Base from U.S. Geological Survey, 1:100,000,

Borah Peak, 1989; Dillon, 1983; Dubois, 1983; Leadore, 1980; Lima, 1987; Salmon, 1981.Projection: Universal Transverse Mercator, zone 121927 North American datum

SCALE 1:100 000

1 0 1 2 3 4 5 6 7 8 MILES

1 0 1 2 3 4 5 6 7 8 9 10 KILOMETERS

CONTOUR INTERVAL 50 METERS

(CONTOUR INTERVAL 40 METERS IN LEADORE QUADRANGLE)

SUPPLEMENTARY CONTOUR INTERVAL 25 METERS IN DUBOIS AND LIMA QUADRANGLES

NATIONAL GEODETIC VERTICAL DATUM OF 1929

113°

112°30'

112°30'

45°05'

45°

112°45'

44°45'

44°30'

112°45'

113°

44°15'

44°15'

MONTANA

IDAHO

MAP EXPLANATION

FAULTS, FOLDS, AND LINEAMENTS

Low-angle normal fault—Tics on hanging wall. Dashed where approximate; dotted where concealedNormal fault that cuts out or reactivates a thrust fault—Fault places older rocks on younger rocks.

Sawteeth on hanging wall. Dashed where approximate; dotted where concealed

Moderately or steeply dipping normal fault—Bar and ball on downthrown side; opposed arrows show

oblique-slip movement. Dashed where approximate; dotted where concealed

Anticline—Arrow indicates plunge direction. Dashed where approximate; dotted where concealed

Syncline—Arrow indicates plunge direction. Dashed where approximate; dotted where concealedMonocline—Arrow on steep limb of the fold

Lineament—Dashed where uncertain

AGES OF NORMAL FAULTS

Fault set Color Magnitude of this Relative and absolute age constraints

extensional event

6 Green Moderate Active or potentially active normal faults in the current tectonic

regime. The age of initial slip is uncertain.

5 Brown Uncertain Many of the east-west striking normal faults cut the Paleogene

normal faults (blue) and are cut by the active Basin and Range faults(green). The east-west normal faults may have formed during morethan one episode of extension. All east-west normal faults arebrown, regardless of their age.

4 Magenta Minor Late early Miocene normal faults with northeast strikes. Little 

Eightmile Creek and Little Sheep Creek faults are the main faults ofthis age. Nicholia School fault zone may have formed at this time.

3 Blue Major, the largest normal Paleogene normal faults—most are middle Eocene to Oligocene,

faults in the region but some may have been active into early or middle Miocene time. 

 formed at this time Cross-cutting relationships show that several systems of normal

faults were active sequentially during this time (VanDenburg andothers, 1998; Blankenau, 1999; Janecke and others, 1999).

2 Purple Minor Coeval with the Challis Volcanic Group, about 49.5 to about 45 Ma1 Red Major? Older than the Challis Volcanic Group, pre 49.5 Ma

Black Variable Faults of uncertain age

Youngest

Oldest

Muddy-Grasshopper fault (M.G.F.)

Armstead fault

M.G.F.

Kissick fault

M.G.F.

Rocky

Canyon fault

M.G.F.

M.G.F.

M.G.F.

Kate Creek fault

Bell Canyon syncline fault

Armstead fault

Red Rock fault

F.S.C.F.

Grouse Canyon 

(east segment)

fault

many

additional

small faults

Grouse Canyon fault

(west segment)

Divide

Creek fault

more Grouse

Canyon fault?

M.G.F.

M.G.F.

M.G.F.

M.G.F.

Kate Creek fault

Deadman fault

Scott 

Canyon 

fault 

Crooked Creek 

fault 

Middle 

Creek 

fault

fault

Black

Canyon

Medicine Lodge

thrust

Former

Lima thrust system

Creek fault

Little Sheep 

Sage Creek fault (F.S.C.F.)

Creek fault 

Deadman fault

Nicholia

West 

fault zone Side 

Deadman fault

Cabin thrust

thrust

Lodge

Medicine

strand of Ellis 

Peak fault zone

M.L.T.

McKenzie Canyon

McKenzie Canyon

thrust

mid basin faults

Monument Hills

fault zone

Sage 

Divide fault

School fault zone

thrust

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

?

? ?

?

?

?

?

?

?

?

?

?

?

?

?

?

? ?

?

?

?

