Cenozoic Block 1

Uplifts in 

East-Central Idaho and 

Southwest Montana 

By EDWARD T. RUPPEL 

GEOLOGICAL SURVEY PROFESSIONAL PAPER 1224 

An interpretation of the linear mountain ranges of 

east-central Idaho as drape-folded, basementcored block uplzfts similar to the block 

uplzjts of southwest Montana 

UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1982 

,f' 
) 

UNITED STATES DEPARTMENT OF THE INTERIOR 

Library of Congress Cataloging in Publication Data 

Ruppel, Edward Thompson, 1925-

JAMES G. WATT, Secretary 

GEOLOGICAL SURVEY 

Dallas L. Peck, Director 

Cenozoic Block Uplifts in East-Central Idaho and Southeast Montana 

(Contributions to Stratigraphy)(Geological Survey Professional Paper I 224) 

Bibliography: p. 24 

Supt.ofDocs. No.: II9. 16:1224 

I. Geology, Stratigraphic-Cenozoic. 2. Geology-Idaho. 3. Geology-Montana. 

I. Title. II. Series. III. Series: United States Geological Survey Professional Paper 1224. 

QE691 .R85 55 I .7'8'09796 80--607875 

For sale by the Branch of Distribution, U.S. Geological Survey, 

604 South Pickett Street, Alexandria, VA 22304 
CON'TENTS 

Abstract - - - - -- -- - -- - -- - - - - - - -- - -- -- -- - -- - - - - - - -- - - -- - - -- -

Introduction - -- - - - - -- - - - - - - - - -- - - - - -- -- - - --- -- - -- - - -- - -- -

Acknowledgments ----------------------------- -------

Summary of earlier studies---------------------------~ 

Regional structural trends ----- ------ ------------ - ---------

Northwest-trending faults -----------------------------

East-trending faults ----------------------------------

Northeast-trending faults ------------------ -- ---------

North-trending faults ---------------------- -- ---------

Differences in the veneer of sedimentary rocks -- - -------- ---

Folds and steep faults in the Lemhi Range and Beaverhead 

Mountains ------ -- -- ----------------- --------------

Folds in the Lemhi Range -------- ------- --- -- ------ --

Faults on the flanks of the Lemhi Range --- ---- --------

Ring fracturing in the central Lemhi Range -------- ----

Page 

1 

3 

3 

3 

5 

5 

6 

7 

7 

8 

8 

8 

9 

Folds and steep faults in the Lemhi Range and Beaverhead 

Mountains-Continued 

Folds and faults in the Beaverhead Mountains ---- -- ---

Interpretation - -- - --- - -- -- - - - ~ - - - ---- - -- - -- -- - --- -- -- -

Age of block uplifts --- --- - -------- -------------------

Relations of block uplifts in east-central Idaho and southwest 

Montana ------- - --- -- -------------------- ---------

Young fault scarps and the debate on basin-ranges in east-central Idaho -- - - - - - --- - -- - - - - - -- -- -- - -- - - - - - -- --- - -- -

Strike-slip faults --------------------------------- --------

Northeast-trending lineaments- -- -- - - -- - -- - - -- - - - - -- - - - - -- -

The relation of intrusive igneous rocks and associated mineral 

deposits to edges of uplifted blocks -------- =---------

Summary and conclusions - ----- --- - - -- --- ---------- -- ----

References cited ------- ---------------- ------ ---- --- ------

ILLUS1,RATIONS 

Page 

9 

11 

14 

15 

15 

17 

17 

19 

22 

22 

Page 

FIGURE 1. Index map of east-central Idaho and southwest Montana ---- --- ---- - -------- --- - ------ --- J ---- - -------------- - --- 2 

2. Sketch map of major steep faults in east-central Idaho and southwest Montana ------ ----------------------------- ~ 4 

3. Map showing major steep faults in the Lemhi Range and Beaverhead Mountains, east-central Ida~o and southwest Montana 10 

4. Structural cross sections of the central part of the Lemhi Range, Idaho --- --- ---- ---------------------------------- 12 

5. Sketch map of ring complex in the Lemhi Range north of Patterson, Idaho------ ----- ----- --- - --- ------- --- -------- 14 

6. Sketch map of domes and basins in southwest Montana in Late Cretaceous and early Tertiar~ time ----------------- 16 

7. Map showing the effects of Pliocene to Holocene arching and faulting north of the Snake River Plain, Montana-Idaho- 17 

8. Sketch showing the faulted course of Baby Joe Gulch, north of Leadore, Idaho - - - --- -- ------------ -------- --------- 18 

9. Map showing northeast-trending lineaments in east-central Idaho and southwest Montana----------------- - --- ------ 19 

10. Map showing distribution of Tertiary intrusive rocks and mineral deposits in east-central IdaHo ---------------------- 20 

III 

CENOZOIC BLOCK UPLIFTS IN I 

EAST-CENTRAL IDAHO AND SOUTHWEST MONTANA 

By EDWARD T . RUPPEL 

ABSTRACT 

The northwest-trending linear mountain ranges of east-central Idaho 

commonly have been considered to be basin-ranges bounded on one or 

both sides by large normal faults. Geologic studies in the Lemhi Range 

and Beaverhead Mountains suggest, however, that these ranges are 

structurally flat-topped block uplifts, flanked by monoclinal folds. The 

monoclinal folds are interpreted as drape folds, in which sedimentary 

rocks, earlier deformed by regional thrust faults, have been passively 

folded over the edges of deeper, rising blocks of crystalline basement 

rocks, although no basement rocks are exposed in these ranges. The 

normal faults that outline the present mountain ranges are thought to 

be relatively minor, secondary faults formed by gravitational collapse 

and sliding of sheets of rock from the steepened monoclinallimbs of t he 

uplifts into the adjacent valleys. The normal faults suggest extensive 

gravitational denudation of sedimentary rocks from the block uplifts, 

and the denudational sheets probably form a significant part of the fill 

in adjacent valleys. 

The basement-cored block uplifts of southwest Montana are known 

to be bounded by steep reverse faults that are partly of Precambrian 

ancestry. The thin veneer of sedimentary rocks above these blocks was 

drape folded, and the ranges largely have been denuded by gravitational sliding. The trends of the ranges in southwest Montana are 

partly similar to those in Idaho, and the Idaho ranges are inferred also 

to be controlled by steep reverse faults, which first formed in the 

Precambrian, and which bound the concealed blocks of basement 

rocks. 

The mountain ranges of both east-central Idaho and southwest Montana thus are inferred to be drape-folded, basement-cored, and 

basement-controlled block uplifts. The main difference is in the thick 

veneer of sedimentary rocks above the basement cores in Idaho, and 

the relatively thin veneer in Montana. 

The major uplift of the mountain blocks took place in Miocene time 

and was essentially vertical because the uplifts are structurally nearly 

flat on top . Most gravitational sliding and denuding of the ranges was 

contemporaneous with uplift, on linear zones of secondary normal 

faults that parallel the monoclinallimbs of the uplifts. Young normal 

faults made conspicuous by Pleistocene and Holocene fault scarps are 

not a product of the major, regional block uplifting, but rather appear 

to be minor extensional faults related to late Tertiary and Quaternary 

regional arching north of the Snake River Plain. 

The northwest- and east-trending steep reverse faults that define 

and bound the basement blocks apparently also controlled the 

emplacement of intrusive igneous rocks and related mineral deposits. 

The principal stocks and all the known major deposits of metallic 

minerals in east-central Idaho are on the monoclinallimbs of the block 

uplifts, above the reverse faults inferred at greater depth. The distribution suggests that magma rose in reverse fault zones to the base of the 

Medicine Lodge thrust system, and there the magma spread laterally 

in sheets in imbricate thrust faults. The principal mineral deposits are 

around necklike stocks, which fed the sheets, and only small mineral 

deposits, few of which have yielded any ore, are known around the 

sheets. Substantial ore bodies ~ not been found in the structurally 

flat, central parts of the block uplifts; and ore bodies are not likely to 

have been deposited in the central parts of the block uplifts because 

the seemingly necessary deep structural controls are not present there. 

INTRO~UCTION The Lemhi and Lost River Ranges in east-central 

Idaho and the Beaverhead Mountains along the IdahoMontana State line are long, northwest-trending ranges 

separated by equally long, linear intermontane valleys. 

In contrast, the ranges in southwest Montana mostly are 

short mountain masses wit~ diverse trends, nearly surrounded by broad, intercoynected basins (fig. 1). The 

different topographic pattefns of the mountain ranges 

are accompanied by changes in structural pattern from 

-central Idaho eastward intd southwest Montana, across 

the leading edge of the Cordilleran fold and thrust belt 

(fig. 2), that seem to indicate that this is a region where 

two significantly differentt structural provinces come 

together and overlap. Wes of the leading edge of the 

thrust belt the structure is ighly complex and includes 

major thrust faults, tight asymmetric folds, steep reverse 

faults, and widespread norr~al faults, but to the east the 

structural pattern is that of basement-cored block uplifts and simple draped folds. These differences 

prompted earlier division of the region into a western 

fold-thrust, or Sevier province, and an eastern foreland 

or Laramide province. 1 

The division into structu al provinces exaggerates the 

differences across the regio 1 and obscures relations that 

suggest that the western province and the eastern 

province are structurally similar. The linear, faultbounded ranges of east-central Idaho appear to be drapefolded uplifts above base

1

ment blocks of crystalline 

metamorphic rocks-similar to th.e basement-cored 

block uplifts of southwest I Montana, and most of the 

post-thrusting steep normr and reverse faults of the 
2 CENOZOIC BLOCK UPUITS, IDAHO, MONTANA 

20 40 60 KILOMETERS 

FIGURE 1.-Index map of east-central Idaho and southwest Montana. Mountain areas are stippled. 
REGIONAL STRUCTURAL TRENDS 3 

region are a result of mainly vertical differential movements on discrete basement blocks, that were, however, 

first defined in Precambrian time. The two structural 

provinces are really only one, of basement-cored block 

uplifts, and the apparent differences in the two regions 

are due mainly to a much thicker sequence of thrustfaulted sedimentary rocks, above crystalline basement 

rocks, in the western province. 

ACKNOWLEDGMENTS 

I am indebted toW. M. Cady, J. H. Dover, the late 

M. R. Klepper, D. A. Lopez, M. W. Reynolds, E. R. 

Verbeek, C. A. Wallace, and I. J. Witkind, all of the U.S. 

Geological Survey, and to Prof. Robert Scholten of Pennsylvania State University, who have helped me with information, ,discussion, and criticism, all freely given. 

SUMMARY OF EARLIER STUDIES 

The great, flat thrust faults that dominate eastcentral Idaho and much of southwest Montana also 

dominate much of the literature on the region; the steep 

normal and reverse faults that are present throughout 

the region (fig. 2) generally have not been discussed in 

much detail. The steep faults bounding the linear ranges 

of east-central Idaho were first noted by Meinzer (1924, 

p. 15) and were discussed in a little more detail a few 

years later by Kirkham (1927, p. 27-29), who considered 

them to be steep thrust faults, bounding wedge uplifts. 

Shenon (1928, p. 5) briefly described faulting in the 

Birch Creek Valley and considered this valley to be a 

graben, bounded by steep, normal faults, a view subsequently extended to the other valleys of east-central 

Idaho by A. L. Anderson (1934, p. 19-21). Anderson 

(1934, p. 26-28) also discussed the age of normal faulting 

and concluded that most faulting had taken place in 

Pliocene and Pleistocene time. Ross (1947, p. 1139) and 

Baldwin (1951), who described normal faults in the Lost 

River and Lemhi Ranges, agreed that much faulting was 

relatively recent, but they suggested that some normal 

fault movement occurred earlier, perhaps in Oligocene 

and Miocene time. Baldwin (1951, p. 885, 899) also concluded that the ranges of east-central Idaho had been 

tilted downward to the northeast, so that the intervening 

valleys are not fault-bounded structural trenches, but 

rather are sediment-filled troughs on the back slopes of 

the tilted fault blocks, a conclusion also reached by R. A. 

Anderson (1948, p. 11) and by Skipp and Hait (1977, p. 

499--509) for the Lemhi Range. 

