks

Contacts Dashed where generally accurate within 400 feet; dotted where inferred (not walked in field)

2 3

Kilometers

44°30' M6°00' Figure 2. Reconnaissance geologic map of part of the and Council 15-min quadrangles. No faults are shown. Meadows 30-min quadrangle and of part of the Cascade may be granodiorite. granitic intrusions of varied mineralogy occur A zone of schist, gneiss, and minor pegmatite with gneiss, migmatite, and considerable peg- extends eastward for about 2 mi from the bor- matite. Dense timber and brush throughout der of the Council Mountain pluton (Fig. 2). much of this belt (Fig. 2) will make detailed Farther to the east, in a zone about 3 mi wide, mapping highly interpretative. The western

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Specimen number 133 134 135 137 142 144

Potassium feldspar 2.4 0.0 0.0 0.0 0.0 0.0 Quartz 18.4 13.8 14.6 20.3 8.2 21.9 Plagioclase 52.2 55.9 53.5 52.2 59.4 53.7 Biotite 9.2 10.6 8.2 13.6 8.2 13.3 Hornblende 12.5 15.6 18.7 9.9 21.8 4.9 Epidote 4.2 2.9 3.6 3.3 1.2 5.3 Opaque accessories 0.1 0.1 0.1 0.0 0.5 0.0 Nonopaque accessories 1.0 1.1 1.3 0.7 0.7 0.9

Each modal analysis is the average of two thin sections.

limit of lens-shaped (?) igneous intrusions is orite of Cascade intrudes the Donnelly pluton selected arbitrarily as the western border of the beneath the reservoir in the same manner that Idaho batholith. No two geologists will place an interior-type granodiorite (or quartz monzo- the "contact" in the same location. The percent nite) intrudes a gneissic quartz diorite that of igneous rocks varies in a north-south direc- resembles the Donnelly unit near the section tion with mostly hornblende-bearing types (Ta- line between sections 3 and 10 in T. 19 N., R. ble 5, specimens 136, 139) in the north, in 4 E., some 32 mi north-northeast of Indian contrast to the common occurrence of garnet- Mountain. bearing types (Table 5, specimens 154, 155, Although the quartz diorite of Donnelly gen- 156, 161) in the south. erally is gneissic near contacts, planar structure The quartz diorite of Donnelly, the major is less strongly defined inward. In contrast to unit of the batholith to the west of the Cascade the distinct planar structure of the Donnelly Reservoir, intrudes the east side of the 3-mi- pluton, planar structure in the granodiorite of wide zone of mixed igneous and metamorphic Cascade is weak or absent. rocks (Fig. 2). The simplest and most clean-cut In summary, field relationships and rock dis- contact of the Donnelly intrusion is about 4 mi tribution within the area of Figure 2 necessitate east-northeast of Indian Mountain (Fig. 2) a rejection of the conclusion that in west to east where a marked color contrast occurs between traverses, the "bedrock systematically and gra- relatively mafic quartz diorite and adjacent dationally changes from schist and gneiss to di- leucocratic country rocks. rectionless granitic rock" (Schmidt, 1964. p. The quartz diorite of Donnelly is an elon- 8). Moreover, the modal data for quartz dior- gated pluton that extends northward for many ites (Tables 3, 4, 5, and 6) show that no sys- miles, to a large extent beneath the surficial tematic mafic to felsic change occurs in west to deposits of Long Valley. The east contact of the east traverses across the area. pluton in the vicinity of Figure 2 must lie Cenozoic deformation within the batholith beneath Cascade Reservoir, because the also is more complex and widespread than granodiorite of Cascade occurs along the east some generalizers have implied. For example, shore of the reservoir. Therefore, the Donnelly the batholith is visualized by Hamilton and My- pluton is less than 6 mi wide in the vicinity of ers (1966, p. 540-542) as a resistant mass that the map area (Fig. 2). Presumably the granodi- defied internal deformation during the Ceno-

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Specimen number 143 149 150 151 152 153 172

Potassium feldspar 0.8 1.7 0.6 2.5 1.3 10.1 1.2 Quartz 30.8 30.4 27.2 30.6 25.2 30.6 33.4 Plagioclase 62.4 60.8 64.3 59.7 65.3 52.0 52.0 Biotite 4.4 5.0 6.1 5.1 6.1 5.4 9.4 Muscovite 0.7 0.8 0.6 0.4 0.6 0.5 1.2 Epidote 0.7 0.9 0.8 1.3 0.7 0.8 2.4 Opaque accessories 0.2 0.2 0.2 0.0 0.2 0.3 0.3 Nonopaque accessories 0.0 0.2 0.2 0.4 0.6 0.3 0.1 * Each modal analysis is the average of two thin sections.

zoic evolution of western North America. Ac- capped by Columbia River Basalt that dips cording to Hamilton and Myers (1966, p. about 15° W. About 2.5 mi to the east, on the 542), "young fault blocks lie north, west, and downdropped side of the fault, a remnant of east of the batholith, but none of consequence Columbia River Basalt is at an elevation of break it." In a brief discussion of a major belt 5400 ft. In the vicinity of Hawley Mountain, of north-trending faults in western Idaho, the fault displacement is at least 2000 ft. The Hamilton (1962, p. 513) stated that "the aver- fault is about 15 mi east of the edge of the age throw on each of the long faults is about gneissic border (Fig. 1) of the batholith. No 1000 or 1500 ft." Contrary to Hamilton and attempt was made to trace this important fault Myers (1966, p. 542), the belt of north-trend- northward, but the impressive alignment of hot ing faults discussed by Hamilton (1962) ex- springs (Stearns and others, 1937, p. 138-139) tends eastward into the interior of the bath- that trends slightly northeast about 45 mi is an olith. indication of the probable location of the fault, About 25 mi north of Boise (Fig. 1), a small or one of its branches. remnant of downdropped Columbia River Ba- No veneer of Columbia River Basalt is avail- salt at an elevation of 2800 ft dips about 20" W. able as a horizon marker for determination of on the east side of a prominent fault reported post-Miocene displacement along faults else- by Anderson (1934a, p. 17). Three mi to the where in the interior of the batholith, but major west, at an elevation of 4000 ft near the crest northerly trending faults have been reported. of the uplifted block, Lindgren's (1898) cross- The Montezuma fault (Anderson, 1939, p. 17; section shows an extensive cap of Columbia Reid, 1963, p. 11) near Atlanta (Fig. 1), proba- River Basalt dipping about 10° W. This fault bly is one of the best documented. This fault qualifies as a major break in the Idaho bath- forms the west boundary of the Sawtooth olith. The fault is about 6 mi east of the edge Mountain fault block. Anderson (1939, p. 17) of the gneissic border (Fig. 1) of the batholith. suggested a vertical displacement of as much as About 25 mi north-northeast of Boise, And- 2000 ft; a cross section by Reid (1963, Fig. 19) erson (1947, p. 170) recognized a major fault indicates a similar displacement. that trends slightly northeast along the west Northwest drift of the Idaho batholith as an side of Boise Basin. On Hawley Mountain, the unbroken plate is a basic part of the Hamilton and fault escarpment at an elevation of 7000 ft is Myers (1966) concept of Cenozoic tensional

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TABLE 5. MODES OF QUARTZ DIORITE WEST OF DONNELLY PLUTON*

(in volume percent)

Specimen number 136 139 154 155 156 161

Potassium feldspar 0.0 0.3 0.3 0.7 0.3 0.2 Quartz 21.4 25.0 22.3 22.9 29.9 33.5 Plagioclase 62.9 59.4 63.3 64.5 58.4 59.7 Biotite 8.8 10.9 10.4 11.3 10.4 6.2 Hornblende 5.7 3.2 1.0 0.0 0.0 0.0 Muscovite 0.0 0.0 0.1 0.1 0.4 0.0 Garnet 0.0 0.0 2.0 0.1 0.2 0.4 Opaque accessories 0.5 0.3 0.0 0.0 0.0 0.0 Nonopaque accessories 0.7 0.9 0.6 0.4 0.4 0.0

Each modal analysis is the average of two thin sections •

rifting and oroclinal bending in the Pacific The occurrence of discrete crystals of epi- Northwest. Appraisal1 of the entire concept dote, as much as 3.0 mm across and commonly requires an extended discussion of relationships associated with unaltered biotite and horn- throughout the Pacific Northwest, but the ma- blende, is a notable feature of satellites which jor fault blocks mentioned in the three preced- are near (generally within 15 mi) the batholith. ing paragraphs invalidate the basic premise that Insofar as the writer knows, discrete crystals of "young fault blocks lie north, west, and east of megascopic epidote occur in the granitic rocks the batholith, but none of consequence break of eastern Oregon, southeastern Washington, it" (Hamilton and Myers, 1966, p. 542). and western Idaho only in plutons and small igneous bodies that border the Idaho batholith. SATELLITES WEST OF THE IDAHO Rocks that contain the conspicuous crystals of BATHOLITH epidote range in composition from mafic quartz From the standpoint of the regional distribu- diorite to leucocratic trondhjemite and tion of plutonic rocks in western Idaho, several granodiorite. Some rocks contain as much as 6 petrographic relationships involving satellites percent epidote. The quartz diorites of Council of the batholith require comment as back- Mountain and Deserette (Fig. 2) are excellent ground for interpretations and conclusions re- examples of the epidote-bearing rocks which garding the location of the contact of the occur northward in satellites for at least 125 mi. batholith south of the Snake River Plain. The unique epidote-bearing rocks occur as plu- 1 Paleomagnetic data for Columbia River Basalt in south- tons of trondhjemite (Hamilton, 1963) in the ern Washington and the over-all trend of basalt dikes in 30-min Riggins quadrangle, some 25 to 55 mi western Idaho, northeast Oregon, and southeast Washington north of Council Mountain. Farther to the indicate that little or no post-Miocene oroclinal bending has north, the rocks are conspicuous about 6 to 10 occurred in these regions (Taubeneck, 1970, p. 92-95). It is mi south of Grangeville2 in plutons sur- emphasized that possible pre-Miocene rotation is irrelevant rounded areally by Columbia River Basalt. to a tectonic model (Hamilton and Myers, 1966) in which normal faulting in the Basin and Range structural province is supposedly accompanied by oroclinal bending in the Pa- 2Grangeville is about 84 mi N. 5° E. from Council Moun- cific Northwest. tain.