Manuscript approved for publication April 6, 2001Any use of trade names in this publication is fordescriptive purposes only and does not imply endorsement by the U.S. Geological SurveyThis map was produced on request, directly fromdigital files, on an electronic plotterFor sale by U.S. Geological Survey Information ServicesBox 25286, Federal Center, Denver, CO 802251-888-ASK-USGSA PDF for this map is available athttp://geology.cr.usgs.gov/greenwood-pubs.html

ULTS AND EXTENSIONAL FOLDS IN THE TENDOY MOUNTAINS AND BEAVERHEAD RANGE, SOUTHWEST MONTANA AND EASTERN IDAHO

By

Susanne U. Janecke

1, James J. Blankenau1*, Colby J. VanDenburg1**, and Bradley S. Van Gosen

2

2001

MISCELLANEOUS FIELD STUDIES MAP MF–2362

Version 1.0

Grasshopper

 basin

Montana

Medicine

Lodge

basin

EllisPeak

Nicholia

Creek

basin

Beaverhead

Mountains

Williams

Creek

Birch Creek

Valley

Bannack

Pass

Idaho

Medicine

Lodge

Valley

Lima

Red Rock valley

Tendoy

Mountains

Muddy

Creek

basin

45°N

Patterson culmination

MPC 

Yellowjacket 

Mine culmination

Hayden Creek culmination

Cobalt culmination

Challis

Salmon

Ellis

Tendoy

Lemhi

Carmen

Lima

Cobalt

Middle Fork Salmon River

Salmon River

Lem Peak fault

Beaverhead Divide fault

IDAHO

MONTANA

Southern Lemhi 

Range

S. Beaverhead 

 Mountains

MLT?

MLT

Lost River Range

Brushy Gulch thrust

Leesburg syncline

foreland

Blacktail-Snowcrest

?

?

?

?

?

?

?

?

?

Grasshopper thrust

Ermont thrust

McKenzie Canyon thrust

MLT

Tendoy Mountains

N. Lemhi 

Range

Salmon River Mtns.

Mesozoic and Paleozoicsedimentary rocks

Swauger and Gunsight Formations and equivalents

Hoodoo Quartzite and Yellowjacket Formation

Proterozoic toArcheanbasement

Idaho and Pioneer batholiths

 Lower Tertiary and Upper Cretaceous Beaverhead Group

Apple Creek Formation

Middle Proterozoic rocks

Yac

AnticlineSyncline

Thrust fault

Low-angle normal fault

Normal faultStrike and dip of bedding

Abbreviations: BDCF, BloodyDick Creek fault; MPF, MaidenPeak fault; MGF, MuddyGrasshopper fault; MPC, 

Maiden Peak culmination; MLT, Medicine Lodge thrust. 

20 KILOMETERS

SCALE

0

BDCF

Iron

Lake thr u st Yac

Yac

Yac

Yac

Yac

Yac

Yac

Modern rivers

Porphyritic rapakivi granite

Ordovician Beaverheadpluton

Salmon basin detachment fault

Island Butte culmination.

MPF

MGF

Divide fault

Deadman fault Divide conglomerate 44°30'N

uplift

Tendoy thrust

Cabin thrust

Study areashown on map

Hawley Creek thrust

 Divide

conglomerate

Carmen culmination

Leadore

Proterozoic to Archean

 rocks

114°W 113°W

Figure 2. Subcrop map of the Idaho-Montana fold-and-thrust belt showing the locations of structural culminations (green labels) and normal faults that 

collapse them (blue and red). Map shows age of bedrock beneath middle Eocene Challis Volcanic Group and was constructed by using principles outlined in Rodgers and Janecke (1992). Complied from 80 sources in the data repository of Janecke and others (2000c). Normal faults of set 1 are red and faults of set 3 are blue. Notice the association between these large normal faults and structural culminations of the fold and thrust belt. The study area shown on the map is outlined in magenta.

HajorT

DCF

MontanaMedicine

Lodge

basin

DILLON

DELL

LIMA

LEADORE

SALMON

Sage Creek basin

Muddy

Creek 

basin

0 10 20

Kilometers

basin

Salmon

Beaverhead 

Mountains

Tendoy

Mountains

Blacktail Mountains

Lemhi Range

Montana

Idaho

Maiden Peak 

 Spur

114°00' 113°30' 113°00' 112°30'

45°15'

44°45'

44°30'

SBF

MGF

MPF BF Montana Idaho Location of figure

UDMGF

DF

LPF 45°00'

BTF

RRF

BDF

Reservoir

Mountain rangeQuaternary-Miocene basin fill

Early Miocene-Eocene basin fill

Mountains

basin

basin

Prairie

Horse

Grasshopper 

Big Holebasin

Medicine

Lodge

basin

Idaho

MF

Nicholia

basin

Creek

Upper Lemhi  Valley

Birch Creek Valley

Red Rock Valley

AYF

BF

44°15'