The block uplifts of southwest Montana were discussed first by Klepper (1950, p. 73-76), who described 

them as domal uplifts first outlined in Late Cretaceous 

time and modified by later Cenozoic doming and 

Pliocene and Pleistocene normal faulting. Pardee (1950, 

p. 375-377, 385-386) described the mountain ranges of 

southwest Montana mostly as tilted fault blocks formed 

as a result of normal faulting in Pliocene and early 

Pleistocene time. 

Scholten, Keenmon, and Kupsch (1955, p. 383-389) 

recognized both normal faults and steep reverse faults in 

the Lima, Mont., region, and, like their predecessors, 

they recognized that at least the normal faults had been 

active in recent time. Scholten (1957; 1968, p. 109-113), 

following Sloss and Moritz (1951, p. 2138), divided 

southwest Montana into shelf and geosynclinal 

provinces separated by a tectonic hinge zone about in 

the southern part of the Beaverhead Mountains, and so 

first clearly suggested the separation into an eastern 

province of basement-cored uplifts and a western 

province of complex folding, thrusting, and basin-range 

faulting. 

The Ruby Range block uplift was mapped and 

described by Tysdal (1970, 1976a, b), who concluded 

that uplift of the range and folding of the sedimentary 

rocks was a result of differential vertical movements of 

discrete basement blocks (Tysdal, 1970, p. 105-119). 

Swanson (1970, p. 670, 718-719) interpreted most of the 

steep faults of southwest Montana as reverse faults 

reflecting vertical movements along ancient shear zones, 

and he attributed much of the folding in this region to 

draping over uplifted, fault-bounded, rigid basement 

blocks of Archean crystalline rocks. 

Most recently, the separation into structural provinces 

first discussed by Scholten (1967) was reaffirmed by 

Beutner (1977, p. 353-354), who applied the names 

Sevier belt to the fold-thrust province on the west, and 

Laramide belt to the region of basement-cored uplifts on 

the east; he stated that "***the terms Laramide and 

Sevier thus become useful in describing the parallel belts 

of contrasting structural character * ** ." 

REGIONAL STRUCTURAL TRENDS 

The steep faults of southeast Montana and eastcentral Idaho include reverse, normal, and strike-slip 

faults that have similar trends throughout the region; 

the dominant trends are near northwest, east, northeast, 

and north (fig. 2). Most of the steep faults seem to reflect 

repeated movement at different times from the 

Precambrian to the near present, and some suggest different kinds of movement at different times. The major 

trends, however, appear to have been established in the 

Archean crystalline rocks before deposition of any of the 

younger sedimentary rocks (Vitaliano and Cordua, 1979, 

p. 6). In southwest Montana, the steep faults reflect 
4 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

., 

20 40 60 KILOMETERS 

FIGURE 2 (above and facing page) .-Sketch map of major steep faults in east-central Idaho and southwest Montana. 
REGIONAL STRUCTURAL TRENDS 5 

EXPLANATION 

1. Willow Creek fault zone 

2. Camp Creek fault 

3. Spanish Peaks fault zone 

4. Gravelly Range thrust zone 

5. Snowcrest-Greenhorn thrust 

6. Centennial fault 

7. Red Rock fault 

8. Horse Prairie fault 

9. Miner Lake-Beaverhead Divide 

fault zone 

---,--- High-angle fault, bar on downthrown side 

Leading edge of Cordilleran fold and thrust belt. 

Sawteeth on upper plate 

....A__....t::...... East margin of Grasshopper thrust plate. Sawteeth on 

upper plate 

.t;,L:;.,.t;,L:;.,L:.£:.. East margin of Medicine Lodge thrust plate. Sawteettton 

upper plate 

Mountain areas 

recurrent movement of Precambrian northwest- and 

east-trending faults, or later development of faults about 

parallel to Pre cam brian northeast-trending foliation or 

faults and north-trending folds and shear zones. 

The structural trends throughout the region are essentially the same as those of central and northwest Wyoming, the type "Wyoming Province" of block uplifts, 

and partly they are a direct extension of the Beartooth 

and Grayling Creek block uplifts into southwest Montana (Foose and others, 1961, p. 1162-1163: Prucha and 

others, 1965; Becraft and others, 1966, p. B8-B10; Ruppel, 1972, p. A49-A50). 

NORTHWEST-TRENDING FAULTS 

The clearest evidence for the antiquity of structural 

trends is in the northwest- and east-trending faults that 

cut Precambrian crystalline rocks in southwest Montana. Northwest-trending faults in the Ruby Range contain diabase dikes of Precambrian age; the earliest 

movement on the faults was left-lateral strike slip, which 

was followed by intrusion of the diabase dikes, and by 

later reverse and normal fault movements in Late 

Cretaceous and Cenozoic time (Mann, 1954, p. 50; 

Heinrich, 1960, p. 16-17; Tysdal, 1970, p. 103-111; 

Garihan, 1973, p. 164-165; 1976, p. 19; Vitaliano and 

Cordua, 1979; Schmidt and Garihan, 1979, p. 301). Late 

Precambrian diabase dikes in the Ruby Range and in the 

southern part of the adjacent Tobacco Root Mountains 

have yielded Rb-Sr intrusive ages of about 1,450 m.y. 

(million years) and 1,120 m.y. (Wooden and others, 1978, 

p. 474). Diabase dikes are not known to intrude most of 

the other northwest-trending faults of the region, but the 

structural trend, reflected in both reverse and normal 

faults is pronounced, and these faults have been the sites 

of some of the most ancient and most recent movements. 

These faults include the great Spanish Peaks reverse 

fault zone in the Madison Range (Iddings, 1904, p. 99; 

Becraft and others, 1966, p. 89), and widely distributed 

subparallel faults that break the crystalline rocks and 

younger sedimentary rocks in the Tobacco Root Mountains (Reid, 1957, p. 17-18; Levandowski, 1956, p. 

184-192; .Vitaliano and Cordua, 1979), Ruby Range 

(Heinrich, 1960, p. 17; Tysdal, 1970, p. 105-111; 

Garihan, 1976), Blacktail Mountains (Klepper, 1950, p . 

73-74), and Highland Mountains (M. R. Klepper, written commun., 1976; Smedes, 1968). Northwest-trending 

reverse and normal faults are common in the Tendoy 

Mountains and in the valley of Medicine Lodge Creek 

(Scholten and others, 1955, p. 383-389), and, in eastcentral Idaho, normal faults, partly accompanied by 

young fault scarps, flank northwest-trending structural 

trenches. Farther north, on the crest and west side of the 

Beaverhead Mountains near Salmon, Idaho, the 

northwest-trending Miner Lake-Beaverhead Divide 

fault zone is the westernmost major reverse fault zone 

known to be exposed in this region (MacKenzie, 1949; 

Tucker, 1975, p. 136-146; Ruppel, 1978, p. 8), and the 

principal fault zone in east-central Idaho known to have 

first formed in Precambrian time. Other reverse faults 

with similar trends but small displacements have been 

described in and near Salmon River Mountains farther 

west (A. L. Anderson, 1956, 1959). 

EAST-TRENDING FAULTS 

The best known east-trending fault zone is the Willow 

Creek zone (fig. 2), which was named and described by 

Robinson (1963, p. 103) as a fault that formed in pre- and 

intra-Lahood Formation (Proterozoic Y) time at the 

north end of the Tobacco Root Mountains. McMannis 

(1963, p. 416-417), Robinson (1963, p. 107), and 

Schmidt, Brumbaugh, and Hendrix, (1977, p. 760) 

described other faults along both east and west extensions of the Willow Creek zone. McMannis and Robinson 

also discussed the evidence for movement on extensions 

of the Willow Creek zone before and during deposition of 

the Belt Supergroup in Proterozoic Y time, for continued 

movement after deposition of the Belt Supergroup but 

before deposition of the Middle Cambrian Flathead 

Sandstone, and for lateral movement in Late Cretaceous 

and early Tertiary time (see also Harrison, 1972, p. 

1216-1218; Harrison, Griggs, and Wells, 1974). Me-
6 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

Mannis (1963, p. 423-424) described the east-trending 

Camp Creek fault, in the southwest part of the Highland 

Mountains, and he suggested that it has had a history of 

Precambrian and later movement similar to that on the 

Willow Creek zone. The Horse Prairie fault zone (Ruppel 

and Lopez, in press), farther south (fig.2), formed the 

south end of the Belt seaway and is similar to both the 

Willow Creek and Camp Creek faults. The latest movements on the Horse Prairie fault zone disrupted the 

courses of present streams, and reintegration of drainage 

across the fault is not complete. All these major eastwest faults controlled southern depositional limits of 

Belt Supergroup or equivalent rocks in part or all of 

Proterozoic Y time, and they were reactivated as strikeslip faults in Late Cretaceous and early Tertiary time. At 

least the Horse Prairie fault zone seems to have moved 

recurrently through Cenozoic time to the present. 

Two east-trending faults north of Bannack, Mont., 

(fig. 2) are only partly mapped and are not well known. 

The southernmost of these has been mapped in Badger 

Pass, northwest of Bannack (Lowell, 1965), and continues from there westward across the south end of the 

Pioneer Mountains to form the north margin of the basin 

terminated on the south by the Horse Prairie fault. The 

fault appears to cut Tertiary rocks and younger deposits 

east of Badger Pass, but it has not been mapped in that 

area; it may extend at least as far east as Dillon, Mont. 

The northern fault is marked by fault scarps on the Big 

Hole Divide, about 15 km east of Jackson, Mont., and 

seemingly extends west from there to control the upper 

end of the Big Hole basin. Both of these faults cutTertiary and Quaternary rocks and deposits, as well as older 

rocks, and partly bound major basins filled with Tertiary 

rocks. They seem likely to be old faults that have moved 

recurrently, like other east-trending faults in southwest 

Montana. 

The Centennial fault, flanking the north side of the 

Centennial Mountains, is the only other major, easttrending fault known in southwest Montana (fig. 2). The 

fault is near the southern margin of cratonic Precambrian crystalline rocks in southwest Montana, and in 

this respect, as well as in its trend, it resembles other 

east-trending faults of the region. Unlike the other easttrending faults, however, the Centennial fault has been 

interpreted to be a normal fault, downthrown on the 

north in Cenozoic time (Pardee, 1950, p. 374-376: 

Witkind, 1977, p. 534); an explanation for its divergent 

trend in a region thought to be characterized by 

northwest- and northeast-trending normal faults has 

remained elusive. Perhaps that explanation is to be 

found by grouping the Centennial fault with the other 

east-trending faults of the region and by interpreting it 

to be an ancient east-trending fault zone, which controlled the southern margin of the craton in southwest 

Montana and which has had a history of recurrent movement from Precambrian time through the Cenozoic like 

the histories of recurrent movement on the Willow Creek 

fault zone, the Camp Creek fault, and the Horse Prairie 

fault zone. 

NORTHEAST-TRENDING FAULTS 

Northeast-trending faults are not so clearly of 

Precambrian origin as the northwest- and east-trending 

faults. The northeast-trending faults described in and 

near the Ruby Range (fig. 2) are considered to be normal 

faults that moved recurrently in Tertiary and Quaternary time (Dorr and Wheeler, 1964, p. 328-335; Tysdal, 

1970, p. 119-123; Garihan, 1976, p. 24-25). The northeast-trending Snowcrest fault, southeast of the Ruby 

Range, was mapped as a moderately steep, northwestdipping thrust of early Tertiary age by Klepper (1950, 

p. 74-75). Hadley (1969) and Mann (1954, p. 52- 53) 

mapped the Greenhorn thrust, north of the Snowcrest 

fault, as a northwest-dipping thrust, and part of its map 

trace suggests that it is steeper than shown in cross section. Mann (1954, p. 55-58) also described the Gravelly 

Range thrust zone, which includes several northeast- to 

north-trending reverse faults that dip 70°-75° west, and 

he considered both the Greenhorn and Gravelly Range 

faults to be Laramide in age. In the Tobacco Root Mountains, Vitaliano and Cordua (1979) mapped and 

described northeast-trending faults that they concluded 

are of probable Precambrian age but which also have 

moved recurrently in post-Precambrian time. Swanson 

(1970, b. 670, 718-719) interpreted all the northeasttrending faults of this region as high-angle reverse faults 

reflecting vertical movements along ancient shear zones. 

