MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE …Compilation of a 1:100,000-scale map of normal...
Transcript of MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE …Compilation of a 1:100,000-scale map of normal...
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 0001 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 QUADRANGLESNATIONAL GEODETIC VERTICAL DATUM OF 1929
113°45'
113°45'
113°
112°30'
112°30'
45°05'
45°
113°30'
113°15'
45°05'
45°
112°45'
44°45'
44°30'
112°45' 113°
44°30'
113°15'
113°30'
44°45'
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 concealed
Normal 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 showoblique-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 concealed
Monocline—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 constraintsextensional event
6 Green Moderate Active or potentially active normal faults in the current tectonicregime. The age of initial slip is uncertain.
5 Brown Uncertain Many of the east-west striking normal faults cut the Paleogenenormal 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 normalfaults 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 Ma
1 Red Major? Older than the Challis Volcanic Group, pre 49.5 Ma
Black Variable Faults of uncertain age
Youngest
Oldest
Salmon
basindetachm
ent
Beaverhead fault
Agenc
y-Yea
rian
fault Lemhi Pass fault
Dan Patch fault
Bloody D
ick Creek fault
Peterson Creek fault
Ree
seC
r. flt
.
Reese
Cre
ek fa
ult
Agency-Yearian
fault
Goat Mountainfault
Eightmile Creek fault
Eightm
ileCreek
fault
Beaverhead fault
Maiden
Peak
fault
Muddy-G
rasshopperfault (M
.G.F.)
Arm
stead fault
M.G
.F.M
.G.F.
Kissick fault
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
manyadditional
small faults
Grouse Canyon fault (west segment)
Divide Creekfault
moreGrouse Canyon fault?
M.G.F.
M.G
.F.
M.G
.F.
M.G
.F.
Kate Creek fault
Deadman fault
Divide fault
Beaverhead fault
Lemhi
fault
Pass
faul
t
Can
yon
Roc
ky
Mou
ntai
nfault
Mid
dle
Creek fault
Little Bear
faul
t
Trai
l
Whe
etip
fault D
Scott Canyon fault
Crooked Creek fault
Middle
Creek fault
fault
Black
Canyon
Medicine Lodge
thrust
Former
Lima thrust system
Creek fault
Little
She
ep
Sag
e C
reek
faul
t (F.
S.C
.F.)
Creek
fault
Deadman fault
Nic
holia
West
fault zone
Side
Deadm
an fault
Cabin thrust thrust
Lodge
Medicine stra
nd o
f Ellis
Peak f
ault z
one
M.L.T.
McKenzie Canyon
McKenzie Canyonthrust
mid basin faults
Monument Hills
fault zone
Sage
fault
Griz
zly
Hill
Phosphoria Klippe
CreekWild Cat
fault
(magenta and purple)
(magenta and purple)
Divide fault
Little
Little
Schoo
l fa
ult
zone
thrust
?
?
?
??
??
?
??
?
?
?
?
? ?
?
??
??
??
??
?
?
?
?
?
??
? ??
?
?
?
?
?
??
? ?? ?
?
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?
Janecke,photointerpretation
Reinterpretation andphotointerpretation of
Sadler (1980), Skipp and Perry
(unpublished mapping),McDowell (1992), Ryder and Scholten (1973), and Perryand others (1988)
Perry and Skipp,unpublished mapping
and
Skipp and others (1979)
Skipp (1984)
Haller(1988)
Scholten and Ram
spott (1968)
and Haller (1988)
Modified fromSkipp and others
(1988)Janecke, reconnaissanceand photointerpretation
Janecke and Skipp,
unpublished mapping
Skipp (1988)
Janecke,
photointerpretation,
aided by Kellogg,
unpublished mapping
and Scholten and
others (1955)
Janecke, photointerpretation andreconnaissance, aided by
Kellogg (unpublished mapping),Landis (1963), and