Faults—Bar and ball on downthrown side of 

steeper normal fault. Rectangle on hanging wall of low-angle normal fault

 Set 1 Set 3 Set 6

Figure 1. Map displaying the geographic setting of the study area (magenta outline), some of the larger normal faults in the region, and locations of rift basins. Basins with predominantly Paleogene basin fill are distinguished from 

basins with mostly Miocene to Quaternary basin fill. The age of the basin fill in the southern Big Hole basin is poorly known. The Sage Creek basin was east of the zone of active rifting during the Paleogene (Janecke, 1994). Abbreviations are: AYF, Agency-Yearian fault; BF, Beaverhead fault; BDF, Bloody Dick Creek fault; BTF, Blacktail fault; DF, Deadman fault; LPF, Lemhi Pass fault; MF, Monument Hills fault zone; MGF, Muddy-Grasshopper detachment fault; MPF, Maiden Peak fault; RRF, Red Rocks fault; SBF, Salmon basin detachment fault; and UDMGF, upper detachment fault of the MuddyGrasshopper detachment fault system.

OVERVIEW

Compilation of a 1:100,000-scale map of normal faults and 

extensional folds in southwest Montana and adjacent Idaho reveals a complex history of normal faulting that spanned at least the last 50 m.y. and involved six or more generations of normal faults. The map is based on both published and unpublished mapping and shows normal faults and extensional folds between the valley of the Red Rock River of southwest Montana and the Lemhi and Birch Creek valleys of eastern Idaho between latitudes 45°05' N. 

and 44°15' N. in the Tendoy and Beaverhead Mountains. Some 

of the unpublished mapping has been compiled in Lonn and others (2000). Many traces of the normal faults parallel the generally northwest to north-northwest structural grain of the preexisting Sevier fold and thrust belt and dip west-southwest, but northeastand east-striking normal faults are also prominent. Northeaststriking normal faults are subparallel to the traces of southeast-directed thrusts that shortened the foreland during the Laramide orogeny. It is unlikely that the northeast-striking normal faults reactivated fabrics in the underlying Precambrian basement, as has been documented elsewhere in southwestern Montana (Schmidt and others, 1984), because exposures of basement rocks in the map area exhibit north-northwest- to northwest-striking deformational fabrics (Lowell, 1965; M’Gonigle, 1993, 1994; M’Gonigle and Hait, 1997; M’Gonigle and others, 1991). The largest normal faults in the area are southwest-dipping normal faults that locally reactivate thrust faults (fig. 1). Normal faulting began before middle Eocene Challis volcanism and continues today. The extension direction flipped by about 90° four times.

INTRODUCTION

Normal faults in southwest Montana and adjacent Idaho were 

divided into distinct fault sets where cross-cutting relationships with datable features indicated the approximate age of slip across them. Absolute and relative ages of normal faults are best constrained at the sites of detailed studies in the southern Beaverhead Mountains (Skipp, 1984, 1985), the Horse Prairie basin (VanDenburg, 1997; VanDenburg and others, 1998), the southeast part of the Salmon basin (Blankenau, 1999), and the Muddy Creek basin (Janecke and others, 1999) (fig. 1). Faults of uncertain age are shown in black on the map.

At least six geometric and temporally distinct sets of normal 

faults are evident in the Tendoy and Beaverhead Mountains. Each set of normal faults, or group of related structures, is described as having formed during a distinct phase of extension, or time period. Fault sets are described from youngest (set 6) to oldest (set 1). VanDenburg and others (1998) described five of these fault sets in the Horse Prairie basin area and Blankenau (1999) detailed the multiple deformational events associated with the third event in the southeast part of the Salmon rift basin. Sets 1–4 correspond to sets 1–4 of VanDenburg and others (1998). Subsequent mapping in the southern Nicholia Creek basin and adjacent areas revealed the presence of an additional set of east-west striking normal faults (set 5) that formed in latest Miocene to Pleistocene(?) time before the currently active system of northwest-striking normal faults developed along the range fronts (Janecke and others, 2000a; and unpublished mapping). The currently active normal faults of set 6 correspond to faults formed during VanDenburg and others’ (1998) phase 5.