The available evidence from mapping in the Snowcrest and Gravelly Ranges suggests that the northeasttrending faults there are probably steep reverse faults 

that locally flatten in their upper parts. (See Mann, 

1954, p. 58; Prucha and others, 1965, p. 969-973.) The 

parallel faults that flank the Ruby Range seem most 

likely also to be steep reverse faults, exposed at a deeper 

structutallevel, along which there might have been some 

later n~rmal fault movement, especially along the west 

flank of the range. The northeast-trending faults in the 

Ruby Range both cut and are cut by the northwesttrending faults, a relation that suggests only that Late 

Cretaceous and younger movements on northeast and 

northwest faults have been nearly contemporaneous. 

The Precambr.ian origin of northeast-trending faults 

and sh~ar zones, inferred by Swanson (1970) and suggested by Vitaliano and Cordua (1979), is not clearly 

demonstrated by the geologic maps and descriptions of 

the region. A prominent northeast trend, parallel to the 

faults, is evident in the foliation and bedding of the 
DIFFERENCES IN THE VENEER OF SEDIMENTARY ROCKS 7 

crystalline metamorphic rocks in both the Ruby Range 

and Tobacco Root Mountains (Levandowski, 1956, p. 

177-180; Heinrich, 1960, p. 16; Reid, 1963, p. 297; 

Burger, 1969, p. 1334-1340), but no northeast-trending 

faults demonstrably of Precambrian age are known. The 

available evidence shows that the faults moved in Late 

Cretaceous time, along lines subparallel to Precambrian 

foliation and further segmented blocks already defined 

and bordered by older northwest- and east-trending 

faults. But the evidence does not show conclusively that 

the faults are controlled by the foliation or by older 

faults. (See Prucha and others, 1965, p. 989-992.) 

NORTH-TRENDING FAULTS 

The north-south structural trend is not as conspicuously developed as the other trends, although 

north-trending strike-slip faults are common in the 

western part of the region, and a few north-trending 

faults cut crystalline rocks farther east along the west 

side of the Tobacco Root Mountains and on the east side 

of the Ruby River basin (fig. 2) (Reid, 1957, p. 19; Dorr 

and Wheeler, 1964, p. 330). All these faults appear to be 

of Tertiary and Quaternary age. 

Reid (1957, p. 19; 1963, p. 299--301) described northtrending open folds, of post-metamorphic Precambrian 

age, and north-trending shear zones of Tertiary age in 

the northern part of the Tobacco Root Mountains; and 

Burger ( 1969, p. 1329) described north-trending open 

folds and joints thought to be of Precambrian age in the 

southern part of the Tobacco Roots. 

In summary, most of the steep faults in southwest 

Montana and east-central Idaho are parallel to Precambrian structural trends developed during or shortly 

after metamorphism in the older Precambrian crystalline basement rocks of this region, but only northwestand east-trending faults clearly reflect recurrent movement along Precambrian faults. Basement control of 

structural features is evident in ·the basement-cored, 

fault-bounded, segmented block uplifts of southwest 

Montana. The mountain ranges farther west are linear 

rather than segmented. The structural trends are the 

same as in the eastern ranges, but basement rocks are 

not exposed, and basement controls can only be inferred. 

DIFFERENCES IN THE VENEER OF 

SEDIMENTARY ROCKS 

The sedimentary rocks east of the leading edge of the 

Cordilleran fold and thrust belt in southwest Montana 

(fig. 2) are very different from those to the west in sequence, age, lithology, and thickness; these differences 

prompted the division of the southwest Montana and 

east-central Idaho region into shelf and geosynclinal 

stratigraphic and tectonic provinces. More recent 

studies have shown that different stratigraphic sequences are present on the craton-east of the thrust 

belt, and in different parts of the thrust belt (Ruppel, 

1978; Skipp and Hait, 1977; Ruppel and Lopez, in 

press)-and that the rocks formerly included in the 

geosynclinal province are indeed geosynclinal, but have 

been transported far east of their depositional region by 

movement in major thrust plates. The rocks of the region 

have been described in many reports, and only a little 

needs to be said about them here, to emphasize the differences between the eastern and western sequences and 

to stress the intense imbricate thrust faulting in the 

western sequence-because the great thickness and complex imbrication of the western rocks were significant 

factors influencing the expression of mid-Cenozoic 

tectonism. 

The sedimentary rocks of the cratonic sequence are at 

most about 3,300 m thick. The basal sedimentary rocks 

are of Cambrian age, overlying Precambrian crystalline 

basement rocks. Paleozoic rocks above the basal Cambrian sandstone are mainly limestone and dolomite interrupted only by a few thin shale formations and interbedded sandstones, in all about 1,600 m thick, and 

these are overlain by sandstone and shale that is mostly 

of Cretaceous age. The cratonic sedimentary rocks are 

drape folded and are cut by steep faults, but the structure is relatively simple. 

Sedimentary rocks in the linear mountain ranges in 

east-central Idaho are in two layers, separated by the 

Medicine Lodge decollement (fig. 2): a lower layer, 

relatively undeformed, of micaceous quartzite and siltstone of the Proterozoic Y Yellowjacket Formation, 

probably more than 6,500 m thick; and an upper layer, 

complexly folded and thrust faulted, of younger but still 

Precambrian quartzitic rocks and Paleozoic quartzite, 

dolomite and limestone, in all more than 15,000 m thick. 

The rocks of the upper layer are relatively massive, but 

the thick units are laced with anastomosing imbricate 

thrust faults, and about half of the rocks are in thrustbroken, overturned limbs of nearly isoclinal asymmetric 

folds; these 'rocks after thrusting, but before high-angle 

faulting, probably are described best as an immensely 

thick sequence of imbricate thrust slices that mostly are 

made up of strongly fractured to shattered quartzite, 

limestone, and dolomite. The Medicine Lodge decollement, between the layers, is as much as 300 m thick. 

Crystalline basement rocks are not exposed in these 

ranges, except possibly in imbricate thrust slices in the 

Beaverhead Mountains (Ruppel, 1978, p. 18), and in two 

large boulders of schist, e~ch several meters in diameter, 

that were inclusions in the Big Timber stock in the 

central part of the Lemhi Range (see fig. 10) (Ruppel and 

Lopez, 1981.) 
8 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

Sedimentary rocks in the Grasshopper thrust plate 

and in the zone of imbricate thrust faults at the east edge 

of the thrust belt resemble those of the craton in some 

respects; but they differ in thickness and partly in composition, and the rocks of the Grasshopper thrust plate 

include Proterozoic Y sandstone and quartzite of the 

Belt Supergroup at least 6,000 m thick. (See Ruppel, 

1978; Ruppel and Lopez, in press.) 

FOLDS AND STEEP FAULTS IN THE 

LEMHI RANGE AND 

BEAVERHEAD MOUNTAINS 

The fault-bounded, northwest-trending linear mountain ranges of east-central Idaho include the Lost River 

and Lemhi Ranges and the Beaverhead Mountains (fig. 

3) and have long been interpreted as basin ranges 

(Meinzer, 1924). The ranges are separated by broad, 

open valleys, which are structural trenches that carry 

divided northwest and southeast drainages as a result of 

arching in late Cenozoic time (Ruppel, 1967). Gravity 

studies in the Lemhi-Birch Creek trench show that it is 

extremely deep-about 3,000 m in its deepest part and 

the deepest structural trench in eastern Idaho 

(Kinoshita and others, 1969). The Lemhi Range rises an 

additional 1,000 m or more above the present valley 

floor, and the Beaverhead Mountains are only a little 

lower, so the present structural relief from mountain 

crest to valley bottom, beneath the fill Tertiary rocks, is 

about 4,000 m (Kinoshita and others, 1969; Ruppel, 

1964, p. C16) . The Pahsimeroi-Little Lost River trench 

appears, on the basis of gravity data, to be only about 

half as deep as the Lemhi-Birch Creek trench (Kinoshita 

and others, 1969), and the total structural relief is about 

2,500-3,000 m. 

FOLDS IN THE LEMHI RANGE 

The central part of the Lemhi Range has been 

described as an anticlinal fold having a complexly folded 

and thrust-faulted, steeply dipping eastern limb 

(Umpleby, 1913, p. 49; Hait, 1965, p. 164-181), but more 

complete mapping indicates that this part of the range is 

a broad, northwest-trending, nearly flat-topped uplift 

flanked by monoclinallimbs that dip steeply into the adjacent basins (Ruppel, 1968, 1980; Ruppel and Lopez, 

1981). The rocks of the range are complexly folded, but 

this folding is related to thrust-faulting, which predates 

uplift of the range, and the thrusts themselves are not 

folded in the axial part of the range. The multiple 

imbricate thrusts in the range dip 5°-15° southwest 

almost everywhere in the range except on its flanks and 

dip into the Medicine Lodge decollement, which is 

nearly flat (Ruppel, 1978; Ruppel and Lopez, 1981; Ruppel and Lopez, in press). Also, a mountaintop remnant 

of Tertiary boulder conglomerate at the head of Sawmill 

Canyon, deposited in what is now the core of the range 

on deewly eroded, thrust-faulted rocks, remains essentially horizontal, neither folded nor tilted (Ruppel and 

Lopez, 1981). The evidence of the nearly flat thrust surfaces and the horizontal boulder conglomerate indicates 

that the axial part of the Lemhi Range is structurally 

flat, and suggests that uplift of the range was essentially 

vertical. 

The monoclinal folding on the flanks of the range commonly is both abrupt d steep. On the west flank of the 

range, the structure i 1 most places is similar to that near 

Patterson (Ruppel, 980) (fig. 4, sec. A-A') where 

imbricate thrust sliJces and thrust faults abruptly 

steepen westward at he crest of the valley wall and dip 

west almost as steep y as the normal faults along the 

west flank of the ran e. On the east flank of the Lemhi 

Range, the dips of i bricate slices and thrust faults, of 

rocks in the imbrie at . slices, and of the Medicine Lodge 

decollement change bruptly to the east about at the 

crest of the valley waLl, and in places the dips are nearly 

vertical (fig. 4, sees. -B' to F-F'). At Gunsight Peak 

(fig. 4, sec. B-B'), im ricate thrust faults that are flat or 

gently west-dipping f rther west are abruptly folded and 

dip 20°-30° east (R ppel, 1980). From Gilmore to the 

south end of the rang · almost all the thrust faults, thrust 

slices, and the rocks in them on the east flank of the 

range dip eastward, n places as steeply as 80° (fig. 4, 

sees. C-C' to E-E') ( uppel and Lopez, 1981; Hait, 1965; 

Beutner, 1968; Ross, 1961). 

FAULTS 0 THE FLANKS OF THE 

MHI RANGE 

The faults that cut he flanks of the central part of the 

Lemhi Range are ste p normal and reverse faults. At 

Patterson Creek (fig. 4, sec. A-A'), and most other 

places, he west flank of the range is bounded by steeply 

west-dipping normal faults that cut even the youngest 

surficia] deposits and are marked by conspicuous fault 

scarps. The folded thrust faults are nearly as steep as the 

range-front faults and could have provided slip surfaces 

appropriate for normal fault movement similar to that 

described by Beutner (1972), who discussed gravitational sliding of older thrust slices from the southern 

part of the Lemhi Range, westward into the Little Lost 

River Valley, on thrust surfaces folded by uplift of the 

range. On the east flank of the range, at Gunsight Peak, 

steep north-northwest-trending, east-dipping normal 

faults bound the range front (fig. 4, sec. B-B'). From 

Gilmore to south of Spring Mountain Canyon (fig. 4, 
FOLDS AND STEEP FAULTS 9 

sees. C-C' to E-E') the complex east-dipping monoclinal 

fold is broken by steep, east-dipping, north-northwestto northwest-trending normal and reverse faults and by 

vertical, north-trending strike-slip faults (Ruppel and 

Lopez, 1981). The largest normal fault near Gilmore is 

4-5 km east of the mountain front and is separated from 

the mountain front by a pediment (Ruppel and others, 

1970, p. 17). Range-front normal faults, broken by northtrending vertical faults that probably are mostly strikeslip, bound the range everywhere to the northwest (fig. 

3), but none have been mapped farther south, even 

though the precipitous east front of the range suggests 

that it probably is fault-controlled (Ross, 1961; Hait, 

1965; Beutner, 1968). 

RING FRACTURING IN THE 

CENTRAL LEMHI RANGE 

A number of curving faults in the north-central part of 

the Lemhi Range, north of Patterson, Idaho (Ruppel, 

1980), seem to differ from the more linear faults that 

flank the Lemhi Range. The curving faults are only 

partly interconnected, but taken together they outline a 

semicircular or semioval area about 10 km in greatest 

radius, centered on the west side of the range (fig. 5). 