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Specimen number 157 158 159 160 162 173

Potassium feldspar 1.0 1.0 2.3 0.1 0.7 0.0 Quartz 19.9 20.3 22.4 18.7 21.3 17.9 Plagioclase 45.5 42.9 44.6 52.4 49.5 48.7 Biotite 17.6 17.3 16.1 14.9 14.4 15.9 Hornblende 14.4 17.9 12.4 13.4 13.1 17.1 Augite 0.8 0.3 2.0 0.0 0.0 0.1 Opaque accessories 0.0 0.0 0.0 0.0 0.0 0.0 Nonopaque accessories 0.8 0.3 0.2 0.5 1.0 0.3

Each modal analysis is the average of two thin sections.

About 5 to 9 mi east-southeast of Grangeville, thin sections of 71 rocks from the batholith and the large crystals of epidote occur in a trondhje- the presence of zeolites in sections from 21 of mite bordered by basalt on the west. Exposures 59 rocks from satellites, however, seem to jus- of pre-Tertiary rocks are not common north of tify the conclusion that zeolites characterize Grangeville. Nevertheless, megascopic crystals granitic rocks of satellites rather than those of of epidote in a tonalite about 27 mi due north the batholith. Interstitial zeolites occur near of Grangeville indicate that the belt of epidote- Council Mountain (Fig. 2) in the Deserette plu- bearing rocks extends northward beneath the ton and in satellites northward for 110 mi, Columbia River Basalt. Hietanen (1962, p. 55) which is near the northern limit of sampling. described large crystals of epidote in tonalite The common occurrence of gabbroic rocks about 40 mi north of Grangeville; the belt of in western Idaho near the batholith is another epidote-bearing rocks probably continues fur- relationship that can be used as an indication of ther northward. South of the quartz diorite of the approximate location of the contact of the Council Mountain, Columbia River Basalt pre- batholith south of the Snake River Plain. The vents any determination of the distribution of gabbroic rocks generally include hypersthene- epidote in concealed satellites that undoubtedly bearing varieties. Near the northwest part of occur between Indian Mountain (Fig. 1) and the batholith, gabbro and norite in the vicinity the Snake River Plain. of Ahsahka occur in close proximity to horn- The distribution of interstitial zeolites blendite (Hietanen, 1962, p. 52). About 43 mi (Taubeneck, 1967, p. 17-19) in the granitic to the south-southeast, gabbro and norite are rocks of western Idaho also is significant be- present near Harpster (Myers, 1968, p. 118). cause zeolites apparently occur only in satellites Some 30 mi south of Ahsahka, gabbroic rocks of the batholith. The zeolites (mostly heulan- near Ferdinand include gabbro, hornblende dite) comprise less than 0.05 percent by melagabbro, and hypersthene gabbro. In the volume of the rocks. Accordingly, the careful Cuddy Mountains, about 25 mi west of Council examination of hundreds of thin sections will Mountain (Fig. 2), gabbro and norite are pre- be necessary to document the distribution of sent in an area of plutonic and low-grade meta- interstitial zeolites in the granitic rocks of west- morphic rocks that is surrounded by Columbia ern Idaho. The absence of interstitial zeolites in River Basalt. The distribution of gabbroic rocks

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in each of the four areas cannot be determined ever, in some rocks east of the foliated border because of the Widespread flows of Columbia zone. River Basalt that cover much of the pre-Terti- ary basement near the west contact of the bath- STRUCTURAL TRENDS IN GRANITIC olith. ROCKS NEAR THE SNAKE RIVER PLAIN STRUCTURAL TRENDS NEAR THE The southern limit of the Idaho batholith tra- WEST CONTACT OF THE ditionally has been placed near the north edge BATHOLITH of the Snake River Plain, although younger The west bof def of the Idaho batholith near rocks everywhere overlie the batholith along the Snake River Pliih is Exposed in greatest this boundary. Larsen and Schmidt (1958, p. 3) detail in the area (Fig. 2) west of Cascade noted that the batholith "may extend south- Reservoir. North of the area of Figure 2 for ward for many miles beneath this cover." Pla- about 10 mi, the border rocks are concealed by nar structures trending roughly north-south in Columbia River Basalt and the Quaternary granitic rocks near the plain strongly suggest deposits of Long Valley. Southwest of the Cas- that the batholith does continue southward cade Reservoir, much of the gneiSsic shell of beneath the Cenozoic rocks. If the batholith the batholith iS covered by Columbia River Ba- terminated near the plain, structures in the gra- salt. Accordingly, the area of Figure 2 merits nitic rocks just north of the plain should be special consideration in documenting structural more nearly east-west in close parallelism to a trends near the west contact of the batholith. concealed contact along or near the northern Over*all structural trend's in the" area west of edge of the plain. In six areas of granitic rocks Cascade Reservoir are north-south (Fig. 2). that were examined eastward from the vicinity Most deviations ritofe than 15" from north- of Boise, planar structure was not detected in south in trends of country rocks are attributable two areas, but northward-striking planar struc- to disruption by forceful emplacement of plu- ture was observed in the following four areas. t6ns stfch as the Council Mountain and Don- About 10 mi north-northwest of Boise (Fig. nelly bodies. Trends in plutons closely parallel 1), faint planar structure in granitic rocks near wall rocks and are essentially north-south, ex- the Snake River Plain trends about N. 5° W. cept along contacts that turn appreciably to the and dips 65° NE. Although the structure is east or west. Within 10 mi to the south of Cas- weak, the trends are consistent with a south- cade Reservoir, structural trends swing from ward continuation of the batholith under the north:south to about S. 20° W.-^—a trend that Cenozoic cover. persists (Fig. 1) southward to the Snake River About 20 mi north-northwest of Mountain Plain. Home (Fig. 1), excellent planar structure in Near the south end of Cascade Reservoir, the rocks bordering the plain trends about N. 15° width of the foliated zone within the batholith W. with 70° dips to the west. About 14 mi north is about 10 mi (Fig. 1). Although the "contact" of Mountain Home, rocks bordering the plain of die batholith is cdhcealed by Columbia River trend mostly between N. 10° E. and N. 10° W.; Basalt south of Indian.Mountain (Fig. 1), fo- dips are generally 70° W. Trends in the areas liated ' rocks of the border zone are exposed north and north-northwest of Mountain Home almost continuously southward to within 8 mi indicate that the batholith extends south under of the Snake Rivef Plain. The inferred location the Cenozoic rocks of the plain. of the contact of the batholith south of Indian Some 32 miles east-northeast of Mountain Mountain i$ arbitrarily drawn (Fig. 1) about 10 Home, good planar structure in granitic rocks mi west of the east margin of the foliated bor- near the county line between Elmore and der rocks. The line denoting the east margin of Camas Counties has an average trend of N. the foliated border rocks should be accurate to 15" W. with dips 80° SW. Planar structure in within 0.5 nit, although the location of the line this vicinity also implies a continuation of the south of Cascade Reservoir is ba^ed on only five batholith to the south. traverses across the border zone. A general ac- General absence of gneissic rocks in the bath- curacy of 0.5 mi is probable because the inten- olith along the north edge of the Snake River sity of foliation diminishes rapidly near the Plain also supports the conclusion that the bath- eastern margin of the foliated border zone. olith continues south beneath the plain. If the Faint to good planar structure does occur, how- south contact of the batholith was near the