Extensional folds are very common in rift basins of southwest 

Montana and eastern Idaho, and formed in association with the many normal faults that have extended the region since at least Eocene time (Janecke and others, 1998). Folds were not separated into distinct sets based on geometry and age because some folds formed over extended periods of time and many have uncertain origins (for example, VanDenburg and others, 1998; Blankenau, 1999; Janecke and others, 1998). Janecke and others (1998) illustrated the widely varying geometries of the extensional folds and showed that many folds are oblique to the associated normal fault. Some folds were active during sedimentation in the rift basins and influenced the facies patterns in the basins (Janecke and others, 1998, 1999; Blankenau, 1999). Other extension folds deform preexisting synrift deposits. 

Rift basins formed during the third and sixth phase of extension in the Tendoy-Beaverhead region (fig. 1). Basins that developed during phase 3 include the Eocene to Oligocene Salmon, 

Horse Prairie, Grasshopper, Montana Medicine Lodge, Muddy Creek, and Nicholia Creek rift basins (Fields and others, 1985; Janecke and others, 1998, 1999) (fig. 1). Deposition in the Horse Prairie half graben locally continued into the early Miocene (Fields and others, 1985; VanDenburg, 1997; VanDenburg and others, 1998). The Red Rock, Birch Creek, upper Lemhi, and Idaho Medicine Lodge valleys formed primarily during phase 6 (fig. 1). Minor localized sediment related to phase 4 is preserved southwest of Lima in the hanging wall of the Little Sheep Creek normal fault (Ryder and Scholten, 1973; Betty Skipp, unpublished mapping) and in the hanging wall of the Little Eightmile Creek fault (VanDenburg, 1997; VanDenburg and others, 1998). Farther to the northeast, however, major half grabens formed during phase 4 (Fritz and Sears, 1993; Sears and Fritz, 1998).

Fault set 6. Miocene(?) to Holocene southwest- and 

northeast-dipping normal faults (shown in green on the 

map)

Fault set 6 includes normal faults that are known or suspected 

to be active in the current tectonic regime. The Tendoy and Beaverhead Mountains lie within the Rocky Mountain Basin and Range Province and the Intermountain seismic belt, along the northern arm of the Yellowstone seismic parabola (Scott and others, 1985; Anders and others, 1989; Smith and Arabasz, 1991; Pierce and Morgan, 1992; Wernicke, 1992). Pardee (1950) and Reynolds (1979) first documented some of these active normal faults in southwest Montana. Extension in this area is currently being accommodated by range-front faults that strike approximately N. 35–50° W. (Crone and Haller, 1991) due to approximately N. 45° E. extension (Stickney and Bartholomew, 1987). Normal faults of set 6 in the northern half of the map area have very consistent northwest strikes. Most of the active range-front faults dip to the southwest, but northeast of a synclinal accommodation zone near the crest of the Tendoy Mountains most of the active range front faults dip to the northeast (Stewart and others, 1998; Stewart, 1998; Sears and Fritz, 1998, plate 1). The southwest-dipping Monument Hills normal fault zone, at the northeast edge of the map area, is an exception to this rule. The Monument Hills fault zone may be antithetic to the Red Rock fault.

Offset Quaternary deposits along the Red Rock, Monument 

Hills, and Beaverhead normal faults show that these normal faults are young and active (Haller, 1988, 1990; Bartholomew, 1989; Crone and Haller, 1991). Some other normal faults lack fault scarps, yet they cut all older structures and are parallel to the known faults of set 6; these faults

—the Kissick, Kate Creek, Rocky 

Canyon, and Bloody Dick Creek faults (Coppinger, 1974; Dubois, 1982; VanDenburg, 1997; S.U. Janecke, unpublished mapping; Lonn and others, 2000)

—were also included in this group.

Fault set 5. Miocene to Pleistocene(?) north- and southdipping normal faults (shown in brown on the map)

A newly identified set of consistently east-west-striking normal 

faults displace all but the youngest range-front normal faults. The east-west faults dip both north and south and typically display small offsets. Most of these normal faults have steep to moderate dips but some faults are listric in the subsurface and produce rollover folds. The east-west normal faults are most numerous in the southern part of the map area, but some occur as far north as the Montana Medicine Lodge basin (M’Gonigle and others, 1991; Dubois, 1982) and Polaris, Montana (Zimbelman, 1984). Few eastwest striking normal faults were documented in the study area prior to this study (for example, Skipp and others, 1979), in part because faults of this orientation are typically small and discontinuous. The large number of normal faults with this orientation, however, shows that they are an important feature of the region. There are more south-dipping than north-dipping normal faults, but both dip directions are present.

The relative age of the east-west-striking normal faults is incompletely known and faults of this orientation may have been active during more than one deformational event. The east-weststriking faults clearly postdate and offset normal faults formed during phase 3, and possibly some faults formed during phase 4. 