Ruppel (1993) Janecke andothers (1999) and
Janecke,unpublished mapping
Haller(1988)
Scholten and others (1955)
and photointerpretation
Modifiedfrom
McDowell(1992)
Dubois(1981)
Janecke,
photointerpretation
W. Perry,unpublished
mapping
Reinterpreted from
Lowell (1965) and
Coryell and Spang (1988)
M’G
onigle and Janecke,
unpublished mapping
M’Gonigle and others(1991)
M’Gonigle and Hait(1997)
Janceke,photo-
interpretationand
reconnaissance
Janecke
M’Gonigle(1993)
M’Gonigle(1994)
INDEX TO SOURCES OF DATA
M’Gonigle andDalrymple
(1996)
Janecke,unpublished
mapping, aided by Dubois (1981,
1982)
Janecke,M’Gonigle,and Perry,
unpublishedmapping
Janecke, reconnaissance mappingand Coppinger (1974)
VanDenburg and Janecke(see VanDenburg, 1997)
andVanDenburg and
others (1998)
Janeckeand
VanDenburg,unpublished mapping
andphotointerpretations
Reinterpretedfrom
Staatz (1973)
Staatz (1979)and
Janecke, reconnaissance
Lucchitta (1966)
Modified fromRuppel (1968) and
Haller (1988)
Reinterpreted fromAnderson (1961)
Blankenau and Janecke(see Blankenuau, 1999)
113°15' 113°30'
113°45'
113°
112°30'
112°45'
44°30'
44°15'
44°45'
45°
Mapping incomplete
Lonn and others
(2000)
Manuscript approved for publication April 6, 2001
Any use of trade names in this publication is fordescriptive purposes only and does not imply endorsement by the U.S. Geological Survey
This map was produced on request, directly fromdigital files, on an electronic plotter
For sale by U.S. Geological Survey Information ServicesBox 25286, Federal Center, Denver, CO 802251-888-ASK-USGS
A PDF for this map is available athttp://geology.cr.usgs.gov/greenwood-pubs.html
MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE TENDOY MOUNTAINS AND BEAVERHEAD RANGE, SOUTHWEST MONTANA AND EASTERN IDAHOBy
Susanne U. Janecke1, James J. Blankenau1*, Colby J. VanDenburg1**, and Bradley S. Van Gosen2
2001
1Department of Geology, Utah State University, Logan, UT2U.S. Geological Survey, Denver, COPresent address:*URS, Salt Lake City, UT**Durango, CO
U.S. DEPARTMENT OF THE INTERIORU.S. GEOLOGICAL SURVEY
MISCELLANEOUS FIELD STUDIES MAP MF–2362Version 1.0
Leadore
Lemhi Valley
Rai
lroa
d C
anyo
n
basin
Prairie
Horse
Lemhi
Pass
Salmonbasin
Grasshopper
basin
Montana
Medicine
Lodge
basin
MaidenPeak
Spur
EllisPeak
NicholiaCreek
basin
Beaverhead
Mountains
Mud Creek
Will
iam
sC
reek
Birch CreekValley
BannackPass
IdahoM
edicineLodge
Valley
Lima
Red R
ock valley
Tendoy
Mountains
Muddy
Creek
basin
45°N
Patterson culmination
MP
C
Yellowjacket
Mine culm
ination
Hayden Creek culmination
Cobalt culm
ination
Challis
Salmon
Ellis
Tendoy
Lemhi
Carmen
Lima
Cobalt
Middle F
ork Salm
on River
Sal
mon
Riv
er
Lem Peak fault
Beaverhead Divide fault
MO
NTA
NA
IDA
HO
Southern Lemhi
Range
S. Beaverhead
Mountains
MLT
?
MLT
Lost River R
ange
Brushy Gulch thrust
Leesburg syncline
foreland
Blacktai
l-Sno
wcr
est
?
?
?
?
?
????
Gra
ssho
pper
thru
st
Erm
ont t
hrus
t
McKenzie Canyon
thrust
MLT
Tendoy Mountains
N. Lem
hi
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
Anticline
Syncline
Thrust fault
Low-angle normal fault
Normal fault
Strike and dip of bedding
Abbreviations: BDCF, BloodyDick Creek fault; MPF, MaidenPeak fault; MGF, Muddy-Grasshopper fault; MPC, Maiden Peak culmination; MLT, Medicine Lodge thrust.
20 KILOMETERS
SCALE
0
BD
CFIron Lake thru
stYac
Yac
Yac
Yac
Yac
Yac
Yac
Modern rivers
Porphyritic rapakivi granite
Ordovician Beaverheadpluton
Salm
on basin detachment fault
Island Butte culmination.