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north edge of the plain, gneissic border rocks the north side of the Snake River Plain. Potas- trending more or less east-west should occur sium feldspar is less abundant in the gneissic along the southernmost exposures of the bath- border rocks than in nongneissic granitic rocks olith. Gneissic rocks do occur in one area, but to the east. Although phenocrysts of potassium trends are not parallel to the margin of the feldspar occur in several areas within the gneis- plain. About 16 mi northeast of Mountain sic border zone, many granitic rocks contain no Home, gneissic granitic rocks near the north- phenocrysts and only small amounts of potas- ern edge of the plain occur with intensely meta- sium feldspar. In contrast, phenocrysts of potas- morphosed country rocks in an area of at least sium feldspar are characteristic of the 15 sq mi. One traverse across this gneissic ter- nongneissic granitic rocks to the east of the bor- rane indicates that trends are mostly within der zone. As is true just north of the Snake 15° of north-south; dips are generally less than River Plain (Table 1), biotite is more abundant 45" E. in the gneissic border rocks, whereas hornblende is confined either to the border rocks or SOUTHERN EXTENSION OF THE to rocks a short distance to the east. Modal data BATHOLITH IN SOUTHWEST IDAHO in Table 7 provide a general approximation of Most of southwest Idaho is characterized by the mineral proportions in the exposed parts of Cenozoic volcanic formations, but many iso- the batholith in southwest Idaho. Specimens lated outcrops of granitic rocks southward 40 116, 117, 118, 249, and 256 ate from the mi from the Snake River (Fig. 1) permit impor- gneissic border zone; the remaining specimens tant conclusions regarding petrographic and are representative of interior rocks. structural relationships of pre-Tertiary rocks Location of the gradational contact (Fig. 1) in throughout an area of about 1000 sq mi. Far- southwest Idaho between gneissic and non- ther to the south, toward the southwest corner gneissic granitic rocks of the batholith poses of Idaho, no pre-Tertiary rocks are exposed. few problems in comparison with uncertainities Uncertainties regarding the characteristics of regarding the location of the "contact" of the concealed pre-Tertiary rocks near the south- batholith. Most of the westernmost exposures west corner of Idaho are increased by the cover of gneissic granitic rocks south of the Snake of Cenozoic volcanic rocks in adjoining parts of River generally cannot be traced continuously Oregon and Nevada. eastward into nongneissic granitic rocks. Nev- Granitic rocks of southwest Idaho are cor- ertheless, exposures are adequate in most related with the Idaho batholith primarily by places to permit a confident location of the east the location and trend of gneissic border rocks margin of the gneissic border rocks within a on either side of the Snake River Plain. The distance of 2 mi or less (Fig. 1). No exposures trend of S. 20° W. in the gneissic border zone of pre-Tertiary rocks occur in southwest Idaho of the Idaho batholith on the north side of the west of the inferred "contact" (Fig. 1) of the Snake River Plain is duplicated 40 to 55 mi batholith, except in the vicinity of South Moun- farther south-southwest where gneissic granitic tain. Pre-Tertiary rocks exposed throughout an rocks reappear in the westernmost exposures of area of about 25 sq mi near South Mountain are pre-Tertiary rocks on the south side of the neither part of the Idaho batholith nor part of Snake River (Fig. 1). Furthermore, the inten- a roof pendant, as was verified by six days of sity of planar structure in the westernmost gra- reconnaissance supplemented by unpublished nitic rocks south of the Snake River rapidly mapping of R. L. Krueger, north of the moun- diminishes to the east, in the same manner as in tain. Metamorphic rocks composed mostly of the westernmost granitic rocks north of the quartzite, schist, and marble (Sorenson, 1927, plain. Coinciding structural relationships in gra- p. 11) are intruded by small granitic bodies nitic rocks on either side of the plain, strength- and, on the south, by a gabbroic complex. East ened by similar mineralogical characteristics, and northeast of South Mountain, scattered warrant the conclusion that the granitic rocks areas of gneissic and nongneissic granitic rocks near the south side of the Snake River in south- within the batholith permit the location of the west Idaho are a southern continuation of the "contact" of the batholith with a maximum er- Idaho batholith. ror of not more than about 5 mi. The inferred Gross mineralogical relationships of granitic width of the gneissic border zone northeast of rocks south of the Snake River resemble the South Mountain is maintained arbitrarily in ex- relationships in the west part of the batholith on tending the "contact" of the batholith north-

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Specimen Potassium Quartz PI agio- Biotite Muscovite Horn- Accessories number feldspar el ase blende Opaque Nonopaqui

116 2.7 28.0 57.3 11.6 0.3 0.0 0.0 0.1 117 1.8 30.5 51.9 14.9 0.9 0.0 0.0 0.0 118 3.1 18.3 57.8 14.6 0.0 4.8 0.0 1.4 249 0.0 30.8 58.6 9.4 1.0 0.0 0.0 0.2 256 3.6 32.9 49.1 13.5 0.8 0.0 0.0 0.1

204 19.2 27.5 45.0 7.8 0.4 0.0 0.0 0.1 245 11.3 30.5 50.9 4.5 2.7 0.0 0.1 0.0 257 14.5 32.2 46.2 4.9 1.9 0.0 0.0 0.3 326 10.7 27.6 52.5 8.3 0.7 0.0 0.0 0.2

Data for specimens 204, 245, 257, and 326 are averages of four thin sections each; other specimens are averages of two thin sections each.

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ward to the Snake River. The abundance of About 4 sq mi of the complex are exposed; gneissic granitic rocks in eastern Oregon in dominant rocks apparently are hornblende gab- conglomerates3 in Cenozoic formations west of bro, hornblende melagabbro, and coarse horn- Marsing (Fig. 1) suggests that the gneissic bor- blendite. Hornblende-augite norite and der zone is at least as wide as is shown in Figure amphibolitized inclusions of country rocks also 1. Gravity data (Bonini, 1963) are consistent are part of the complex. Locally, the gabbroic with the general location of the inferred "con- rocks are intruded by quartz diorite. tact" of the batholith throughout the 55-mi in- Data summarized in Figure 1 indicate that terval from east of South Mountain northward southwest structural trends in granitic rocks to the Snake River; the correlation seems best near the Snake River commence a swing to the within 25 mi of South Mountain. southeast in an area about 25 mi due south of Mineralogical and structural features of the Marsing. Farther to the south, trends within the igneous masses near South Mountain confirm batholith in the most westerly rocks are about that the bodies are satellites of the Idaho bath- S. 20° E. for 28 mi, beyond which the batholith olith. Gneissoid quartz diorites characterized is concealed by Tertiary volcanic rocks. Gra- by discrete crystals of epidote occur as lens- nitic rocks are exposed for a total of only 10 mi shaped intrusions in schist along a ridge almost along the 28-mi interval, but the general S. 1 mi west of the lookout tower on South Moun- 20° E. trend suggests that a similar southeast tain. The small igneous bodies have a consider- trend characterizes the intervening parts of the able range of color index, as shown by modal batholith that are overlain by volcanic rocks. data (Table 8, nos. 259, 261, 264) for rocks Furthermore, structural trends in the meta- from three intrusions. The large amount of epi- morphic rocks near South Mountain are also to dote (average content 2.7 percent) in the three the southeast; dips are to the southwest. Trends rocks is typical of many satellites of the bath- of schistose inclusions within the gabbroic com- olith on the north side of the Snake River Plain. plex, as well as local banding in the gabbroic However, as most epidote in the granitic rocks rocks, are also to the southeast. Accordingly, west of the lookout is less than 0.5 mm across, structural trends in the pre-Tertiary wall rocks the crystals are not as conspicuous as in the near South Mountain are similar to the south- satellites of west-central Idaho. east trends in the Idaho batholith to the east and The largest granitic intrusion in the vicinity northeast. The southeast trends within and out- of South Mountain is a zoned stock about 6 sq side of the batholith prevail throughout an area mi in outcrop area. This intrusion is north of the of sufficient size to conclude that a significant mountain. Four specimens (Table 8, nos. 265, change in structural direction occurs in south- 266, 267, 268) from within 0.35 mi of the west Idaho in the region near South Mountain. contacts of the mass are quartz diorites, whereas two specimens (nos. 269, 270) from near the CONTACT OF BATHOLITH NEAR center of the pluton are granodiorites. All rocks IDAHO-NEVADA BOUNDARY carry hornblende, but they are structureless, The Idaho batholith cannot be traced south- unlike hornblende-bearing rocks of the border ward from the exposures (Fig. 1) east of South zone of the batholith. Mountain by means of surface geology because Gabbroic complexes that occur as satellites of of a widespread cover of Tertiary volcanic the batholith on the north side of the Snake rocks. The nearest known pre-Tertiary rock to River Plain have a counterpart near South the south is a granodiorite (Fig. 1, loc. 1) that Mountain that supports the conclusion that the crops out for several miles along Cottonwood contact of the batholith lies to the east. On the Creek near the headwaters of this stream. This south and southeast sides of South Mountain, a granodiorite, surrounded areally by Tertiary gabbroic complex extends for 5 mi along an volcanic rocks, is exposed on either side of the overlapping contact of Tertiary volcanic rocks. Idaho-Nevada state line. Paleozoic strata, intruded by small granitic plutons, occur from 3 *The conglomerates also contain nonfoliated clasts of the to 10 mi south of the state line. The pre-Terti- typical muscovite-bearing granodiorite (Table 7) of the interior of the batholith. The common occurrence of quart- ary rocks in Nevada suggest that the granodior- zofeldspathic gneisses in the conglomerates, as well as many ite along Cottonwood Creek is either part of clasts of gneissic granitic rocks, indicates that the source area the border of the Idaho batholith or part of a includes a larger region near the border of the batholith than satellitic stock of the batholith. present exposures of pre-Tertiary rocks represent. Structural and mineralogical features of the