Faults of set 4 are relatively rare in the Tendoy and Beaverhead Mountains, and unequivocal cross-cutting relationships between fault sets 5 and 4 have not been observed. Northwest-striking normal faults of set 6 appear to displace some of the east-weststriking normal faults. For example, the southwest-dipping Crooked Creek normal fault of set 6 displaces the eastern and western parts of the south-dipping Grouse Creek normal fault (set 5) in the southern part of the Beaverhead Mountains (see map; Skipp, 1984). These data suggest that some (or most?) of the east-west-striking normal faults are older than the currently active northwest-striking range-front normal faults. Elsewhere, faults of set 6 appear to reactivate segments of some east-west-striking normal faults. This could explain the pronounced east-west jog in the Beaverhead fault north of Leadore, Idaho, and the presence of fault scarps on south-dipping faults in the Williams Creek to Mud Creek area in the southwest part of the map (Haller, 1988, 1990; Skipp and others, 1988).

Offset latest Cenozoic rocks indicate a young age for the eastwest-striking normal faults. Near the continental divide around 

Bannack Pass the east-west striking normal faults displace the 

6.62-Ma tuff of Blacktail (age from Morgan and others, 1998) and 

overlying undated gravel deposits (Skipp, 1984; Skipp and others, 

1979). Skipp (1984) assigned a Pliocene to early Pleistocene(?) 

age to these gravel deposits. Altogether the data suggest a latest 

Miocene to Pleistocene(?) age for the normal faults of set 5.

The east-west-striking normal faults are parallel to the Centennial normal fault (Witkind, 1975) to the east of the map area and 

probably have a common origin (Janecke and others, 2000a). 

Normal faults with east-west strikes are currently active in a 100- 

to 150-km-wide belt west of the Yellowstone caldera, in a region 

directly east of the map area (Janecke and others, 2000a). Farther to the west and northwest, east-west-striking normal faults are 

no longer active. Instead, the active normal faults strike northwest 

(set 6) and appear to record “typical” northeast-southwest Basin 

and Range extension. The east-west-striking normal faults probably reflect transient stress reorientation in the vicinity of the Yellowstone hot spot due to subsidence toward the growing eastern 

Snake River Plain. The inactive east-west-striking normal faults in 

the Tendoy and Beaverhead Mountains may have formed when 

the Yellowstone hot spot was southwest of its current position (Janecke and others, 2000a).

Fault set 4. Early to middle Miocene northwest-dipping 

normal faults (shown in magenta on the map)

Normal faults of fault set 4 strike northeast, at a high angle to 

the overall northwest structural grain of the fold and thrust belt 

and to most of the normal faults of the region, but parallel to the 

foreland uplifts to the east. These faults appear as widely spaced 

fault zones. Faults of set 4 formed in early to middle Miocene 

time (for example, Fritz and Sears, 1993; Sears and Fritz, 1998), 

and are relatively rare in the Tendoy and Beaverhead Mountains. 

Similar normal faults to the northeast bound the southeast margin 

of the Miocene Ruby and Beaverhead half grabens and appear to 

reactivate southeast-vergent basement-cored thrust sheets of the 

foreland (McBride, 1988; Fritz and Sears, 1993; Sears and Fritz, 

1998). Three northwest-dipping normal faults were included in 

this fault set: the Little Eightmile Creek fault (VanDenburg and 

others, 1998), and the newly identified Little Sheep Creek and 

Nicholia School fault zones. Further mapping is needed to confirm the Miocene age of the latter two normal faults. Other northeast-striking normal fault zones that may have developed at this 

time include swarms of northeast-striking normal faults around Ellis Peak in the western Tendoy Mountains (the Ellis Peak fault system), and spaced normal faults in the Beaverhead Mountains between Lemhi Pass and Railroad Canyon.

The Little Sheep Creek normal fault southwest of Lima, Montana, is here interpreted as a southwest continuation of the Sage 

Creek normal fault that is exposed on the northwest flank of the 

Snowcrest Range (for example, McBride, 1988). Perry and others 

(1988) had earlier interpreted the north-striking “Former Sage 

Creek fault” as the southwest continuation of the Sage Creek fault 

that is in the Snowcrest Range. The revised interpretation is preferred because the Little Sheep Creek fault is directly along strike 

and has the identical orientation as the Sage Creek fault to the 

northeast. The Sage Creek–Little Sheep Creek fault system probably reactivated northwest-dipping foreland thrusts of the BlacktailSnowcrest trend (McBride, 1988; Perry and others, 1988). The 

origin of the other north-striking normal faults is less certain. It is 

unlikely that the northeast-striking normal faults reactivated fabrics 

in the underlying Precambrian basement, as has been documented 

elsewhere in southwestern Montana (Schmidt and others, 1984), 

because exposures of basement rocks in the map area exhibit 

north-northwest- to northwest-striking deformational fabrics (Lowell, 1965; M’Gonigle 1993, 1994; M’Gonigle and Hait, 1997; 

M’Gonigle and others, 1991).