The displacement on the faults where it can be determined, most commonly is down on the inside of the 

semicircular area. The faults are concealed in places by 

Quaternary surficial deposits and appear to be older 

than other steep faults in the range, but younger than 

the thrust faults, which they cut. 

The semicircular area of curving faults includes nested 

within it two other semicircles that are strikingly outlined by deep, curving glaciated canyons. The outermost 

of the topographic rings coincides or merges with the 

fault-controlled semicircle on the south, near Patterson, 

and also on its northern edge in Morse Creek valley, and 

elsewhere it is at least partly fault-controlled. The innermost topographic ring is not so obviously fault-controlled but instead probably reflects erosion controlled 

by fracturing. The nested rings thus defined by curving 

faults, inferred fracturing, and topography, are also sites 

of igneous intrusions (fig. 5). All the small Tertiary 

granitic stocks of the area are in the outer rings, including the concealed stock at the lma mine near Patterson with its associated deposits of tungsten, 

molybdenum, copper, and silver minerals. The innermost ring is intruded by andesitic and basaltic dikes 

related to the Challis Volcanics, as are the outer rings in 

a few places. The association of Challis dikes and the 

curving structures suggests that this complex of nested 

rings is related to Challis volcanism-the remnant of a 

Challis caldera from which the extrusive volcanic rocks 

have been completely removed, so that only the curving 

faults, reflecting collapse above a magma chamber, remain. Only the eastern half of the caldera is exposed. If 

the missing western half is restored, the caldera center is 

about on the southwest flank of the Lemhi Range, 8-10 

km north of Patterson. The oval shape of the caldera 

seems likely to be the result of later deformation during 

mid-Tertiary block uplift. (See p. 11.) 

The Withington Creek caldera (D. A. Lopez, oral commun., 1979) occupies a similar position on the northeast 

flank of the Lemhi Range, about 15 km south of Salmon, 

Idaho. This small caldera, about 10 km in maximum 

diameter, is marked by massive Challis ash-flow tuffs at 

least 600 m thick within the caldera, in contact across 

curving faults with only 50-100 m of thinly layered ashflow tuff and andesitic flows outside the caldera. The 

eastern part of the caldera is cut by northwest-trending 

faults along the range front. 

FOLDS AND FAULTS IN THE 

BEAVERHEAD MOUNTAINS 

The structure of the Beaverhead Mountains is similar 

to that of the Lemhi Range in most respects, and the 

Beaverheads, like the Lemhi Range, appear to be a flattopped uplift flanked by monoclinally folded limbs that 

are cut by steep faults (fig. 3). The southern part of the 

range has been described by Scholten and Ramspott 

(1968, p. 27-40) and Lucchitta (1966, p. 92-131) as an 

elongate domal uplift, flanked by steep reverse faults on 

the east and by younger normal faults on both sides. The 

axial part of the range is underlain by nearly flat, preuplift thrust faults that steepen on the range flanks, in 

places to as much as 80°. The central part of the range, 

lying east of the communities of Leadore and Tendoy, 

Idaho, is much the same as the southern part, but the 

central part is broken by north-trending strike-slip faults 

(Ruppel, 1964, 1968). On the top of Grizzly Hill, north of 

Leadore, imbricate thrust slices dip gently south and 

southwest at angles of 10° or less; at the crest of the 

Lemhi valley wall, they steepen to 35°, the same dip as 

that of the normal fault system in the east-trending segment of the mountain front north of Leadore (Ruppel, 

1968; Staatz, 1973). The eastern edge of the central 

Beaverhead Mountains seems to be bounded by a 

northwest-trending, steeply east-dipping fault zone, but 

it has not been mapped. 

In the northern part of the Beaverhead Mountains, the 

Medicine Lodge thrust plate is widely preserved and 

shows the same structural relations in the mountain 

block as farther south-and as in the Lemhi Range. The 

Medicine Lodge decollement and overlying imbricate 

thrust slices are nearly flat for many tens of kilometers 
10 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

25 

15 

MONT 

IDAHO 

50 KILOMETERS 

30 MILES 

Base from U.S. Geological Survey 

topographic maps of Montana 

and Idaho, scale 1:500,000 

FIGURE 3 (above and facing page).-Major steep faults in the Lemhi Range and Beaverhead Mountains, east-central Idaho 

and southwest Montana. 
FOLDS AND STEEP FAULTS 11 

EXPLANATION 

Fault, steeply dipping, bar on downthrown side, although a 

few faults have late movement in opposite direction. 

Dashed where inferred 

• • • Reverse fault, sawteeth on upthrown block. Dashed where 

inferred 

Caldera margin or ring-fracture zone 

A--A' Line of section shown in figure 4 

11 

lWOO' 

113°00' 

INDEX OF PRINCIPAL SOURCES 

Geology modified from: 

1. David A. Lopez (oral commun _, 1979) 

2. Tucker (1975) 

3. Staatz (1972, 1973) 

4. Ruppel (1980) 

5. Ruppel (1968) 

6. Lucchitta (1966) 

7. Scholten and others (1955), Scholten and Ramspott (1968) 

8. Ruppel and Lopez ( 1981 ) 

9. Mapel and Shropshire (1973) 

10. Skipp and Hait (1977), Beutner (1968) 

11. Ruppel (unpublished mapping) 

northward along the top of the range and dip more 

steeply on the flanks of the range, which are broken by 

steep normal faults on both sides of the range (Tucker, 

1975, p. 168-172; Coppinger, 1974, p. 181-185; Ruppel 

and Lopez, in press). East of Salmon, Idaho, the 

northwest-trending Miner Lake- Beaverhead Divide 

zone of reverse faults, dipping 70° or more west, parallels 

the range crest (fig. 3) (Tucker, 1975, p. 136---146; 

MacKenzie, 1949, p. 34-36; Ruppel, 1978, p. 8) and 

places allochthonous rocks of the Medicine Lodge and 

Grasshopper thrust plates on the east against autochthonous rocks of the Yellowjacket Formation on the 

west. However, the fault zone originated in the 

Precambrian as a normal fault, and only the Cenozoic 

movement is reverse (Ruppel and Lopez, in press). Thin 

erosional remnants of the Medicine Lodge plate overlie 

the Yellowjacket Formation on the west side of the 

range, and dip westward as much as 25°. The east side of 

the Beaverhead Mountains in this region has not been 

mapped, but some imbricate thrust slices high in the 

range are known to dip eastward, commonly at 10° or so. 

INTERPRETATION 

The folds and steep faults in the Lemhi Range and 

Beaverhead Mountains have been attributed in earlier 

reports to episodes of compression and extension in the 

upper crust, and the regional structural elements have 

been thought of as more or less simple, broad anticlines 

and synclines, broken by basin-range normal faults and 

by a few steep reverse faults whose origin remained 

enigmatic. But the "anticlines" that form the ranges actually are flat-topped and have monoclinal flanks that 

are broken by normal faults almost everywhere and by 

known steep reverse faults in a few places (fig. 4, sec. 

F-F'). The tilted limbs of the monclinal folds commonly 

dip 20°-60° into the adjacent valleys but in some places 

are nearly vertical. The structural configuration suggests 

that the linear mountain ranges of east-central Idaho are 

complex block uplifts in which the earlier thrust-faulted 

sedimentary rocks of the region have been folded, more 

or less passively, over vertically rising basement blocks. 

The structurally flat, central parts of the mountain 

blocks are thus interpreted as being above the central 

parts of uplifted basement blocks. The monoclinal folds 

are interpreted as drape folds over the edges of rising 

basement blocks, a folding process facilitated by the 

multiple slip surfaces in imbricate thrust faults and by 

the thick Medicine Lodge decollement, as well as by slip 

between beds and within shaly formations. (See Stearns, 

1978, p. 22-30.) The few major reverse faults of the 

region are interpreted as shallow extensions of reverse 

faults bounding the basement blocks, that continue into 

the overlying veneer of sedimentary rocks. These faults 

are known (fig. 3) along the east flank of the southern 

Beaverhead Mountains, where the sedimentary veneer is 

thin; in the Miner Lake-Beaverhead Divide reverse fault 

zone, in the northern part of the Beaverhead Mountains, 

the faults are known where Cenozoic reverse movement 

occurred along a Precambrian fault zone; and the reverse 

faults are known south of Gilmore , some of which appear 

to be local features, perhaps formed by local stresses during folding (fig. 4, sec. C-C') (Prucha and others, 1965, p. 

991). 

The Patterson ring-fracture complex and the Withington Creek caldera are both on the edges of the Lemhi 

Range in positions that suggest that basement fractures 

could have controlled the location and development of 

these Challis calderas, before major block uplift. The 
12 

A 

West 

METERS 

3500 

3000 

2500 

2000 

c 

West 

METERS 

3500 

3000 

2500 

2000 

1500 

B 

West 

METERS 

3500 

3000 

2500 

2000 

D 

West 

METERS 

3500 

3000 

2500 

2000 

1500 

CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

B' 

East 

Geology modified from 

Hait (1965, pl. 2, sec. K-K') 

4 5 KILOMETERS 

FEET 

12,000 

10,000 

8000 

6000 

C' 

East 

D' 

A' 

East 

FEET 

12,000 

10,000 

8000 

6000 

FEET 

12,000 

10,000 

8000 

6000 

East 

FEET 

12,000 

10,000 

8000 

6000 

FIGURE 4.-Structural cross sections of the central part of the Lemhi Range, Idaho. A -A', across the west-dipping monoclinal west flank 

the Gilmore area: C-C', Long Canyon; D-D', Spring Mountain Canyon; E-E', Warm Creek to Coal 
F 

E 

West 

METERS 

3500 

3000 

2500 

2000 

1500 

FOLDS AND STEEP FAULTS 

E' 

East 

FEET 

12,000 

10,000 

8000 

6000 

----------------------------------------------------------------------------------~4000 

Geology modified from Hait (1965, pl. 2, sec. 1-1') 

3 4 KILOMETERS 

Lemhi Range F' 