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granodiorite along Cottonwood Creek favor northeast to northeast. Trends of country rocks the interpretation that the granodiorite is part generally are more or less concordant with con- of a satellite of the batholith. Most granitic tacts of bordering batholiths. Therefore, the rocks along Cottonwood Creek are without pla- anomalous deviation of the north-northeast re- nar structure. Absence of planar structure is gional trend of the pre-Tertiary rocks of north- atypical of border rocks of the batholith and ern Nevada to east-west in the Mountain City suggests, instead, that the rocks are part of a quadrangle and to east-northeast in the Row- satellite. The rather high percentage of potas- land quadrangle may reflect the nearness of the sium feldspar in three specimens (Table 8, nos. south contact of the Idaho batholith. 315, 316, 317) also is not typical of border rocks of the batholith. Furthermore, mega- EAST CONTACT OF BATHOLITH scopic crystals of epidote (average content 1.8 NEAR SNAKE RIVER PLAIN percent) are characteristic of satellites rather than border rocks of the batholith. Also, the The inferred location of the east contact (Fig. three rocks contain about 0.1 percent of inter- 1) of the batholith near the north side of the stitial zeolites (Taubeneck, 1967, p. 17-19), Snake River Plain is influenced by die character which have been observed in Idaho only in of poor exposures of granitic rocks that are sur- rocks satellitic to the batholith. rounded near location 4 by Cenozoic forma- South of the southernmost granodiorite tions (Malde and others, 1963). The granitic along Cottonwood Creek, 4 to 5 mi, four speci- rock near location 4 is a porphyritic granodior- mens from the elongated Hicks Mountain stock ite or quartz monzonite of a type that character- (Coats and others, 1965) were collected for izes the interior of the batholith. Therefore, the comparison with granitic rocks of Cottonwood contact of the batholith is east of location 4. Creek. The specimens (Table 8, nos. 307, 308, About 12 mi to the north-northeast of location 310, 311) contain megascopic epidote (rather 4, in an area partly covered by volcanic rocks, small crystals and not abundant) and about 0.1 the contact of the batholith can be located to percent of interstitial zeolites—minerals that within a few miles. From this vicinity the con- commonly distinguish rocks of the satellites of tact is arbitrarily extended almost due south to the Idaho batholith. pass about 6 mi east of location 4. A few hundred yards south of the drainage The location of the east contact of the bath- divide between Cottonwood Creek and Little olith on the south side of the Snake River Plain Salmon Creek, thermally metamorphosed gab- involves by far the greatest uncertainty in the broic rocks several miles north of the Hicks attempt (Fig. 1) to define the approximate Mountain stock are overlain by Tertiary vol- boundaries of the southern part of the Idaho canic rocks. Together with cited features of the batholith. Two small areas of Ordovician sedi- Hicks Mountain and Cottonwood Creek mentary rocks (Youngquist and Haegele, granodiorites, the presence of gabbroic rocks 1956, p. 10-11; Crosthwaite, 1969, p. 10) near the headwaters of Cottonwood Creek in about 18 mi south of Twin Falls (Fig. 1) imply northernmost Nevada can be used as an argu- by their unmetamorphosed condition that the ment that the south contact of the Idaho bath- nearest contact of any large batholith is many olith is not far to the north. miles away. As other Paleozoic strata also sur- The east to east-northeast trends of Paleozoic rounded by Cenozoic volcanic rocks occur rocks in the 15-min Mountain City and Row- within 12 mi to the southeast of the Ordovician land quadrangles of northernmost Nevada are sediments, the east contact of the Idaho bath- compatible with the possibility that the south olith must be west of the Ordovician rocks. contact of the Idaho batholith is not far north of Exposures of the Ordovician strata are suffi- the Nevada-Idaho state line. Paleozoic strata ciently good, especially in the larger area to the are exposed almost continuously between loca- west, to assure that trends shown in Figure 1 are tions 2 and 3, Figure 1. Over-all trends in the representative throughout an area of at least Mountain City quadrangle are nearly east-west several square miles.. As a general parallelism (R. R. Coats, 1967, oral comm.) although fur- between the trend of the Ordovician sediments ther east in the adjoining Rowland quadrangle, and the contact of the batholith is a reasonable trends are east-northeast (Bushnell, 1967, PI. assumption, the postulated configuration of the 1). Where pre-Tertiary strata occur farther batholith, as suggested in Figure 1, is consistent south in Nevada, the regional trend is north- with the attitude of the Ordovician rocks.

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SIERRA NEVADA BATHOLITH zone of Mesozoic granitic rocks in the western Many comprehensive investigations during United States. In any modern synthesis of the the past 20 yrs by members of the U.S. Geolog- Mesozoic evolution of western North America, ical Survey have made the Sierra Nevada bath- however, the northward divergence (Kistler, olith the best known Mesozoic batholith of the 1970, p. 597) of the Jurassic and Cretaceous circum-Pacific belt. The most fundamental rela- belts of granitic rocks from the general region tionship from the standpoint of this paper is that of Yosemite National Park must be evaluated. the axis of the Sierra Nevada batholith does not SIERRA NEVADA BATHOLITH IN parallel the Sierra Nevada Mountains, which HUMBOLDT COUNTY, NORTHWEST trend north-northwest. The axis of the bath- NEVADA olith trends northerly across the Sierra Nevada at an acute angle and continues northward into Knowledge of the granitic rocks of western Nevada (Bateman and Wahrhaftig, 1966, p. Humboldt County in northernmost Nevada is 107). desirable as background for any attempt to Willden (1963, p. 6) reported that the Jack- evaluate the possibility of a connection between son Mountains of northwest Nevada "lie just the Idaho and Sierra Nevada batholiths. Ac- east of what might be considered the east mar- cordingly, reconnaissance studies were made of gin of the Sierra Nevada batholith." The pres- the plutonic rocks in Humboldt County that are ence of the batholith in northwest Nevada was within 45 mi of the Nevada-Oregon boundary. The approximate location of the east border established by chemical and radiometric studies 4 of the granitic rocks in western Pershing of the Sierra Nevada batholith in northwest County by Tatlock and Marvin (1967) of the Humboldt County is shown by the dashed line U.S. Geological Survey and by reconnaissance in Figure 3. This line represents the east margin mapping of Willden (1964) in Humboldt of a belt in which granitic rock is more abun- County. On a generalized geological map that dant than stratified rock in scattered areas of showed the configuration of the Sierra Nevada pre-Tertiary terrane. Widespread Tertiary vol- batholith, Bateman and Eaton (1967, Fig. 1) canic rocks and Quaternary alluvium necessi- extended the east border of the batholith as far tate that the dashed line be a generalized line north as the Nevada-Oregon state line. Moore with a possible error of as much as 5 mi. The (1969, Fig. 5) also extended the east limit of west margin of the batholith cannot be deli- the batholith northward to the Nevada-Oregon neated with assurance because Cenozoic forma- boundary. tions extend continuously for scores of miles to A recent analysis of radiometric age data for the west of the granitic plutons (Fig. 3). How- granitic rocks of California and western ever.Denio may be near the west border be- Nevada revealed that the Sierra Nevada bath- cause only a few small granitic intrusions occur olith is composed of five belts of rock ranging in the pre-Tertiary rocks to the west and north in age from Middle and Late Triassic to Late of this small community (Fig. 3). Low grade Cretaceous (Evernden and Kistler, 1970). The metamorphism of greenschist facies in the pre- belt of Middle and Late Triassic age, not well Tertiary rocks to the west and north of Denio defined in the western United States, will be ignored in this discussion. The four remaining *The belt of granitic rocks that extends from Baja Cali- fornia on the south, northward through the Sierra Nevada, belts in the batholith are combined into a Juras- and into western and northwest Nevada customarily is subdi- sic belt and a Cretaceous belt, to permit sum- vided into individual batholiths, but "a single name, such as mary statements of age relationships of granitic Cordilleran batholith, could be applied" (Bateman and oth- rocks in areas to the north of the central Sierra ers, 1963, p. 2) to these plutonic rocks. Sierra Nevada bath- Nevada. The Cretaceous belt of granitic rocks olith, however, is the name used in this paper for the belt of in the batholith extends northward across the granitic rocks in western and northwest Nevada, as well as Sierra Nevada and into Nevada. The Jurassic for the granitic rocks of the High Sierra in California. belt of granitic rocks diverges north-northwest The granitic rocks of northwest Humboldt County might from the Yosemite region of the batholith and be subdivided into several batholiths in a detailed petrologic investigation because continuity is uncertain (Fig. 3). Rather extends into the Klamath Mountains of north- than introduce one or more new names for the mass or ern California and southwest Oregon (Lan- masses of granitic rocks, Sierra Nevada batholith is used for phere and others, 1968). Traditionally, the belt convenience, but the writer emphasizes that the major zone of Jurassic plutons has dominated concepts re- of granitic rocks in Humboldt County could be a discontinu- garding the location and trend of the major ous belt of small batholiths.