Fault set 3. Late middle Eocene to early middle Miocene 

west-southwest-dipping listric low-angle normal faults 

(shown in blue on the map)

North-northwest-striking normal faults are among the largest in 

the region and include several major, middle Eocene to Miocene, 

low-angle normal faults of regional extent (detachment faults) (fig. 

1). From west to east, these are the Salmon basin, Agency-Yearian, Lemhi Pass, Maiden Peak, Deadman, and Muddy-Grasshopper low-angle normal faults (M’Gonigle and Dalrymple, 1993, 

1996; VanDenburg and others, 1998; Blankenau, 1999; Janecke 

and others, 1999). The Divide fault, which may also have formed 

during this phase of extension (Karl Kellogg, written commun., 

2000), is distinctly steeper than these other normal faults (Lucchitta, 1966). West-southwest-dipping normal faults of set 3 are the 

main basin-forming normal faults in the study area (fig. 1). The 

Muddy-Grasshopper normal fault is interpreted as the main breakaway fault for Eocene to Oligocene extension within a northtrending rift zone (Janecke, 1994; Janecke and others, 1999). 

The Armstead normal fault in its footwall (Lowell, 1965; Coryell 

and Spang, 1988) is probably a smaller localized normal fault. 

Cross-cutting relationships with syntectonic basin-fill deposits show 

that several generally west-southwest-dipping low-angle normal 

faults were active sequentially during this major pulse of normal 

faulting (Blankenau, 1999; VanDenburg, 1997; VanDenburg and 

others, 1998). As much as 2.5 km of syntectonic deposits are 

still preserved in the hanging walls of these normal faults despite 

subsequent uplift and exhumation by tributaries of the Missouri 

and Salmon Rivers systems (Janecke and others, 1999; Blankenau, 1999; VanDenburg and others, 1998).

Faults of sets 3 and 6 might be mistaken for one another because they have similar orientations. Most of the older normal 

faults of set 3, however, strike in a more northerly direction, and 

unpublished seismic data show that faults of set 3 are significantly 

shallower than the active normal faults of set 6. The presence of 

fault scarps, the cross-cutting relationships with other faults of 

known age, the age of the basin-fill deposits in the hanging walls 

of the faults (VanDenburg, 1997; VanDenburg and others, 1998; 

Janecke and others, 1999; Blankenau, 1999), the presence of 

pediments, and the flow directions of modern streams adjacent to 

the normal faults were all considered when the faults were assigned to a particular set. Minor reactivation of parts of some 

faults of set 3 might have occurred during phase 6.

Fault set 2. Middle Eocene northwest-dipping normal 

faults (shown in purple on the map)

Widely scattered, small northeast-striking normal faults extended the Beaverhead Mountains during middle Eocene Challis volcanism. Overlap relationships and along-strike changes in the 

Tertiary stratigraphy in the footwall and hanging walls of these 

northeast-striking normal faults show that they were active during 

or slightly after middle Eocene Challis magmatism (M’Gonigle and 

others, 1991; VanDenburg and others, 1998). Some faults of this 

set may have been reactivated during phase 4. This tectonic 

event was a relatively minor one in this region, in contrast to regions to the west near the core of the Challis volcanic field (Kiilsgaard and others, 1986; Janecke, 1992) and northwest in the Bitterroot metamorphic core complex (Foster and Fanning, 1997).

Fault set 1. Pre-middle Eocene low-angle normal faults 

with original southwest dips (shown in red on the map)

Normal faults of set 1 are the oldest normal faults in the Tendoy and Beaverhead Mountains and are exposed northeast of Leadore, Idaho, on both sides of the continental divide. Ruppel 

(1968), Staatz (1973, 1979), and Lucchitta (1966) originally mapped these structures as thrust faults and Skipp (1988) noted that 

they were in fact normal faults. Critical, but localized, overlap relationships show that these low-angle normal faults formed before 

middle Eocene Challis volcanism (VanDenburg and others, 1998). 