X X )( X 

)( )( )( X 

: : :~: : : x 

X Xr x X X ~: X X X :y: X 

~~~~l~\:x: x ~ X X X X 

X X 

X )( )( )( X 

Dip of reverse faults between basement 

blocks is unknown, and is shown as 

vertical only to suggest that the faults 

must be ve 

10 

SCALES APPROXIMATE 

EXPLANATION 

Tuff and gravel in valley fill of Tertiary age 

Paleozoic sedimentary rocks in Medicine Lodge thrust plate 

Proterozoic sedimentary rocks in Medicine Lodge thrust 

plate 

Paleozoic and Precambrian sedimentary rocks in imbricate 

thrust slices in Medicine Lodge thrust plate (sec. FF 'only) 

Proterozoic sedimentary rocks of Yellowjacket Formation, 

beneath Medicine Lodge thrust plate 

1/ 

1/ 

X X X 

X : r ~: X X X )( X X 

20 KILOMETERS 

Contact or bedding 

Medicine Lodge decollement 

Imbricate thrust faults in Medicine Lodge plate 

Reverse fault 

Normal fault 

C/) 

ffi 

t-

:;g 

a 

a 

a 

~ 

13 

~ 

~ Granodiorite and quartz monzonite stocks and sheets of 

Tertiary age 

Strike-slip fault; arrows show relative displacement 

Precambrian basement crystalline metamorphic rocks 

(sec. F-F' only); arrows show inferred differential displacement of fault-bounded basement blocks Sources: Sections A-A', and 8-8' , Ruppel (1980); sections C-C', 0-D' , E-E' 

Ruppel and Lopez (1981); 0-D' , Hait (1965). 

near Patterson. B-B', across the east-dipping monoclinal east flank near Gunsight Peak. Across the east-dipping monoclinal east flank in 

Kiln Canyon. F-F', Composite schematic section across the Lemhi Range and Lemhi Valley. 
14 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

D .. .. 

10 KILOMETERS 

5 MILES 

EXPLANATION 

Ring-fracture zone 

Fault, steeply dipping. Bar and ball on downthrown side 

Zone of normal faults at mountain front. Bar and ball on 

downthrown side 

Tertiary stocks, quartz monzooite and granodiorite, except 

lma stock, which is granite 

Area intruded by dikes similar in composition to Challis 

Volcanics 

FIGURE 5.-Sketch map of ring-fracture complex in the Lemhi Range 

north of Patterson, Idaho. 

elongate, oval shape of the Patterson ring-fracture complex (fig. 5) suggests that it might have been stretched. If 

it originally was more nearly circular, as the curving 

faults and curving canyons imply, the present elongate 

shape could be a result of stretching over the edge of a 

rising block of basement crystalline rock. 

Most of the normal faults along the mountain fronts 

appear to be relatively minor faults, which outline and 

emphasize the mountain blocks, but which are secondary structures formed by gravitational collapse of the 

monoclinal shoulders of uplifted blocks into adjacent 

basins. Some gravitational sliding probably accompanied the early stages of block uplift, as older thrust 

faults were steepened by monoclinal folding along the 

range flanks, and provided readymade slip surfaces for 

gravity sliding. As uplift continued, the thrust surfaces 

were progressively steepened, and so made more unstable, and other, normal faults formed as the monoclinal shoulder was raised higher and masses of increasingly unsupported rocks failed and slid into the adjacent basins. Gravitational collapse of unsupported 

monoclinal block shoulders seems likely to have contributed significantly to denudation of the uplifted 

blocks, to stripping of earlier Tertiary Challis Volcanics 

from the ranges, and to filling of the adjacent basins. 

The young normal faulting marked by conspicuous 

scarps on the southwest sides of the block uplifts, and 

strike-slip faulting, reflect movements during regional 

warping later than the main block uplift. These movements discussed in more detail later in this report (p. 

16--17). 

In summary, the linear, northwest-trending mountain 

ranges of east-central Idaho appear to be block uplifts, 

composed of drape-folded sedimentary rocks above stillconcealed blocks of basement crystalline rocks that have 

been vertically uplifted. Monoclinal drape folds, modified by secondary normal faults, form the flanks of each 

mountain range, and they more or less outline the shapes 

of the underlying basement blocks, which suggests that 

the basement blocks are bounded mainly by northwestand east-trending reverse faults. The northwest-trending 

reverse faults in a few places reach up into the drape 

folds. The region is thus inferred to be one of flat-topped 

uplifts and flat-bottomed basins, separated by 

monoclinal folds and minor, secondary normal faults; 

this is in contrast to the conclusions of some earlier 

reports that the ranges owe their origin to basin-range 

normal faulting. The youngest normal faults and strikeslip faults suggest a small amount of block tilting related 

to late regional warping, rather than to the block uplifts 

that formed the ranges. 

AGE OF BLOCK UPLIFTS 

The time of major block uplifts in east-central Idaho 

and in southwestern Montana west of the edge of the 

thrust belt can be fixed only within rather broad limits. 

A maximum age is indicated by the time that thrust 

faulting was completed, about 76 m.y. ago (Ruppel and 

Lopez, in press), because the thrust faults are folded 

over the uplifted blocks. A still younger age is suggested 

by relations of Challis Volcanics of Eocene age, for these 

rocks commonly are folded at the mountain fronts and 

dip steeply into the valleys, and the volcanics are broken 

by normal faults at the range fronts (Ruppel, 1968, 1980; 

Ruppel and Lopez, 1981). 

A late Tertiary, perhaps Miocene age for the major uplift in east-central Idaho seems most likely. A. L. 

Anderson (1961, p. 71) concluded that uplift of the 

ranges near Salmon, Idaho, might have begun in very 

late Oligocene time and continued through the Miocene, 
YOUNG FAULT SCARPS AND THE DEBATE ON BASIN-RANGES 15 

with relatively little uplift since then. Major midMiocene uplift is suggested by abundant coarse, locally 

derived conglomerate beds in mid-Miocene tuffs 

(Nichols, 1975) in the upper part of the Lemhi Valley, 

and some of the normal faults on the east side of the 

Lemhi Range have been inactive at least since the late 

Pliocene (Ruppel, 1964, p. C16; 1967, p. 657-658), suggesting major uplift from mid-Miocene to middle or late 

Pliocene. An about similar span of faulting, but mainly 

Miocene, was suggested by Scholten, Keenmon, and 

Kupsch (1955, p. 396-398) and Scholten and Ramspott 

(1968, p. 34-38, 53) for the southern Beaverhead Mountains. None of these estimates for the age of major uplift 

is definitive, and starting and ending points are not well 

known. The available evidence suggests, however, that 

most uplift took place in Miocene time, but that uplift 

may have started in late Oligocene time and may have 

continued to middle Pliocene time. 

The latest movement on northwest-trending normal 

faults in east-central Idaho, partly marked by very 

young scarps cutting all but the youngest alluvial 

deposits, is almost all along the southwest sides of the 

uplifted blocks. North-trending strike-slip faults of 

similar age occur on both sides of uplifted blocks, where 

the range-trends zigzag from northwest to east-west, and 

back to northwest. These youngest movements probably 

started in the late Pliocene and have continued almost to 

the present (Ruppel, 1964, p. C16; 1967, p. 658). 

RELATIONS OF BLOCK UPLIFTS IN 

EAST-CENTRAL IDAHO AND 

SOUTHWEST MONTANA 

The block uplifts of the cratonic region in southwest 

Montana, and those farther west in Mont~na and eastcentral Idaho, are similar in most respects. Throughout 

the region, the uplifts trend in generally similar directions and are parallel to faults and structural trends 

formed in Precambrian time. The exposed basement 

blocks in the eastern part of the region are at least partly 

bounded by Precambrian faults that moved recurrently 

in Late Cretaceous and Cenozoic time. By inference, the 

basement blocks beneath the more linear uplifts of eastcentral Idaho zalso are bounded by Precambrian faults, 

and the pattern of the ranges suggests that the dominant 

bounding faults at depth must trend northwest and 

east-the same directions as those of demonstrable 

Precambrian faults farther east. Drape folding accompanied uplift of blocks in both southwest Montana and 

east-central Idaho, as did reverse faulting and subsequent normal faulting on the mountain flanks, and as 

did gravitational sliding of rocks from the top and sides 

of the uplifts (Tysdal, 1970, p. 124-129). The time of rnajor uplift is the same throughout the region-with 

perhaps some uplift in the late Oligocene, with major uplift in the Miocene probably continuing into the 

Pliocene, and with conspicuous but relatively minor uplift or block tilting from late Pliocene to the present 

(Klepper, 1950, p. 75-76; Pardee, 1950, p. 366-367; 

Tysdal, 1970, p. 105-130; Dorr and Wheeler, 1964, p. 

328-335). 

The differences between the block uplifts seem to be 

more apparent than real. The reverse faults that commonly bound and cut the crystalline basement cores of 

the eastern uplifts are not evident farther west, where 

only a few reverse faults are known. The difference 

probably is a consequence of the difference in thickness 

of the sedimentary veneer above the basement blocks. 

Little veneer is left on the eastern block uplifts, and the 

uplifts are exposed to a much deeper structural level 

than the uplifts to the west, where the veneer, originally 

greatly thickened by major thrust plates, is still many 

thousands of meters thick and no crystalline cores are 

exposed. The basement reverse faults inferred to be present at depth, beneath the western block uplifts, probably 

die out upward in the thick sedimentary veneer (for example, Steams, 1971, p. 128-132); they are suggested, 

however, by the few known major reverse faults (figs. 3, 

4), and especially by the Miner Lake-Beaverhead Divide 

fault zone. 

The initial uplift of the eastern blocks was in Late 

Cretaceous time (Klepper, 1950, p. 74), when northeasttrending domes rose in a foreland bulge in front of the 

thrust belt (fig. 6) (Ruppel and Lopez, in press). These 

initial domes differ in trend and magnitude from the 

Tertiary block uplifts, and they differ in being closely 

related to thrust faulting. The main block uplifting, 

throughout the region, was mostly in a Miocene time and 

is distinct from the earlier, Cretaceous uplifts. 

YOUNG FAULT SCARPS AND THE 

DEBATE ON BASIN-RANGES IN 

EAST-CENTRAL IDAHO 

Brief discussions or assertions on the origin of the 

linear mountain ranges of east-central Idaho have been a 

part of almost all reports on the geology of different areas 

in the region, and a debate of sorts has limped along 

almost from the time the ranges were first described as 

products of regional uplift and erosion (Umpleby, 1913, 

p. 48-49). As earlier summarized (p. 3), Meinzer (1924, 

p. 15) first recognized normal faults on the flanks of the 

ranges, and subsequent reports by others have yielded 

almost as many interpretations as there are reports: as 

thrust-bounded wedge uplifts (Kirkham, 1927, p. 27-29); 

as tilted fault blocks (R. A. Anderson, 1948; Baldwin, 
16 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

46oJJr1

00_' -~;::-:---.::r--3T-0' ____ __:1_:_::12r

0

0:::.._0' ____ --'-C11~1 ~30:._' -------, 

Butte 

Ul 

~ 

() 

30' 0 

Ul 

:::::1 

0 

c: 

0 

~ 

~ 

() 

0 

45°00' 

<( 

0 

Craton 

25 50 KILOMETERS 

EXPLANATION 

Mesozoic rocks 

Paleozoic rocks 

Precambrian crystalline rocks 

Leading edge of thrust belt. 

Sawteeth on upper plate 

FIGURE 6.-Domes and basins in southwest Montana in Late 

Cretaceous and early Tertiary time, before major uplift of mountain 

blocks in Miocene time (modified from Scholten, 1967, fig. 3). 

1951; Skipp and Hait, 1977, p. 499, 509); as folded belts 

(Ross, 1961, p. 236; and earlier reports); as faultbounded basin-ranges (Shenon, 1928, p. 5; Ruppel, 1964, 

p. C16); and, in the present report, as drape-folded block 

uplifts flanked by minor, secondary normal faults that 

are slip surfaces of gravitational slide sheets. My conclusion, that the ranges are block uplifts, suggests that 

Kirkham (1927, p. 27-29), who proposed the wedge uplift 

interpretation, was more nearly correct than most of his 

successors, but neither the wedge uplift nor folded belt 

interpretations survived for long, because steep, normal 

faults clearly do exist in many places along the range 

fronts. The debate has centered until now on the basinrange and tilted fault-block interpretations and has been 

based almost entirely on evidence drawn from the 

southern parts of the Lemhi Range and Beaverhead 

Mountains. These are areas where young normal fault 

scarps are conspicuous on the southwest sides of the 

ranges, and the differing interpretations of the ranges are 

based largely on differing interpretations of the 

significance of the young faults. 

The debate has had several interrelated parts: (1) are 

the ranges bounded by faults on both sides, or on only 

one side; (2) what is the evidence for block tilting, and is 

it Valid; and (3) when did uplift, tilting, or both take 

place? The first two parts are closely related. The most 

obvious normal faults are those marked by young scarps 

on the southwest sides of all the linear ranges in eastcentral Idaho. No similar young faults have been 

mapped on the northeast sides of the southern Lemhi 

and Lost River Ranges, and so the ranges were interpreted to be tilted blocks. No other evidence for block 

tilting has been advanced. However, the Beaverhead 

Mountains are outlined by nearly continuous faults on 

both sides (fig. 3), as are the central and northern parts 

of the Lemhi Range. At Gilmore, on the east-central side 

of the Lemhi Range, the principal normal fault is several 

kilometers east of the mountain front; the fault is 

beveled by a pediment and is partly concealed by a 

veneer of pediment and glacial gravels (Ruppel and 

others, 1970, p. 16-17). Gravity studies (Kinoshita and 

others, 1969) show, too, that the valleys are symmetrical 

and deepest in the middle, and not deepest along the 

southwest-facing scarpline as would be true if they were 

on the back slope of an eastward-tilted fault block. And 

finally, the Medicine Lodge decollement and related 

imbricate thrust faults above it are nearly flat or dip 

gently west in the inner parts of the Lemhi Range and 

Beaverhead Mountains, but they should dip at least 

moderately and consistently to the east if the ranges 

were tilted fault blocks uplifted thousands of meters on 

the west. The available evidence thus indicates that the 

uplifts of east-central Idaho are symmetrical, two-sided, 

and not significantly tilted whether or not they are 

bounded by normal faults on both sides and suggests 

that some normal faults might be away from the mountain front, as at Gilmore, and hidden by Pliocene and 

Pleistocene gravels. 

The explanation for the young faults on the southwest 

sides of the Idaho ranges is partly to be found in the third 

element of the basin-range debate-when did uplift, 

tilting, or both take place? As earlier discussed, block 

uplift was mostly in Miocene time and was essentially 

vertical because the blocks are not significantly tilted. 

The movement that produced the young fault scarps on 

the southwest sides of the Idaho ranges probably began 

in the late Pliocene and continued into the Holocene 

(Ruppel, 1964, p. C16; 1967; Hait and Scott, 1978). 
NORTHEAST-TRENDING LINEAMENTS 17 

The young fault scarps in Idaho are paralleled in 

southwest Montana by similarly young, but mostly 

northeast-facing fault scarps (Pardee, 1980, p. 403-404; 

Scholten, 1968, p. 118-119). 

The late Pliocene to Holocene normal faulting reflected in the young fault scarps was accompanied by the rise 

of northeast-trending arches north of and parallel to the 

Snake River Plain (fig. 7) (Kirkham, 1927, p. 11; Ruppel, 

1967). The concordance in timing suggests a relation in 

origin. The best known arch, just north of the Snake 

River Plain, is an elongate, northeast-trending dome, 

that, in its uplift, divided the earlier southeast-draining 

intermontane basins of the region and reversed the 

drainage in the northern parts of the basins, so that the 

present rivers north of the arch flow to the northwest. 

(See Ruppel, 1967.) The highest part of the arch lies on 

the Montana-Idaho boundary and separates the region 

of the southwest-facing scarps in Idaho, from the region 

of northeast-facing scarps in Montana. The scarps 

mostly are at high angles to the long axis of the dome, a 

relation that suggests that they reflect extensional movements away from the crest of the dome, or, put another 

way, that they reflect stretching parallel to the long axis 

of the dome. The extension does suggest some rotation of 

-the mountain blocks, not related to the original block 

uplift, but the rotation is so slight as to be undetectable 

except in the young faults. Some indication of the 

amount of rotation is suggested by the apparent total 

displacements on young fault systems: at Patterson, on 

the west side of the Lemhi Range, the young faults have 

an aggregate displacement of about 100-150 m, judging 

from reconstruction of prefault stream courses. On 

similar evidence, the young faults north of Leadore, on 

the west side of the Beaverhead Mountains, have a total 

vertical displacement of about 60 m (fig. 8) (Ruppel, 

1964, p. C16). Reverse displacement on the fault near 

Gilmore, on the east side of the Lemhi Range, is about 

120m (Ruppel, 1964, p. C16; Ruppel and others, 1970, p. 

17) and suggests that the entire Lemhi Range block 

rotated up on the southwest and down on the northeast 

by about the same amount-a little more than 100m, in 

late Pliocene through Holocene time. 

STRIKE-SLIP FAULTS 

Vertical, north -trending strike-slip faults are widely 

distributed in east-central Idaho and the adjacent part 

of southwest Montana (fig. 3) (Ruppel, 1964), although 

just how widely is still in some dispute (Beutner, 1968, p. 

54; Skipp and Hait, 1977, p. 499). The faults are concentrated in those areas where the ranges change trend 

from northwest to east. The latest movements along the 

faults are Pliocene through Holocene, but some earlier 

movements, not clearly recognizable now, seem likely. A 

50 100 KILOMETERS 

EXPLANATION 

-,---,--r-r-1 r-1 Young fault scarps 

.. • Long axis of domal uplift 

+-----+~ Short axis of domal uplift 

Mountain area 

Intermontane valley 

FIGURE 7 .-Map showing the effects of Pliocene to Holocene arching 

and faulting north of the Snake River Plain, Montana-Idaho. 

satisfactory explanation for the faults remains elusive 

(Ruppel, 1964, p. C18; Scholten and Ramspott, 1968, p. 

47), but their locations, where block uplifts change 

trend, suggest that they are tear faults in the sedimentary veneer above different basement blocks that moved 

or tilted differentially. 

NORTHEAST-TRENDING LINEAMENTS 

East-central Idaho and southwest Montana are 

crossed by broad, northeast-trending bands of different 

rocks, separated by lineaments that coincide in places 

with known faults (fig. 9), and which also suggest basement structural controls (Hyndman and others, 1977). 

The lineaments are conspicuous on space imagery of this 
18 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

. ,, ' ~ ' 

-- __ - -~-:_;_"'-?,~ __ :::---- ~- .. -- ~ ~-

-- ----- ' 

--------