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118° 30'

119 00 118° 30' Figure 3. Semireconnaissance map showing general- Willden (1964) and from Walker and Repenning ized subdivisions of bacholithic rocks of northern Hum- (1965). Capital letters indicate plutons dominantly of boldt County, Nevada. Map is modified slightly from quartz diorite.

suggests that the rocks are not part of a large Plutons with mesozonal attributes in north- screen or roof pendant of a batholith which ern Humboldt County imply that the dashed includes concealed granitic rocks a few miles to line (Fig. 3) representing the east border of the the west. Therefore, the pre-Tertiary rocks near batholith cannot be invalidated by the possibil- Denio probably are a short distance west of the ity that inadequate erosion has exposed only border of the Sierra Nevada batholith. Satellitic the upper part of a much larger batholith that plutons may lie beneath the Cenozoic volcanic underlies country rocks to the east. The gneissic formations for scores of miles to the west, as to gneissoid texture of most of the quartz dior- granitic clasts partly verify, which occur in a ite, as well as some granodiorite, characterizes conglomerate near the base of the Warner mesozonal and catazonal plutons, rather than Mountain fault block about 9 mi west of the plutons of the epizone (Buddington, 1959). California-Nevada boundary. The regional metamorphism of the greenschist

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fades (Willden, 1963, p. 17), however, is in- tions of four specimens from the monzodiorite dicative of the mesozone rather than the cata- northeast of pluton G contain an estimated 2 to zone (Buddington, 1959, p. 714). Moreover, 9 percent of quartz and from 6 to 12 percent of the schistose structure in country rocks border- potassium feldspar. ing the plutons and the absence of granophyre The distribution and composition of the plu- confirm a mesozonal classification for the plutonic rocks of northwest Humboldt County tons. Mesozonal plutons provide an adequate suggest that the quartz diorite boundary line of depth of erosion to permit an accurate assess- Moore (1959) may trend northeast to Denio, ment of the approximate location of the east although Moore (1959, p. 199) placed this border of the batholith. boundary about 160 mi west of Denio. Loca- In comparing Figure 3 with the reconnais- tion of the line in this part of the United States sance map of Humboldt County by Willden is uncertain because of widespread Cenozoic (1964), students of Nevada geology will note formations in northeast California, northwest minor differences. For example, granitic rocks Nevada, and southern Oregon. Moreover, mo- are not shown in Figure 3 in the Pueblo Moun- dal data for granitic rocks in northwest Nevada tains to the west and southwest of Denio be- were not available in 1959- Ralph J. Roberts cause only two small intrusions were observed has informally suggested in recent years that in traverses across the pre-Tertiary part of this the quartz diorite boundary line is near Denio, range. Another difference in the maps is that approximately parallel with boundaries (Rob- Figure 3 omits a dioritic pluton in the pre-Terti- erts, 1966, Fig. 3) farther east for facies belts in ary rocks west of the Happy Creek Ranch. This the Cordilleran geosyncline. Modal data in Ta- pluton, unlike the diorite and monzodiorite ble 9 strengthen the suggestion of Roberts that plutons shown in Figure 3, was subjected to the the quartz diorite boundary line is near Denio. greenschist-facies regional metamorphism A relocation of the quartz diorite boundary (Willden, 1963, p. 16) that preceded emplace- line is supported by the occurrence of boulders ment of the batholithic rocks. (as much as 5 ft across) of quartz diorite among In the barren mountain ranges of northwest the plutonic and metamorphic clasts in a con- Humboldt County, distinct color differences in glomerate in the Warner Mountains, about 90 the plutonic rocks as seen from a distance of mi west-southwest of Denio. Clasts in the con- many miles permit a rough subdivision be- glomerate provide the only indication of the tween dark-colored bodies on the west that in- character of the pre-Tertiary basement clude much quartz diorite, as opposed to throughout an area of more than 10,000 sq mi. leucocratic intrusions on the east that are com- Additional justification for a relocation of the posed of granodiorite and quartz monzonite. quartz diorite boundary line is the occurrence Because few contacts were walked, detailed of boulders of quartz diorite and epidote-bear- field studies will necessitate changes in the ing trondhjemitic rocks (closely akin to the configuration of most bodies shown in Figure 3. trondhjemite of west-central Idaho) in a basal Moreover, as at least some of the intrusions are conglomerate of Cenozoic age about 5 mi north composite, a subsequent subdivision of several of Denio. If the boundary line is moved about bodies will be possible. Enough samples were 160 mi eastward to Denio, the line should collected, however, to confirm the abundance trend northeast through southeast Oregon to of quartz diorite in the designated plutons. Ex- the vicinity of South Mountain before swinging cept for pluton B (Fig. 3), modal analyses of north-northeast along the western margin of rocks from intrusions that are dominantly of the Idaho batholith. Southwest of Denio a relo- quartz diorite are given in Table 9. Additional cated line apparently should trend about S. samples from pluton E may disclose that 45° W. to the northernmost part of the Sierra granodiorite comprises part of this body, as sug- Nevada Mountains, where Diller (1908, p. 89) gested by specimen 41. Diorite and gabbroic reported that granitic rocks in the Taylorsville rocks are associated with some masses of quartz region are "generally quartz diorite, but locally diorite, as in pluton C. The diorite body west the orthoclase may increase and the rock passes of pluton E, as judged by thin sections from into granodiorite." If modern petrographic stu- four samples, contains an estimated 1 to 6 per- dies confirm a large proportion of quartz dior- cent of quartz and from 0 to 4 percent of potas- ite among the granitic rocks in the Taylorsville sium feldspar. Several dioritic rocks from region, the boundary line in northern Cali- pluton C are of similar composition. Thin sec- fornia and southern Oregon should be moved

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eastward to the vicinity of Taylorsville, Denio, POSSIBLE CONNECTION BETWEEN and South Mountain. IDAHO AND SIERRA NEVADA The probability that the quartz diorite BATHOLITHS boundary line is near Denio is another factor Many earth scientists during the last quarter that favors the supposition that Denio approxi- century speculated on the continuity of Meso- mately marks the west border of the batholith. zoic granitic rocks between the Idaho batholith The quartz diorite boundary line in east-central in central Idaho and the Sierra Nevada bath- California is in the west margin of the Sierra olith in the High Sierra of California. If the Nevada batholith, and the line in west-central batholiths are connected, granitic rocks of the Idaho is in the west part of the Idaho batholith connecting link must occur beneath the Ceno- (Moore, 1959, p. 199). Accordingly, it can be zoic volcanic rocks of southeast Oregon, south- argued by analogy that the location of the line west Idaho, and northernmost Nevada. in northwest Nevada coincides with the west Structural data (Fig. 1) from exposures of border of the batholith in Nevada. If so, the pre-Tertiary rocks in southwest Idaho weaken batholith in northwest Humboldt County is the possibility of a connection between the only about 40 percent as wide as in the central Idaho and Sierra Nevada batholiths. If the bath- Sierra Nevada Mountains. A much narrower oliths are connected, the Idaho batholith south- batholith in northwest Nevada is reasonable be- east of South Mountain must veer sharply west cause the batholith does not include the large beneath the Cenozoic volcanic formations and plutonic belt of Jurassic age that contributes to reach the Idaho-Nevada boundary between its mass in the region of Yosemite National long 116" and long 117° (Fig. 4). Between long Park. 117°30' and long 117°45', however, Mesozoic It does not follow from the northward diver- metasedimentary rocks in the Santa Rosa gence (Kistler, 1970, p. 597) of the Jurassic Range (Compton, I960) extend northward to and Cretaceous plutonic belts from the central within 15 mi of the Nevada-Oregon boundary Sierra Nevada that all granitic rocks in western (Willden, 1964). Accordingly, if the two bath- Humboldt County are of Cretaceous age. Ran- oliths are connected, granitic rocks of the con- domly distributed plutons of Jurassic age are necting link must be confined to a relatively present east of the batholith in north-central narrow belt (Fig. 4) that extends east-northeast Nevada(McKee and Silberman, 1970, p. 613), for about 75 mi near lat 42° N. and they also may occur in western Humboldt The distribution of granitic rocks in northern County as small intrusive units that are early in Humboldt County indicates that the Sierra the emplacement sequence. Moreover, pre- Nevada batholith does swing eastward near lat Cretaceous granitic rocks definitely occur in the 42° N. As shown by Moore (1969, Fig. 5), the region as is demonstrated by clasts of metamor- east contact of the batholith swings through an phosed granodiorite and quartz monzonite in a arc of about 40° (Fig. 3) before it is covered by basal Ceriozoic conglomerate about 5 mi north Tertiary volcanic rocks. of Denio, by small bodies of regionally meta- Structural trends in gneissic and schistose morphosed granitic rocks in the southern Pueb- country rocks about 11 mi east of Denio are blo Mountains near Denio, and by restricted consonant with an eastward swing in the axis of exposures of metamorphosed granodiorite and the batholith near the Nevada-Oregon bound- quartz monzonite about 55 mi south-southwest ary. The highly metamorphosed rocks are ex- of Denio. The metamorphosed granitic rocks posed in an area of slightly more than 2 sq mi are characterized by finely disseminated grains (Fig. 3) on the north side of a pluton of of epidote. Small flakes of recrystallized biotite granodiorite. The intricate contact has a gen- are common, and some rocks are highly frac- eral trend of about N. 60° E. (Fig. 3). Trends tured. Perhaps the metamorphosed granitic in the country rocks vary within a range of N. rocks belong to the Lee Vining (Evernden and 20° E. to S. 75° E.; the average of 27 determina- . Kistler, 1970, p. 19) intrusive sequence of Mid- tions is N. 59° E. The average trend is much dle and Late Triassic age. Regardless of the age more easterly than the over-all trend of the or ages of the metamorphosed granitic rocks, Sierra Nevada batholith in Nevada. Exposures their restricted distribution is in harmony with are inadequate to determine whether the in- the conclusion that the Cretaceous plutonic belt tensely metamorphosed country rocks east of dominates the Sierra Nevada batholith in Denio are along the northern border of the northwest Nevada. batholith or are part of a screen between two