These normal faults are potentially the largest faults in the region, 

because they omit a large stratigraphic section in a hanging-wallflat geometry, and consistently place upper Paleozoic rocks on 

Middle Proterozoic Belt Supergroup rocks (VanDenburg and others, 1998). The Goat Mountain, Grizzly Hill, Phosphoria Klippe, 

and Wild Cat Creek faults are probably offset segments of a once 

continuous normal fault. The continuity of the Middle Proterozoic 

footwall and upper Paleozoic hanging wall adjacent to the younger 

Divide normal fault of Lucchitta (1966) suggests that the Divide 

fault probably cuts out the southeast continuation of the Goat 

Mountain–Grizzly Hill–Phosphoria Klippe–Wild Cat Creek fault 

system (Goat-Cat fault system). 

VanDenburg (1997) and VanDenburg and others (1998) incorrectly named this Goat-Cat low-angle normal fault system the Divide Creek fault, based on an erroneous correlation with the Divide Creek fault of Skipp (1984, 1985) in the southern Beaverhead Mountains. Subsequent mapping (S.U. Janecke, unpublished) shows that the Divide Creek fault is too young to correlate 

with these pre-middle Eocene normal faults because it cuts the 

middle Eocene Challis Volcanic Group. The Divide Creek fault 

(sensu strictu) is now included in fault set 5.

Presently, the Goat Mountain, Grizzly Hill, Phosphoria Klippe, 

and Wild Cat Creek normal faults dip gently south or southwest 

(Lucchitta, 1966; VanDenburg 1997). The Goat Mountain fault 

segment, north of Little Eightmile Creek, restores to a gentle 

southwest dip after the effects of subsequent tilting are removed 

(VanDenburg and others, 1998). These normal faults thus indicate that northeast-southwest extension had begun in the region 

prior to middle Eocene volcanism (VanDenburg, 1997). Some of 

the normal slip on the enigmatic south-dipping fault D of 

M’Gonigle (1993, 1994) may also date from this period. Fault D 

has been interpreted as a depositional contact (Skipp, 1988), as a 

thrust fault (Ruppel, 1978), and as a thrust that was later reactivated as a normal fault (M’Gonigle, 1993, 1994).

DISCUSSION AND REGIONAL IMPLICATIONS

The history of Tertiary extension in the Tendoy and Beaverhead Mountains is complex, but detailed mapping and analysis 

show that the seemingly random array of normal faults represents a fairly orderly sequence of more than six temporal and geometric 

sets of normal faults (Janecke, 2000). Within each phase of normal faulting a consistent extension direction prevails. Extension was northeast-southwest during phases 1, 3, and 6, whereas northwest-southeast extension characterized phases 2 and 4. The newly identified fifth phase of extension accommodated northsouth extension during latest Miocene to Pleistocene(?) time.

The largest normal faults to form in the region accommodated 

northeast-southwest extension during phases 1, 3, and 6. These three phases also appear to be the most protracted phases of extension. Phase 3, which produced among the largest faults in the area, persisted from the waning phases of Challis volcanism (about 45 Ma) into early Miocene time (about 20 Ma) (VanDenburg and 

others, 1998; Janecke and others, 1999), possibly until about 15 Ma (S.U. Janecke and M. Perkins, unpublished data). 

The overall parallelism between the largely southwest-dipping 

normal faults of sets 1, 3, and 6 and the preexisting contractional structures of the Sevier belt (Janecke and others, 1999) suggests that all three sets are due to gravitational collapse of the Sevier orogenic belt. Royse and others (1975) and Constenius (1996) documented the strong structural control of thrust belt structures on younger normal faults in the Basin and Range province. Gravitational collapse of the fold and thrust belt cannot explain the northeast-southwest extension that is affecting the foreland farther to the east, in the Ruby Mountains and adjacent areas. It is noteworthy that the same northeast-southwest extension direction persists into the Ruby Range in the foreland east of the Sevier fold 

and thrust belt during the current tectonic regime (set 6) (Stickney and Bartholomew, 1987; Fritz and Sears, 1993).

Two sets of normal faults show an especially strong relationship with preexisting contractional structures. Both the prevolcanic normal faults of set 1 and the basin-forming normal faults of set 

3 are preferentially localized within structural culminations that formed during the Late Cretaceous to early Tertiary Sevier orogeny (fig. 2). Janecke and others (2000c) briefly outlined the evidence for and positions of these structural culminations, and they showed that the region between Salmon, Idaho, and the present 

Nicholia Creek basin contained numerous structural highs in Late Cretaceous to early Tertiary time. The prevolcanic normal faults of set 1 bound the southwest margin of the combined Island Butte–Carmen culminations (fig 2; Janecke and others, 2000c). Farther to the west, outside the map area, pre-volcanic normal faults in the northern Lemhi Range (Tysdal, 1996a, b; Tysdal and Moye, 1996) extend the Hayden Creek culmination in a similar manner (fig. 2; Janecke and others, 2000c). The Hayden Creek culmination is the next major culmination southwest of the Carmen and Island Butte culminations (Janecke and others, 2000c).