~~~~~~~~-- _. ~. C:..' (..' 

FIGURE 8.-The faulted course of Baby Joe Gulch, north of Leadore, 

Idaho, looking north. The stream course in the foreground is several 

hundred meters wide but is cut off by young faults along which uplift has taken place at the mountain front; Baby Joe Gulch is the 

narrow, V-shaped gulch entering the stream course on the right_ 

region. The southernmost lineament is the northwest 

edge of the Snake River Plain, separating the volcanic 

rocks of the plain from the mainly Paleozoic sedimentary 

rocks to the north. No faults are known along the lineament (M. L. Kuntz, oral commun., 1979). The middle 

lineament extends across the north end of the linear 

ranges of east-central Idaho into Horse Prairie in 

southwest Montana, and farther northeast along the 

southeast side of the Highland Mountains. The lineament coincides with a known steep fault in the head of 

Horse Prairie and a possible fault along the Highland 

Mountains, an area not mapped. It separates the region 

of Paleozoic and Proterozoic Y sedimentary rocks on the 

south from one to the north that is underlain mostly by 

Tertiary Challis Volcanics and older Proterozoic Y 

sedimentary rocks, but which includes younger 

Paleozoic and Mesozoic sedimentary rocks and the 

Upstream about 300 m, and behind the two sharp bends in the 

gulch, Baby Joe Gulch widens to fit the down-faulted segment of its 

original channeL The vertical displacement between the faulted 

segments of the original channel is about 60 m. Sketch by Ouida B. 

Fleenor. 

Boulder batholith at its east end in Montana. The 

northernmost lineament, described by Hyndman, 

Badley, and Rebal (1977), trends northeast through an 

area of northeast-trending faults near Gibbonsville, 

Idaho, to the northwest side of the Big Hole Basin in 

Montana, a part of the basin that is bounded by a 

northeast-trending fault. The region north of this lineament is composed dominantly of batholithic rocks. The 

lineament, extended, falls more or less on the west side 

of the Boulder batholith. 

The lineaments are crossed by many of the faults of 

this region, with no apparent effect, but nonetheless they 

seem to outline regions of somewhat different overall 

structure, as well as of different rocks. The region immediately north of the Snake River Plain encompasses 

most of the known block uplifts in east-central Idaho and 

southwest Montana, although both the Lemhi Range 
RELATION, INTRUSIVE IGNEOUS ROCKS AND MINERAL DEPOSITS TO UPLIFTED BLOCKS 19 

and Beaverhead Mountains extend beyond it to the 

northwest. Most of the known major faults farther 

northwest trend northeast, at right angles to the dominant northwest trend in the region north of the Snake 

River Plain, but northwest-trending block uplifts, not as 

obvious, probably are present as well. The northwest 

region also includes the northeast-trending dike swarms 

described by Hyndman, Badley, and Rebal (1977). 

The lineament-bounded regions seem to confin~ the 

elongate domes that rose in late Tertiary and Quaternary time (p. 17) and so seem to have influenced the late 

Cenozoic regional uplifts north of the Snake River Plain. 

The lineaments are so dimly seen, and the structure of 

some of the region so poorly known, that their origin is 

uncertain. The most prominent faults along them are in 

the oldest, and so the deepest rocks, and these oldest 

rocks also are partly covered by abundant volcanic rocks 

and are intruded by dike swarms parallel to the linea50 100 KILOMETERS 

EXPLANATION 

Lineament 

Mountain area 

Intermontane valley 

I FIGURE 9.-Northeast-trending lineaments in east-central Idaho and 

southwest Montana. 

ments. The relations suggest that the lineaments reflect 

deeply buried major fractures in the underlying crystalline basement, as Hyndman, Badley, and Rebal suggested (1977). If so, the fractures seem most likely to 

have originated in Precambrian time. 

THE RELATION OF 

INTRUSIVE IGNEOUS ROCKS AND 

ASSOCIATED MINERAL DEPOSITS 

TO EDGES OF UPLIFTED BLOCKS 

The principal known deposits of metallic minerals in 

east-central Idaho are almost all near the margins of the 

mountain ranges and so are on the monoclinal flanks of 

uplifted mountain blocks (fig. 10). The distribution of 

these mineral deposits and of associated quartz monzonite and granodioritic intrusives was discussed in an 

earlier report (Ruppel, 1978, p. 4-5; 18-20), in which I 

concluded that the Medicine Lodge thrust system had 

restricted the upward movement of intrusive magmas. 

and mineralizing solutions to the lower part of the thrust 

plate. The geologic relations of some of the intrusive 

bodies now are better known and show that some of the 

intrusive masses are not stocks, but rather that they are 

sheets intruded into thrust faults. 

In the central part of the Lemhi Range, all of the principal mineral deposits are associated with stocks on the 

mountain-block flanks, and only a few smaller deposits 

are associated with sheets of igneous rock. The relation 

of stocks, sheets, and mineral deposits is most evident in 

the Gilmore mining district (fig. 10) (Ruppel and others, 

1970; Ruppel and Lopez, 1981), for only the central part 

of the Gilmore intrusive mass seems to be truly a stock, 

and the most productive mineral deposits are in a small 

area adjacent to this part of the intrusive. The north part 

of the intrusive is a thick, short sill, intruded into the 

lower part of the Devonian Jefferson Formation, with 

only a few associated deposits of metallic minerals. The 

south part of the intrusive is a thick sheet intruded into 

one or more thrust faults that extends about 10 km 

southward from Gilmore to Spring Mountain Canyon. 

Only small deposits of metallic minerals have been 

found in the sedimentary rocks enclosing the sheet, and 

none of them have yielded appreciable amounts of ore. 

The relations of igneous rocks and associated mineral 

deposits around the Gilmore stock and its peripheral 

sheets suggest that the stock was the main intrusive 

center and the main center for mineralization. The 

sheets were fed from the stock and are fairly simple intrusive masses that did not carry significant mineralizing solutions. Perhaps their main importance is in showing that magma spread laterally into the lower part of 
20 

0 

I 

10 

I 

I 

10 

20 

I 

30 

I 

I 

20 

40 

I I 

50 KILOMETERS 

I 

30 MILES 

CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

INDEX 

1. Carmen stock 

2. Miner Lake-Beaverhead Divide 

fault zone 

3. Bloody Dick stock 

4. Saginaw mine 

5. Pope-Shenon mine 

6. Harmony mine 

7. Withington Creek caldera 

8. Little Eightmile stock and Little 

Eightmile mining district 

9. Junction mining district 

10. Alder Creek stock 

11. Big Eightmile (Blue Jay) stock and 

Blue Jay mine 

12. Falls Creek intrusive 

13. North Fork intrusive 

14. Patterson ring-fracture complex 

15. lma mine and lma stock (concealed) 

16. Park Fork intrusive 

17. Big Timber stock 

18. Gilmore stock 

19. Gilmore mininq district 

20. Spring Mountain sheet 

21. Spring Mountain mining district 

22. Sawmill Canyon sheet 

23. Nicholia mining district 

24. Birch Creek mining district 

25. Dome mining district 

EXPLANATION 

Tertiary intrusive igneous rocks in 

stocks and sheets 

Reverse fault, sawteeth on upthrown 

side 

Caldera margin or ring-fracture zone 

Mine or mining district 

Mountain area 

FIGURE 10.-Map showing distribution of Tertiary intrusive rocks and mineral deposits in east-central Idaho. 
RELATION, INTRUSIVE IGNEOUS ROCKS AND MINERAL DEPOSITS TO UPLIFTED BLOCKS 21 

the Medicine Lodge thrust plate, mostly as sheets intruding imbricate thrusts. The abunda;nce of imbricate 

thrusts in the Medicine Lodge plate, and so of available 

avenues for lateral spread of magma, restricted intrusive 

activity to the lower part of the plate and seems to explain why quartz monzonite-granodiorite stocks and 

sheets-and associated mineral deposits-are only found 

low in the thrust plate. 

Only a few of the other intrusive masses in the Lemhi 

Range seem clearly to be stocks. The largest of these is 

the Big Eightmile stock, which is a crosscutting composite intrusive suggesting repeated intrusive pulses. 