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Denio, although continuity in the strict sense may not persist for each mile of the concealed S /H/ interval between the two batholiths. / POST-OLIGOCENE DISPLACEMENT OF IDAHO AND SIERRA NEVADA BATHOLITHS The Idaho and Sierra Nevada batholiths NEVADA • . ?' were more nearly in a north-south alignment before post-Oligocene deformation in the Ba- sin and Range structural province produced an estimated east-west change of as much as 50 mi in the relative position of the two batholiths. A positional change of such a magnitude is consistent with the combined displacements from normal faulting, dike intrusion, and right-lat- Figure 4. Map showing possible connection between eral faulting, which are features of the Basin the Sierra Nevada batholith and the Idaho batholith. and Range structural province. Interpretations of the Cenozoic evolution of plutons. Either possibility is consistent with an the western United States in terms of plate tec- eastward swing of the batholith because screens tonics must emphasize the tectonic significance commonly strike parallel to the axis of a bath- of the intersection of the North America plate olith. The country rocks probably are part of a with the crest of the East Pacific Rise in late screen, as suggested by the intense metamor- Oligocene time. Annihilation of the eastern- phism and by the geometry (Fig. 3) of the plu- most sector of the rise with concomitant initia- tons near the Oregon-Nevada boundary. tion of the northern portion of the San Andreas Structural trends in the large area of pre- fault system is closely related to collision of the Tertiary rocks west and north of Denio are North American and Pacific plates (Atwater compatible with an eastward swing of the bath- and Menard, 1970, p. 449). Extensional Basin olith, although more easterly trends in the and Range faulting throughout much of the rocks would provide stronger confirmation of a western United States commonly is regarded as change in direction of the batholith. The aver- a consequence of the subsequent interaction of age trend of pre-Tertiary rocks west and south- the two plates. west of Denio is N. 42° E., as judged by Extension in the Basin and Range structural observations at 14 localities in an area of 3 sq province must be a major factor in the east-west mi. Massive volcanic rocks north of Denio in change in alignment of the Idaho and Sierra the northern part of the region of pre-Tertiary Nevada batholiths. Available radiometric ages rocks (Fig. 3) provided little evidence of struc- of dikes in the western United States suggest an tural trends during two traverses across the age of about 25 m.y. for the inception of impor- area. However, isolated exposures of pre-Terti- tant normal faulting of the Basin and Range ary rocks that are not shown in Figure 3, about structural province (Taubeneck, 1970, p. 88). 13 mi north of Denio, include a nearly vertical Accordingly, Basin and Range extension is clas- greenstone that trends about N. 40° E. sified as post-Oligocene. The close agreement is In summary, relations in northernmost noteworthy between the 27 m.y. date (Atwater Nevada indicate that the Sierra Nevada bath- and Menard, 1970, p. 449) when the North olith may extend eastward near lat 42° N. to American continent impinged on the crest of connect with the Idaho batholith in the manner the East Pacific Rise and the beginning of Basin suggested in Figure 4. This possibility is fa- and Range faulting as determined by radiomet- vored by the virtual continuity of the Creta- ric ages of dikes. ceous plutonic belt, wherever pre-Tertiary rocks The Sierra Nevada batholith in the vicinity, are exposed, northward from the central Sierra about lat 36°45' N., of Fresno, California, is Nevada Mountains to the Oregon-Nevada west of the Basin and Range structural prov- boundary. Logic suggests that the belt of gra- ince; the Idaho batholith is mostly east of the nitic rocks does not terminate beneath the normal faults of the structural province. Re- Cenozoic volcanic formations east-northeast of gional relationships can be visualized from a

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map of Basin and Range faults (Gilluly, 1963, rocks generally are characterized by negative p. 158). In addition to faults shown by Gilluly, relief; the dike; are not easily seen unless bed- the writer includes (see also Schmidt and Mac- rock is well exposed. Even iin fertwiiry volcanic kin, 1970, p. 4) within the structural province rocks, in contrast to textbook concepts and the north-south faults of western Idaho (Ander- mental images, many dikes dq hot exhibit posi- son, 1934b, p. 23-26). Many of the larger tive relief. For example, niost dike? of a basaltic faults, mostly near the west side of the Idaho swarm near Lakeview, Oregon, not fajr froni the batholith, were shown on a sketch map by northwest corner of Nevada, vovUd remain un- Hamilton (1962, p. 512). The major fault ap- known without roadcpts. A 75-ft-w^fe dike in parently has at least 9000 ft of displacement as Warner Canyon, exposed pnjy in a roadcut judged from gravity interpretations of valley fill along highway 140, is an excellent example of in the Long Valley graben (Kinoshita, 1962, p. a large dike that woyld not be noticed without 6). Farther south, on the southwest side of the man-ma,de excavations- The writer has found at Snake River Plain, a detailed map by Kittleman least some dikes in nearly every mountain and others (1967) shows the abundance of range in southeast Oregon, western Idaho, and north-south normal faults near the Idaho-Ore- northern Nevada. Published ?pd unpublished gon state line. data of many workers indicate tjbat dikes occur The effect of Basin and Range faulting on the in most parts of the Basin and Range structural position of the Sierra Nevada and Idaho bath- province. Although dike smarms occur north of oliths is readily seen from a map5 by Gilluly the Snake River 1>lain in the Jdpho Batholith, (1970, p. 61) that shows the percentage of dis- they apparently are absent in th&t'portion of the tention across the width of the structural prov- Basin and Range; province that is e$st of the ince. If an extension of 50 mi is assumed across batholith. Considering the distribution pf dikes northern Utah, northern Nevada, and of all compositions, rhyolitic as well as basaltic, northeast California, the distribution of faults in dike intrusion in the structural province proba- the structural province implies roughly 30 mi of bly produced from 3 to 5 mi of east-west post-Oligocene east-west change in the relative change in the pver-aU position of the Sierra position of the Sierra Nevada and Idaho bath- Nevada and Idaho bajholiths. oliths. Westward displacement of much of the Part of the east-west flexure near lat 42° N. Sierra Nevada^ batholith. (mostly the part in in the possible connection (Fig. 4) between the California) during rigbt-lateral faulting in the Idaho and Sierra Nevada batholiths can be ex- western Great Basin ^Jso ij a factor thai must be plained by differential extension on either side evaluated in reconstructing the late Oligocene of the Idaho-Nevada state line. The rather position of the two batholiths. Major north- abrupt change in fault density (Gilluly, 1963, p. west-trending structural'/Sones that require con- 158) in passing northward from northern sideration are the t^s Ve$as-Walker Lane and Nevada into southern Idaho would accentuate Death Valley-Furnace Creek systems. Data an original flexure in the belt of plutonic rocks. summarized by Stewart and others (1968, p. Post-Oligocene dikes and dike swarms of the 1411) indicate about 30 to 35 mi pf right-lat- Basin and Range structural province (Taube- eral displacement (rotational drag 35 well as neck, 1969) also have contributed to the fault slip) along the Las Vegas shear zone. change in position of the Idaho and Sierra Bending and faulting along the Las Vegas shear Nevada batholiths, but the magnitude of the zone are post-Qligocerie according to Fleck change is uncertain, partly because many of the (1970, p. 333). Nielsen (i?65, p. 1305) re- dikes are so poorly exposed that they com- ported about 12 mi of aggregate right-lateral monly escape detection (Taubeneck, 1970, p. displacement in the Soda Springs region of the 74, 78). Basaltic dikes that cut pre-Tertiary Walker Lane, sprne-240 mi northwest ,of Las Vegas. Lateral faulting in the Soda Springs re- 'The map was compiled from the tectonic map of the gion probably began after middle Miocene United States by considering each 10-mi-long normal fault time (Nielsen, 1965, p. 1305,1306). Estimates segment. As pointed out by Gilluly (1970, p. 59-60), no consideration was given to fault segments shorter than 10 mi, of the amount of right-lateral displacement nor does the map provide for the many miles of normal faults along the Death Valley-Furnace Creek fault sys- concealed beneath the- alluvium of the intermontane basins. tem vary from ,50 mi (Sfcewajt, 1967, p. 133) tp Accordingly, actual distention is greater than indicated by the between 10 and 20 nti (Bright and Jroxej, map. 1970, p. 2173) to even less ifi the southern