The detachment faults of set 3 further collapsed the preexisting 

culminations. The Salmon basin detachment fault collapsed the 

Carmen culmination on the northeast side of the Leesburg syncline (fig. 2). The Muddy-Grasshopper and Maiden Peak normal faults collapsed the Maiden Peak culmination and its poorly characterized continuation in the northern Tendoy Mountains (Hait and M’Gonigle, 1988; S.U. Janecke, unpublished mapping; Lonn and others, 2000), whereas the Deadman fault reversed the structural relief produced by the Island Butte culmination (fig. 2). The association between the largest normal faults and the culminations suggests that these normal faults were localized within these major uplifts in the Cordilleran fold and thrust belt. Lesser extensional strains characterize areas adjacent to the culminations.

Cross faults are those normal faults at a high angle to the overall north-northwest structural grain of the major contractional and 

extensional features in the area. They developed during phases 2, 4, and 5, in middle Eocene, late early to early middle Miocene, and latest Miocene to Pleistocene(?) time. The cross faults in the Tendoy and Beaverhead Mountains are typically small-offset normal faults when compared with the structures that developed during phases 1, 3, and 6. The cross faults appear to record brief interruptions in a protracted interval of northeast-southwest extension (VanDenburg and others, 1998). Phase 2, for example, began after 49.5 Ma and ended before 45 Ma (Janecke, 1992; VanDenburg and others, 1998). Phase 4 is best dated east of the Tendoy Mountains as a late early to early middle Miocene event 

(Sears and others, 1995; Sears and Fritz, 1998). In the Horse Prairie rift basin, VanDenburg and others (1998) showed that the northwest-dipping Little Eightmile Creek normal fault was active during a short interval around 17 Ma, in late early Miocene time. Phase 5 may represent a transient change in the extension direction as this part of North America migrated past the Yellowstone hotspot (Janecke and others, 2000a).

The tectonic significance of these three “cross” phases of extension is incompletely understood. Phase 2, during Challis volcanism, may be related to arc-parallel extension during northeastdirected subduction of the Farallon plate beneath North America 

from 50 to 45 Ma (Janecke, 1992). Phase 4 was a time of gravitational collapse of the northeast-trending basement cored uplifts 

of the southwest Montana foreland province (Sears and others, 1995). These uplifts first formed in Cretaceous time and may have been active into early Tertiary time (Scholten and others, 1955; Ryder and Scholten, 1973; Perry and others, 1988; McBride, 1988; Janecke and others, 2000b). It is unclear why the subsequent collapse along northwest-dipping normal faults occurred more than 30 m.y. after the end of shortening during a relatively short time period. Sears (1995) proposed that initiation of the Yellowstone hot spot about 16.5 Ma may have triggered this event. Phase 5 produced small east-west-striking normal faults over a broad geographic region. Flexure both along and toward 

the northeast-trending axis of the eastern Snake River Plain may have produced small bending-related normal faults in the vicinity of the Yellowstone hot spot that nucleated the normal faults of set 5 (Janecke and others, 2000a). The north-dipping Centennial and eastsoutheast-dipping Teton normal faults are examples of such faults that are forming around the current position of the hot spot (Hamilton, 1960; Hamilton and Myers, 1966). The older east-west-striking normal faults in the Tendoy and Beaverhead Mountains may have formed when the hot spot was southwest of it current position at the northeast edge of the Heise volcanic field (Janecke and others, 2000a).

Although the extension direction changed five times in the 

Tendoy and Beaverhead Mountains during the Cenozoic, the overall pattern can be viewed as a more than 50-m.y. history of largely northeast-southwest extension, which was punctuated by brief intervals of fairly minor northwest-southeast and north-south extension.

ACKNOWLEDGMENTS

Betty Skipp, William Perry, John M’Gonigle, and Karl Kellogg, 

each of the U.S. Geological Survey (USGS) in Denver, kindly shared some of their unpublished mapping. Mapping was partially supported by a grant from the National Science Foundation (EAR 

9317395) and two contracts with the USGS. Personnel of the Dillon office of the USDA Forest Service provided aerial photos.

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