The core of the Big Eightmile stock is hydrothermally 

altered and contains disseminated pyrite, chalcopyrite, 

and some molybdenite. Only a few small vein deposits of 

lead and silver minerals are known around the Big 

Eightmile stock, probably because the enclosing quartzitic rocks were not as favorable a host for replacement as 

were the sandy carbonate rocks of the lower part of the 

Jefferson Formation that contain the major ore bodies at 

Gilmore (Ruppel and others, 1970, p. 19-23; Umpleby, 

1913, p. 63-69, 89-109). The Big Timber stock intrudes a 

steep reverse fault and is probably truly a stock, but its 

westward extensions are sheets or sills (Ruppel and 

Lopez, 1981; Ruppel, 1968). No mineral deposits are 

known around it. The Ima intrusive mass, at Patterson, 

is not exposed at the surface, but the results of exploration studies for deposits of associated tungsten and 

molybdeum minerals suggests that it is a stock; it and 

two small stocks farther north (fig. 5) probably were intruded in a ring-fracture zone (p. 9.). 

Other intrusive masses in the Lemhi Range are 

characterized by nearly flat roofs, and occur in or near 

imbricate thrust faults, a relation that suggests that they 

are sheets. The largest of these is the Sawmill Canyon 

sheet, whose intrusive center is probably concealed by 

glacial deposits; a few small deposits of lead-silver 

minerals and a barite deposit are known in the host rocks 

above the sheet. The intrusive mass in the Park Fork of 

Big Creek probably is also a sheet, perhaps fed from the 

Big Timber stock. The other intrusives in the Lemhi 

Range are small, semiconcordant bodies roofed in thrust 

faults, and most of them probably are small sheets. 

Small deposits of metallic minerals are widely distributed along the west flank of the Beaverhead Mountains (fig. 10), but only the deposits in the central and 

northern parts of the range are associated with exposed 

intrusive igneous rocks-the Little Eightmile and 

Carmen stocks (Staatz, 1972, 1973; MacKenzie, 1949, p. 

15-21; A. L. Anderson, 1959, p. 33-40). The map pattern 

of the southern part of the Little Eightmile stock 

(Staatz, 1973) suggests that it is a sheet intruding a 

thrust fault. The Carmen stock is a compound, 

inclusion-rich granodiorite that probably does not have 

any peripheral sheets; but it has not been studied in 

much detail and its contact relations are only partly 

known (MacKenzie, 1949; A. L. Anderson, 1957, 1959). 

Small deposits of gold, and of copper, lead, silver, and 

tungsten minerals, are also distributed along the crest of 

the northern Beaverhead Mountains; most of them are 

near the Beaverhead Divide-Miner Lake reverse fault 

zone and are associated with dioritic dikes in and near 

the fault zone. 

Gold deposits on the east slope of the Salmon River 

Mountains, west of Salmon, Idaho (fig. 10) are associated with a quartz monzonite stock (A.~· Anderson, 

1956, p. 39-42; 1959, p. 37-40, 73-75) that apparently 

was intruded along a northwest-trending steep reverse 

fault. 

The fairly consistent relation in east-central Idaho of 

stocks and associated mineral deposits on the flanks of 

mountain ranges, and along reverse faults in some 

places, suggests that the structures that are inferred to 

control the block uplifts in this region also controlled the 

emplacement of stocks and mineral deposits. I thi~k 

that the controlling structures are steep reverse faults 

that bound uplifted blocks of crystalline rocks at depth, 

and I suggest that these faults were primary avenues 

utilized by magma rising from deep in the crust. The 

magma spread laterally into imbricate thrusts near the 

bottom of the Medicine Lodge thrust plate, to form 

sheets like those exposed in the central part of the Lemhi 

Range. The major mineral deposits of the region were 

deposited around and above necklike, central stocks that 

fed the sheets. Small deposits of metallic minerals occur 

in many places around the sheets, but they have yielded 

relatively little ore. The search for new mineral deposits, 

therefore, might be concentrated most usefully in areas 

known or inferred to be above the edges of buried crystalline blocks, where conduits in reverse faults, now stocks, 

have channeled rising magma to the base of regional 

thrust plates and there fed laterally spreading sheets. 

Substantial ore bodies have not been found and are not 

likely to have formed in the structurally flat central 

parts of the block uplifts, because the seemingly necessary deep structural controls are not present there. 

The ring-fracture complex at Patterson and the 

Withington Creek caldera are on the edges of the Lemhi 

Range, and their location suggests that Tertiary centers 

of volcanism also were fed by magma rising along the 

reverse faults at depth. 

The structural framework of southwest Montana is 

similar in most respects to that of east-central Idaho, 

and the primary structural controls on the emplacement 

of intrusive igneous rocks and deposition of associated 

mineral deposits probably are also similar. 
22 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

SUMMARY AND CONCLUSIONS 

The northwest-trending linear mountain ranges of 

east-central Idaho traditionally have been considered to 

be basin ranges bounded on one or both sides by large 

normal faults, but they are better described as flattopped block uplifts flanked by monoclinal folds. The 

normal faults that outline the present mountain ranges 

are relatively minor, secondary faults formed by 

gravitational collapse and sliding of sheets of rock from 

the steepened monoclinal limbs of the uplifts into the 

adjacent valleys. The mountains and valleys thus owe 

their structural relief not to normal faulting but to essentially vertical relative uplift of the mountain blocks. The 

normal faults suggest extensive gravitational denudation 

of sedimentary and volcanic rocks from the block uplifts, 

and the plates of denuded rocks must form a substantial 

part of the fill in the adjacent valleys. 

The monoclinal folds on the flanks of the block uplifts 

are interpreted as drape folds, in ~hich sedimentary 

rocks, earlier deformed by regional thrust faults, have 

been passively folded over the edges of deeper, rising 

blocks of crystalline rocks. The older, imbricate thrust 

faults seem likely to have provided multiple slip surfaces 

that made possible the abrupt draping of the sedimentary veneer over the crystalline blocks, and also provided 

at least some of the surfaces utilized by sheets of rock 

that slid from the steepened limbs into the valleys. 

The block uplifts of east-central Idaho are similar to 

those east of the thrust belt in southwest Montana, and 

the apparent differences between the two regions are 

largely a result of eastward thrust faulting, which preceded major block uplift and carried a greatly deformed 

and thickened sequence of sedimentary rocks into eastcentral Idaho and part of southwest Montana. However, 

these apparent differences earlier led to a suggested division of the region into an eastern Laramide province of 

block uplifts and a western Sevier province of folded and 

thrust-faulted rocks cut by basin-range normal faults-a 

division that now seems to be only partly appropriate. In 

terms of post-thrusting Cenozoic tectonism, the region is 

a single structural province-one of flat-topped, 

basement-cored block uplifts with monoclinal, drapefolded flanks. 

The block uplifts east of the thrust belt are cored by 

Precambrian crystalline metamorphic rocks, and the uplifts are in blocks -defined partly by faults clearly of 

Precambrian age, but which have been the sites of recurrent movement almost to the present time. The 

northwest and east trends of Precambrian faults in 

southwest Montana are repeated in the trends of the 

block uplifts in east-central Idaho. The similarity of 

structural trends suggest that the Idaho uplifts, like 

those east of the thrust belt, are cored by crystalline 

metamorphic rocks in blocks defined by steep reverse 

faults of Precambrian age. The crystalline block cores 

are not exposed in east-central Idaho, but a few steep 

reverse faults are present, and some of these like the 

large Miner Lake-Beaverhead Divide fault zone east of 

Salmon, Idaho, are probably shallow extensions of the 

still larger, steep reverse faults at depth. The development of calderas and the emplacement of intrusive igneous rocks and deposition of associated mineral 

deposits appear to have been controlled by the steep 

reverse faults between basement blocks, because the 

known calderas and the major stocks and mineral 

deposits are on the flanks of the block uplifts, above the 

inferred basement faults. 

The block uplifts of east-central Idaho and southwest 

Montana suggest that Cenozoic faulting, folding, and 

mountain building is almost entirely a vertical response 

to forces acting on the bottom of basement blocks at 

great depth. The nature of the forces driving the basement blocks. vertically is unknown. (But see Dickinson 

and Snyder, 1978; Sales, 1968; Stearns, 1971, 1978, p. 

31-34.) The response of the rocks above the vertically 

and differentially rising basement blocks has been passive drape folding and gravitational sliding; there is no 

clear indication in the exposed rocks of the region of 

repeated episodes of compression and extension in the 

upper crust alternating through Cenozoic time. 

REFERENCES CITED 

Anderson, A. L., 1934, A preliminary report on recent block faulting in 

Idaho: Northwest Science, v. 8, p. 17-28. 

__ 1956, Geology and mineral resources of the Salmon quadrangle, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 106, 102 p. 

__ 1957, Geology and mineral resources of the Baker quadrangle, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 112, 71 p. 

__ 1959, Geology and mineral resources of the North Fork 

quadrangle, Lemhi County, Idaho: Idaho Bureau of Mines and 

Geology Pamphlet 118, 92 p. 

__ 1961, Geology and mineral resources of the Lemhi quadrangle, 

Lemhi County, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 124, 111 p. 

Anderson, R. A., 1948, Reconnaissance survey of the geology and ore 

deposits of the southwestern portion of the Lemhi Range, Idaho: 

Idaho Bureau of Mines and Geology Pamphlet 80, 18 p. 

Baldwin, E. M., 1951, Faulting in the Lost River Range area of Idaho: 

American Journal of Science, v. 249, p. 884-902. 

Becraft, G. E ., Pattee, E. C., Weldin, R. D., and Roche, J. M., 1966, 

Mineral resources of the Spanish Peaks primitive area, Montana: U.S. Geological Survey Bulletin 1230-B, p. B1-B45. 

Beutner, E. C., 1968, Structure and tectonics of the southern Lemhi 

Range, Idaho: University Park, Pennsylvania, The Pennsylvania State University, Ph. D. thesis, 106 p. 

__ 1972, Reverse gravitative movement on earlier overthrusts, 

Lemhi Range, Idaho: Geological Society of America Bulletin, v. 

83, p. 839-846. 

__ 1977, Causes and consequences of curvature in the Sevier 

orogenic belt, Utah and Montana: Wyoming Geological Association, 29th Annual Field Conference Guidebook, p. 353-365. 
REFERENCES CITED 23 

Burger, H. R., ITI, 1969, Structural evolution of the southwestern 

Tobacco Root Mountains, Montana: Geological Society of 

America Bulletin, v. 80, p. 1329-1341. 

Coppinger, Walter, 1974, Stratigraphic and structural study of Belt 

Supergroup and associated rocks in a portion of the Beaverhead 

Mountains, southwest Montana and east-central Idaho: Oxford, 

Ohio, Miami University, Ph. D. thesis, 266 p. 

Dickinson, W. R, and Snyder, W. S., 1978, Plate tectonics of the 

Laramide orogeny, in Vincent Matthews, Ill, ed., Laramide 

folding associated with basement block faulting in the western 

United States: Geological Society of America Memoir 151, 

p. 355-366. 

Dorr, J. A., Jr., and Wheeler, W. H., 1964, Cenozoic paleontology, 

stratigraphy, and reconnaissance geology of the upper Ruby 

River basin, southwestern Montana: Contribution from Museum of Paleontology, University of Michigan, Ann Arbor, v. 13, 

no. 12, p. 297-339. 

Foose, R. M., Wise, D. U., and Garbarini, G. S., 1961, Structural 

geology of the Beartooth Mountains, Montana and Wyoming: 

Geological Society of America Bulletin, v. 72, p. 1143-1172. 

Garihan, J. M., 1973, Structural and metamorphic history of the PreBeltian rocks of the central Ruby Range, southwestern Montana 

[abs.]: Geological Society of America Abstracts with Programs, 

v. 5, no. 2, p. 164-165. 

___ 1976, Geologic road log from Dillon to Alder, covering the 

Precambrian geology of the central Ruby Range, Madison 

County, southwestern Montana: Montana Bureau of Mines and 

Geology Special Publication 73, p. 15-26. 

1 Hadley, J. B., 1969, Geologic map of the Cameron quadrangle, 

Madison County, Montana: U.S. Geologic Survey Geologic 

Quadrangle Map GQ-813. 