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Death Valley region (Wright and Troxel, 52' N. Geochronologic data indicate that the 1967, p. 947). McKee (1968, p. 512) sug- Lee Vining intrusive epoch is of Middle to Late gested 30 mi of right-lateral displacement along Triassic age. The trend of N. 10° W. of the east the northern part of the Death Valley-Furnace margin of the Lee Vining rocks is in the same Creek fault zone. Faults along the Death Val- region where 100 mi of bending involving ley-Furnace Creek zone have been active in late Jurassic formations (Albers, 1967, p. 151) is Tertiary and Holocene time, but present evi- postulated. Accordingly, westward displace- dence does not warrant a conclusion that all ment of the Sierra Nevada batholith by sigmoi- strike-slip displacement is post-Oligocene. Ap- dal bending between iats 37° N. and 39° N. is praisal of data for right-lateral faulting in the not substantiated by the distribution of granitic western Great Basin, primarily along the Las rocks in the eastern part of the batholith. Vegas-Walker Lane and Death Valley-Furnace In conclusion, an appreciable change in the Creek systems, permits a tentative suggestion of relative position of the Idaho and Sierra as much as 10 to 15 mi of post-Oligocene east- Nevada batholiths has occurred since Oligo- west change in the relative, position of the cene time. If the suggested magnitudes of dis- Sierra Nevada and Idaho batholiths. placement from normal faulting, dike intrusion, The possibility that sigmoidal bending in and right-lateral faulting are approximately cor- western Nevada and eastern California is re- rect, an east-west change of as much as 50 mi in sponsible for about 100 mi of horizontal dis- the alignment of the batholiths has occurred placement (Albers, 1967, Fig. 4) must be since the North American continent intersected considered in any attempt to determine the the easternmost sector of the crest of the East original relative position of the two batholiths. Pacific Rise. Between lats 37°N. and 39° N., some40 to 185 mi south-southeast of Reno, a generalized map CHARACTER OF EARTH'S CRUST IN by Albers (1967, Fig. 4) suggests that sigmoi- SOUTHWEST IDAHO dal bending has displaced Paleozoic and Meso- Hamilton and Myers (1966, p. 539) cited zoic rocks about 100 mi westward. Although interpretations of seismic refraction data in an Albers (1967, p. 151) concluded that most of abstract by Hill and Pakiser (1963, p. 890) as the postulated sigmoidal bending occurred justification for the conclusion that "low- before Miocene time, the effect of a horizontal velocity ('granitic') continental crust is proba- displacement of as much as 100 mi on the posi- bly wholly lacking beneath at least the western tion of the Sierra Nevada batholith justifies con- part" of the Snake River Plain, "which has a sideration regardless of whether or not most of thick but high-velocity ('basaltic') crust." The the suggested displacement is pre-Miocene. seismic data are for a north-south profile that The east border (Moore, 1969, Fig. 5) of the extends approximately from Boise, Idaho, Sierra Nevada batholith between lats 37° N. through Mountain City (Fig. 1), Nevada, to and 39° N. trends without deviation through Eureka, Nevada. Along the line of the seismic the west-central part of the area of suggested profile, the Snake River Plain is considered by sigmoidal bending. The border of the batholith Hamilton and Myers (1966, Fig. 1) and by Hill as shown by Moore (1969, Fig. 5) is a general- and Pakiser (1967, p. 686) to extend south- ized boundary, rather than a contact, between ward into northernmost Nevada. These work- mostly granitic rock on the west and country ers include a much larger area within the 6 rock on the east, but the state maps of Nevada plain than is shown in Figure 1. and California indicate that the border of the batholith between lats 37° N. and 39° N. can be 6 Physiographers (for example, Fenneman, 1931; Freeman drawn with an accuracy of about 5 mi. The and others, 1945; Hunt, 1967) commonly have differed by absence of distortion along the east border of more than 35 mi regarding the location of the southern edge the batholith suggests that post-batholith orocli- of the Snake River Plain, mostly because the region south- nal bending has not occurred. This conclusion west and west-southwest of Twin Falls has no well-defined is strengthened by the N. 10° W. alignment topographic boundaries. Fenneman and Johnson (1946) included much of southern Idaho and all of southwest Idaho (Evernden and Kistler, 1970, PI. 1) of the east- within their Columbia Plateau province—an unfortunate ern margin of the Lee Vining intrusive se- decision that influenced some workers to include most, or all, quence. Rocks of this sequence occur in eastern of southwest Idaho in a western Snake River Plain subprov- California and western Nevada in a belt about ince of the Columbia Plateau. As pointed out by Malde 115 mi long between lats 37°12' N. and 38° (1965, p. 255), the Snake River Plain is not an appendage

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The preferred model of Hill and Pakiser gravity high which could coincide with a postu- (1967, p. 697) for crustal structure along the lated "pull-apart."7 Idaho part of the seismic profile includes a 5.2- South of the Boise shot point, 104 mi, gra- kps layer about 10 km thick above a 6.7-kps nitic rocks near location 1 (Fig. 1) in southern- layer about 40 km thick. The model includes no most Idaho are several miles east of the line of granitic layer (6.0 kps) in Idaho; the granitic the seismic profile. Available geological and layer in Nevada pinches out northward near the geophysical data include no compelling evi- Nevada-Idaho border. Hill and Pakiser (1967, dence for eliminating a granitic layer from the p. 701) did note, however, that "It is not clear seismic profile for at least 68 mi north of loca- from the seismic refraction data if a 6.0 km/sec tion 1. Farther north, the seismic profile crosses layer exists between the intermediate layer and the central part of a prominent gravity high the 5.2 km/sec layer under the Snake River about 20 mi wide (Hill, 1963, p. 5808). The Plain." Significantly, Pakiser (1968, written preferred model of Hill and others (1961, p. commun.) stated that the 5.2 kps layer in south- 250) for explaining the gravity high is a tabular west Idaho and the uppermost part of the un- body of basalt about 90 mi long, from 4 to 6 mi derlying intermediate (6.7 kps) layer might be in width, and extending from about 5000 ft to granitic, making it permissible to extend the 60,000 ft below sea level. According to this granitic layer under the Snake River Plain. interpretation, a 4 to 6 mi "pull-apart" of the Surface geology in Idaho indicates that a gra- granitic layer would occur along the seismic nitic layer is present along at least part of the profile in the vicinity of the gravity high. seismic profile. The Boise shot point (Fig. 1, Exposures of granitic rocks to the west of the loc. 5) was in the batholith at Lucky Peak Reser- seismic profile prove that a granitic layer voir (Hill and Pakiser, 1967, p. 686). The bath- roughly parallels the profile for at least 35 mi. olith extends south from the shot point for Granitic rocks at location 7 (Fig. 1) are within about 2 mi (Lindgren, 1898) before the gra- 14 mi of the profile. Gravity data (Bonini, nitic rocks disappear beneath Cenozoic depos- 1963; Hill, 1963) east and southeast of location its. Poor exposures of the batholith occur at 7 impose no restrictions on extending the gra- location 6 (Fig. 1), about 4 mi south-southeast nitic rocks eastward beneath the Cenozoic for- of the shot point. Where granitic rocks crop out mations to the line of the seismic profile. along and near the profile, as in the vicinity of In summary, gravity and seismic data consid- the Boise shot point, the exclusion of a granitic ered in terms of surface geology and the distri- layer from the crustal profile seems unrealistic. bution of granitic rocks are consistent with the Mineralogical and structural features in the granitic rocks south and southeast of the shot point 7 The Snake River Plain is interpreted by Hamilton and are consistent with the interpretation that the Myers (1966, p. 535, 540) as a "lava-filled tensional rift rocks continue southward beneath the Snake formed in the lee of the northwestward drifting plate of the River Plain. Furthermore, as geophysical data Idaho batholith" which supposedly is bounded on the north (Hill, 1963, p. 5808) do not support the possi- by the Osburn fault system. The batholith is visualized as bility of a "pull-apart" for about 18 mi south of drifting northwest accompanied by Basin and Range faulting the shot point, the granitic rocks south and on the east (Hamilton and Myers, 1966, p. 535). Tension southeast of the shot point can be inferred to was oblique in the northwest-trending western half of the continue southward beneath the plain for at Snake River Plain and direct in the northeast-trending half of the plain (Hamilton and Myers, 1966, p. 540). If so, a "pull- least as far as the northern edge of a large apart" more logically would occur in the eastern part of the plain rather than in the western part. The three en echelon of the Columbia Plateau. Descriptions by Malde and Powers gravity highs, however, are confined to the western half of (1962, p. 1200) and by Malde (1965, p. 255) suggest that the plain where strike-slip faulting is postulated (Hamilton these geologists visualize the western Snake River Plain and Myers, 1966, p. 540) rather than extensional thinning more or less as shown in Figure 1. The Owyhee Mountains, and normal faulting. In addition to the anomalous location of south of Marsing (Fig. 1), form a natural boundary for the the gravity highs, the Hamilton and Myers (1966) model is plain near the Idaho-Oregon line. Along the northern edge weakened by the failure of geologists to recognize strike-slip of the Snake River Plain, most workers agree on the bound- displacement along the faults of the western Snake River ary except in the area that is east of Mountain Home (Fig. 1). Plain and, on the north, by lack of evidence for post-Miocene In areas on either side of the Snake River where topography strike-slip displacement along the St. Joe fault (R. R. Reid, provides no unequivocal border for the plain, I selected 1968, oral commun.), the Osburn fault (Hobbs and others, boundaries (Fig. 1) that exclude pre-Tertiary rocks from the 1965, p. 128), and the Hope fault (King and others, 1970, plain. p. 4).

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interpretation that models of crustal structure Bateman, P. C., and Wahrhaftig, C., 1966, Geology should show a layer of low-velocity ("gra- of the Sierra Nevada: Calif. Div. Mines and nitic") continental crust underlying most of the Geology Bull. 190, p. 107-172. western Snake River Plain and nearly all of Bonini, W. E., 1963, Gravity anomalies in Idaho: southwest Idaho. This conclusion is strength- Idaho Bur. Mines and Geology Pamph. 132, 10 ened by the large volume and widespread dis- P. Buddington, A. F., 1959, Granite emplacement with tribution of arkose in Cenozoic formations in special reference to North America: Geol. Soc. eastern Oregon, as well as by the distribution in America Bull., v. 70, p. 671-747. Cenozoic formations in eastern Oregon of con- Bushnell, K., 1967, Geology of the Rowland quad- glomerates that contain a more diverse suite of rangle, Elko County, Nevada: Nevada Bur. pre-Tertiary salic rocks than is represented Mines Bull. 67, 38 p. among the present exposures of continental Coats, R. R., Marvin, R. F., and Stern, T. W., 1965, crust in southwest Idaho. Reconnaissance of mineral ages of plutons in Elko County, Nevada, and vicinity: U.S. Geol. ACKNOWLEDGMENTS Survey Prof. Paper 525-D, p. 11-15. Compton, R. R., 1960, Contact metamorphism in The study of dikes in the Basin and Range Santa Rosa Range, Nevada: Geol. Soc. America structural province was facilitated by two grants Bull., v. 71, p. 1383-1416. from the General Research Fund of Oregon Crosthwaite, E. G., 1969, Water resources of the State University. A third grant from the same Salmon Falls Creek basin, Idaho-Nevada: U.S. fund permitted the study of pre-Tertiary rocks Geol. Survey Water-Supply Paper 1879-D, 33 near the Oregon-Nevada state line. L. R. Kittle- P- Diller, J. S., 1908, Geology of the Taylorsville re- man provided helpful information regarding gion, California: U.S. Geol. Survey Bull. 353, locations in eastern Oregon of conglomerates 128 p. that contain clasts of pre-Tertiary rocks. The Evernden, J. F., and Kistler, R. W., 1970, writer is indebted to C. W. Hulbe for com- Chronology of emplacement of Mesozoic bath- munications pertaining to the location of con- olithic complexes in California and western glomerates in the Warner Mountains, northeast Nevada: U.S. Geol. Survey Prof. Paper 623,42 California. P- Fenneman, N. M., 1931, Physiography of the western United States: New York, McGraw-Hill REFERENCES CITED Book Co., Inc., 534 p. Fenneman, N. W., and Johnson, D. W., (with Physi- ographic Committee, U.S. Geol. Survey), 1946, Albers, J. P., 1967, Belt of sigmoidal bending and Physical divisions of the United States: U.S. right-lateral faulting in the western Great Basin: Geol. Survey Map, scale 1:7,000,000. Geol. Soc. America Bull., v. 78, p. 143-156. Fleck, R. J., 1970, Age and possible origin of the Las Anderson, A. L., 1934a, Geology of the Pearl- Vegas Valley shear zone, Clark and Nye Coun- Horseshoe Bend gold belt, Idaho: Idaho Bur. ties, Nevada Geol. Soc. America, Abs. with Pro- Mines and Geology Pamph. 41, 36 p. grams (Ann. Mtg.), v. 2, no. 7, p. 333. 1934b, A preliminary report on recent block Freeman, O. W., Forrester, J. D., and Lupher, R. L., faulting in Idaho: Northwest Sci., v. 8, p. 17-28. 1945, Physiographic divisions of the Columbia 1939, Geology and ore deposits of the Atlanta intermontane province: Assoc. Am. Geogra- district, Elmore County, Idaho: Idaho Bur. phers Annals, v. 35, p. 53-75. Mines Geology Pamph. 49, 71 p. Gilluly, J., 1963, The tectonic evolution of the west- 1947, Geology and ore deposits of Boise Basin, ern United States: Geol. Soc. London Quart. Idaho: U.S. Geol. Survey Bull. 944-C, p. 119- Jour., v. 119, p. 133-174. 319. 1970, Crustal deformation in the western Atwater, T., and Menard, H. W., 1970, Magnetic United States, p. 47-73 in Johnson H., and lineations in the Northeast Pacific: Earth and Smith, B. L., eds., The megatectonics of conti- Planetary Sci. Letters, v. 7, p. 445-450. nents and oceans: New Brunswick, New Jersey, Bateman, P. C., Clark, L. D., Huber, N. K., Moore, Rutgers Univ. Press, 282 p. J. G., and Rinehart, C. D., 1963, The Sierra Goldschmidt, V. M., 1916, Ubersicht der Eruptiv- Nevada batholith: A synthesis of recent work gesteine im Kaledonischen Gebirge zwischen across the central part: U.S. Geol. Survey Prof. Stavanger und Trondhjem: Kristiania Viden- Paper 414-D, 46 p. skaps Skr., 1. Math.-Naturv. Klasse, no. 2, 140 Bateman, P. C., and Eaton, J. P., 1967, Sierra P. Nevada batholith: Science, v. 158, p. 1407- Hamilton, W., 1962, Late Cenozoic structure of 1417. west-central Idaho: Geol. Soc. America Bull., v.

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placement along fault and shear zones in the naissance geologic map of the Adel quadrangle, Death Valley-Las Vegas area, California arid Lake, Harney, and Malheur Counties, Oregon: Nevada: Geol. Soc. America Bull., v. 78, p. U.S. Geol. Survey Misc. Geol. Inv. Map 1-446. 131-142. Willden, R., 1963, General geology of the Jackson Stewart,;. H., AlbersJ. P., and Poole, F. G., 1968, 'Mountains, Humboldt County, Nevada: U.S. Summary of regional evidence for right-lateral Geol. Survey Bull. 1141-D, 65 p. displacement in the western Great Basin: Geol. 1964, Geology and mineral deposits of Hum- Soc. America Bull., v. 79, p. 1407-1414. boldt County, Nevada: Nevada Bur. Mines Taubeneck, W. H., 1967, Petrology of Cornucopia Bull. 59, 154 p. tonalite unit, Cornucopia stock, Wai Iowa Moun- Wright, L. A., and Troxel, B. W., 1967, Limitations tains, northeastern Oregon: Geol. Soc. America on right-lateral, strike-slip displacement, Death Spec. Paper 91, 56 p. Valley and Furnace Creek fault zones, Cali- 1969, Post-Oligocene dikes and dike swarms of fornia: Geol. Soc. America Bull., v. 78, p. 933- the Basin-Range structural province: Geol. Soc. 950. America Abstracts for 1968, Spec. Paper 121, p. 1970, Summary of regional evidence for right- 293. lateral displacement in the western Great Basin: 1970, Dikes of Columbia River basalt in Discussion: Geol. Soc. America Bull., v. 81, p. northeastern Oregon, western Idaho, and 2167-2174. southeastern Washington, p. 73-96 in Gilmour, Youngquist, W., and Haegele, J. R., 1956, Geologi- E. H., and Stradling, D., eds., Proceedings of cal reconnaissance of the Cassia Mountains re- the second Columbia River Basalt symposium: gion, Twin Falls and Cassia Counties, Idaho: Cheney, Wash., Eastern Washington State Col- Idaho Bur. Mines and Geology Pamph. 110, 18 lege Press, 333 p. U. S. Geological Survey, 1967, Northeast extension of Sierra Nevada granitic rocks; p. 86 in Geo- logical Survey Research: U.S. Geol. Survey MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 19, Prof. Paper 575-A, 377 p. 1970 Walker, G. W., and Repenning, C. A., 1965, Recon- REVISED MANUSCRIPT RECEIVED JANUARY 11, 1971

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