Hait, M. H ., Jr., 1965, Structure of the Gilmore area, Lemhi Range 

Idaho: University Park, Pa., The Pennsylvania State University, Ph. D. thesis, 173 p. 

1 Hait, M .H., Jr., and Scott, W. E., 1978, Holocene faulting, Lost River 

I Range, Idaho [abs.]: Geological Society of America Abstracts 

I with Programs, v. 10, no. 5, p. 217. 

Harrison, J. E., 1972, Precambrian Belt basin of northwestern United 1 

States; its geometry, sedimentation, and copper occurrences: 

Geological Society of America Bulletin, v. 83, p. 1215-1240. 

' Harrison, J. E., Griggs, A. B., and Wells, J.D., 1974, Tectonic features 

of the Precambrian Belt basin and their influences on post-Belt 

structures: U.S. Geological Survey Professional Paper 866, 15 p. 

Heinrich, E. W., 1960, Pre-Beltian geology of the Cherry Creek and 

Ruby Mountain areas, southwestern Montana-Pt. · 2, Geology 

of the Ruby Mountains and nearby areas in southwestern Montana: Montana Bureau of Mines and Geology Memoir 38, 25 p. 

I 

Hyndman, D. W., Badley, R., and Rebal, D., 1977, Northeast-trending 

· early Tertiary dike swarm in central Idaho and western Montana [abs.]: Geological Society of America Abstracts with 

; I Programs, v. 9, no. 6, p. 734. 

, Iddings, J. P., 1904, A fracture valley system [Montana]: Journal of 

. I Geology, v. 12, p. 94-105. 

, 1Kinoshita, W. T., Davis, W. E., and Peterson, D. L., 1969, Principal 

facts for gravity stations in Lemhi and Birch Creek Valleys, 

Lemhi County, Idaho: U.S. Geological Survey Open-file report, 

6 data sheets. 

-_]Kirkham, V. R. D., 1927, A geologic reconnaissance of Clark, and Jefferson, and parts of Butte, Custer, Fremont, Lemhi, and 

Madison Counties, Idaho: Idaho Bureau of Mines and Geology 

Pamphlet 19, 47 p. 

··!Klepper, M .R., 1950, A geologic reconnaissance of parts of Beaverhead 

and Madison Counties, Montana: U.S. Geological Survey Bulletin 969-C, p. 55-85. 

Levandowski, D. W., 1956, Geology and mineral deposits of the 

Sheridan-Alder area, Madison County, Montana: Ann Arbor, 

Mich., University of Michigan, Ph. D. thesis, 318 p. 

Lowell, W. R., 1965, Geologic map of the Bannack-Grayling area, 

Beaverhead County, Montana: U.S. Geological Survey Miscellaneous Geologic Investigations Map I-433. 

Lucchitta, B. K., 1966, Structure of the Hawley Creek area, IdahoMontana: University Park, Pa., The Pennsylvania State 

University, Ph. D. thesis, 235 p. 

MacKenzie, W. 0., 1949, Geology and the ore deposits of a section of 

the Beaverhead Range east of Salmon, Idaho: Moscow, Idaho, 

University of Idaho, Ph. D. thesis, 52 p. 

McMannis, W. J ., 1963, Lahood Formation-A coarse facies of the Belt 

Series in southwestern Montana: Geological Society of America 

Bulletin, v. 74, p. 407-436. 

Mann, J. A., ~54, Geology of part of the Gravelly Range, Montana: 

The Yellowstone-Bighorn Research Association, Inc., Yellowstone-Bighorn Research Project Contribution 190, 92 p. 

Mapel, W. J., and Shropshire, K. L., 1973, [1974], Preliminary geologic 

map and section of the Hawley Mountain quadrangle, Custer, 

Butte, and Lemhi Counties, Idaho: U.S. Geological Survey 

Miscellaneous Field Studies Map MF-546. 

Meinzer, 0. E., 1924, Ground water in Pahsimeroi Valley, Idaho: Idaho 

Bureau of Mines and Geology Pamphlet 9, 35 p. 

Nichols, Ralph, 1975, Early Miocene mammals from the Lemhi Valley 

of Idaho: The Journal of the Idaho State University Museum, v. 

18, no. 2, p. 9-47. 

Pardee, J. T., 1950, Late Cenozoic block faulting in western Montana: 

Geological Society of America Bulletin, v. 61, p. 359-406. 

Prucha, J. J., Graham, J. A., and Nickelsen, R. P., 1965, Basementcontrolled deformation in Wyoming province of Rocky Mountains foreland: American Association of Petroleum Geologists 

Bulletin, v. 49, p. 966-992. 

Reid, R. R., 1957, Bedrock geology of the north end of the Tobacco Root 

Mountains, Madison County, Montana: Montana Bureau of 

Mines and Geology Memoir 36, 26 p. 

___ 1963, Metamorphic rocks of the northern Tobacco Root Mountains, Madison County, Montana: Geological Society of 

America Bulletin, v. 74, p. 293-305. 

Robinson, G. D., 1963, Geology of the Three Forks Quadrangle: U.S. 

Geological Survey Professional Paper 370, 143 p. 

Ross, C. P., 1947, Geology of the Borah Peak Quadrangle, Idaho: 

Geological Society of American Bulletin, v. 58, p. 1085-1160. 

___ 1961, Geology of the southern part of the Lemhi Range, Idaho: 

U.S. Geological Survey Bulletin 1081-F, p. 189-260. 

Ruppel, E. T., 1964, Strike-slip faulting and broken basin-ranges in 

east-central Idaho and adjacent Montana: in Geological Survey 

Research 1964: U.S. Geological Survey Professional Paper 

501-C, p. C14-C18. 

___ 1967, Late Cenozoic drainage reversal, east-central Idaho, and 

its relat,ion to possible undiscovered placer deposits: Economic 

Geology, v. 62, p. 648-663 . 

___ 1968, Geologic map of the Leadore quadrangle, Lemhi County, 

Idaho: U.S. Geological Survey Geologic Quadrangle Map 

GQ-733. 

___ 1972, Geology of pre-Tertiary rocks in the northern part of Yellowstone National Park, Wyoming: U.S. Geological Survey 

Professional Paper 729-A, p. A1-A66. 

___ 1978, Medicine Lodge thrust system, east-central Idaho and 

southwest Montana: U.S. Geological Survey Professional Paper 

1031, 23 p. 

___ 1980, Geologic map of the Patterson quadrangle, Lemhi County, 

Idaho: U.S. Geological Survey Geologic Quadrangle Map 

GQ-1529. 
24 CENOZOIC BLOCK UPLIFTS, IDAHO, MONTANA 

Ruppel, E. T. , and Lopez, D. A., 1981, Geologic map of the Gilmore 

quadrangle, Lemhi County, Idaho: U.S. Geological Survey 

Geologic Quadrangle Map GQ-1543. 

__in press, The thrust belt in southwest Montana and east-central 

Idaho: U.S. Geological Survey Professional Paper 1278. 

Ruppel, E. T., Watts, K. C., and Peterson, D. L., 1970, Geologic, 

geochemical, and geophysical investigations in the northern 

part of the Gilmore mining district, Lemhi County, Idaho: U.S. 

Geological Survey Open-file report, 56 p. 

Sales, J. K., 1968, Crustal mechanics of Cordilleran foreland deformation-A regional and scale-model approach: American Association of Petroleum Geologists Bulletin, v. 52, p. 2016-2044. 

Schmidt, C. J., Brumbaugh, D. S., and Hendrix, T. E., 1977, Fault 

movements and origin of the salient-reentrant pattern in the 

disturbed belt of southwestern Montana [abs.]: Geological 

Society of America Abstracts with Programs, v. 9, no. 6, p. 760. 

Schmidt, C. J., and Garihan, J. M. , 1979, A summary of Laramide 

basement faulting in the Ruby, Tobacco Root and Madison 

Ranges and its possible relationship to Precambrian continental 

uplifting [abs.]: Geological Society of America Abstracts with 

Programs, v. 11, no. 6, p. 301. 

Scholten, Robert, 1957, Paleozoic evolution of the geosynclinal margin 

north of the Snake River Plain, Idaho-Montana: Geological 

Society of America Bulletin, v. 68, p. 151-170. 

___ 1967, Structural framework and oil potential of extreme southwestern Montana: Montana Geological Society Guidebook, 

1967, p. 7-19. 

___ 1968, Model for evolution of Rocky Mountains east of Idaho 

batholith: Tectonophysics, v. 16, no. 2, p. 109- 126. 

Scholten, Robert, Keenmon, K. A., and Kupsch, W. 0., 1955, Geology 

of the Lima region, southwestern Montana, and adjacent Idaho: 

Geological Society of America Bulletin, v. 66, p. 345-404. 

Scholten, Robert, and Ramspott, L. D., 1968, Tectonic mechanisms indicated by structural framework of central Beaverhead Range, 

Idaho-Montana: Geological Society of America Special Paper 

104, 70 p. 

Shenon, P. J., 1928, Geology and ore deposits of the Birch Creek district, Idaho: Idaho Bureau of Mines and Geology Pamphlet 27, 

25 p. 

Skipp, Betty, and Hait, M . H., Jr., 1977, Allochthons along the 

northeast- margin of the Snake River Plain, Idaho: Wyoming 

Geological Association, 29th Annual Field Conference Guidebook, p. 499- 515 . 

Sloss, L. L., and Moritz, C. A., 1951, Paleozoic stratigraphy of 

southwestern Montana: American Association of Petroleum 

Geologists Bulletin, v. 35, p. 2135-2169. 

Smedes, H . W., 1968, Geologic map of the Butte South quadrangle, 

Montana: U.S. Geological Survey Open-file map, scale 1:24,000. 

Staatz, M. H., 1972, Geology and description of the thorium-bearing 

veins, Lemhi Pass quadrangle, Idaho and Montana: U.S. 

Geological Survey Bulletin 1351, 94 p. 

___ 1973, Geologic map of the Goat Mountain quadrangle, Lemhi 

County, Idaho, and Beaverhead County, Montana; U.S. 

Geological Survey Geologic Quadrangle Map GQ-1097. 

Stearns, D. W., 1971, Mechanisms of drape folding in the Wyoming 

province: Wyoming Geological Association, 23rd Annual Field 

Conference Guidebook, p. 125-143. 

___ 1978, Faulting and forced folding in the Rocky Mountains 

foreland, in Vincent Matthews, III, ed., Laramide folding associated with basement block faulting in western United States: 

Geological Society of America Memoir 151, p. 1-37. 

Swanson, R. W., 1970, Mineral resources in Permian rocks of southwest 

Montana: U.S. Geological Survey Professional Paper 313-E. p. 

661-777. 

Tucker, D. E., 1975, Stratigraphy and structure of Precambrian Y 

(Belt?) metasedimentary and associated rocks, Goldstone 

Mountain quadrangle, Lemhi County, Idaho, and Beaverhead 

County, Montana: Oxford, Ohio, Miami University, Ph. D. 

thesis,· 221 p. 

Tysdal, R. G., 1970, Geology of the north end of the Ruby Range, 

southwestern Montana: Missoula, Montana University, Ph. D. 

thesis, 187 p. 

___ 1976a, Geologic map of northern part of Ruby Range, Madison , 

County Montana; U.S. Geological Survey Miscellaneous 

Investigations Map 1-951. 

___ 1976b, Paleozoic and Mesozoic stratigraphy of the northern part 

of the Ruby Range, southwestern Montana: U.S . Geological 

Survey Bulletin 1405-1, p. 11-125. 

Umpleby, J. B., 1913, Geology and ore deposits of Lemhi County, 

Idaho: U.S. Geological Survey Bulletin 528, 182 p . 

Vitaliano, C. J., and Cordua, W. S., 1979, Geologic map of southern 

Tobacco Root Mountains, Madison County, Montana : Geological Society of America Map and Chart Series, MC-31. 

Witkind, I. J., 1977, Structural pattern of the Centennial Mountains, 

Montana-Idaho: Wyoming Geological Association, 29th Annual 

Field Conference Guidebook, p. 531-536. 

Wooden, J. L., Vitaliano, C. J., Koehler, S. W., and Ragland, P. C., 

1978, The late Precambrian mafic dikes of the southern Tobacco 

Root Mountains, Montana; geochemistry, Rb-Sr geochronology 

and relationship to Belt tectonics: Canadian Journal of Earth 

Sciences, v. 15, no. 4, p. 467-479.