MP
F MG
F
Divide fault
Deadm
an fault Divide con-glomerate
44°30'N
uplift
Tendoy thrust
Cabin thrust
Study areashown on map
Hawley Creek thrust
Divideconglomerate
Carmen culm
ination
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
Montana
Medicine
Lodge
basin
DILLON
DELL
LIMALEADORE
SALMON
Sage Creek basin
MuddyCreek
basin
0 10 20
Kilometers
basinSalmon
Beaverhead
Mountains
Tendoy
Mountains
BlacktailMountains
Lemhi
Range
Montana
Idaho
Maiden P
eak
Spur
113°30' 113°00' 112°30'114°00'
45°15'
44°45'
44°30'
SBF
MG
F
MP
FBF
Montana
IdahoLocation of figure
UD
MG
F
DF
LPF45°00'
BTF
RRF
BDF
Reservoir
Mountain range
Quaternary-Miocene basin fill
Early Miocene-Eocene basin fill
Mountains
basin
basi
n
Prairie
Horse
Gra
ssho
pper
Big Holebasin
MedicineLodge
basin
Idaho
MF
Nicholia
basin
Creek
Upper Lemhi Valley
Birch Creek Valley
Red R
ock Valley
AY
FBF
44°15'
Faults—Bar and ball on downthrown side of steeper normal fault. Rectangle on hang-ing wall of low-angle normal fault
Set 1 Set 3 Set 6
Figure 1. Map displaying the geographic setting of the study area (magenta out-line), some of the larger normal faults in the region, and locations of rift ba-sins. 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). Ab-breviations 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 detach-ment fault; MPF, Maiden Peak fault; RRF, Red Rocks fault; SBF, Salmon ba-sin detachment fault; and UDMGF, upper detachment fault of the Muddy-Grasshopper 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 oth-ers (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 northeast- and east-striking normal faults are also prominent. Northeast-striking normal faults are subparallel to the traces of southeast-di-rected thrusts that shortened the foreland during the Laramide or-ogeny. It is unlikely that the northeast-striking normal faults reac-tivated 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 deforma-tional fabrics (Lowell, 1965; M’Gonigle, 1993, 1994; M’Gonigle and Hait, 1997; M’Gonigle and others, 1991). The largest nor-mal faults in the area are southwest-dipping normal faults that lo-cally reactivate thrust faults (fig. 1). Normal faulting began before middle Eocene Challis volcanism and continues today. The exten-sion 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 peri-od. 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 map-ping in the southern Nicholia Creek basin and adjacent areas re-vealed the presence of an additional set of east-west striking nor-mal faults (set 5) that formed in latest Miocene to Pleistocene(?) time before the currently active system of northwest-striking nor-mal 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 separat-ed into distinct sets based on geometry and age because some folds formed over extended periods of time and many have uncer-tain origins (for example, VanDenburg and others, 1998; Blanke-nau, 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 exten-sion in the Tendoy-Beaverhead region (fig. 1). Basins that devel-oped 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; Ja-necke 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 (Van-Denburg, 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 Bea-verhead 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 oth-ers, 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 approximate-ly 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 con-sistent 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; Stew-art, 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 south-dipping 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 roll-over 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; Du-bois, 1982) and Polaris, Montana (Zimbelman, 1984). Few east-west striking normal faults were documented in the study area pri-or to this study (for example, Skipp and others, 1979), in part be-cause faults of this orientation are typically small and discontinu-ous. 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 in-completely known and faults of this orientation may have been ac-tive during more than one deformational event. The east-west-striking faults clearly postdate and offset normal faults formed dur-ing 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-west-striking 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 east-west-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 Centen-nial 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). Far-ther 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 proba-bly reflect transient stress reorientation in the vicinity of the Yel-lowstone 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 (Ja-necke 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 con-firm the Miocene age of the latter two normal faults. Other north-east-striking normal fault zones that may have developed at this time include swarms of northeast-striking normal faults around El-lis Peak in the western Tendoy Mountains (the Ellis Peak fault sys-tem), and spaced normal faults in the Beaverhead Mountains be-tween Lemhi Pass and Railroad Canyon.
The Little Sheep Creek normal fault southwest of Lima, Mon-tana, 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 pre-ferred 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 prob-ably reactivated northwest-dipping foreland thrusts of the Blacktail-Snowcrest 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 (Low-ell, 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-Yeari-an, Lemhi Pass, Maiden Peak, Deadman, and Muddy-Grasshop-per 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 (Lucchit-ta, 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 break-away fault for Eocene to Oligocene extension within a north-trending 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; Blanke-nau, 1999; VanDenburg and others, 1998).
Faults of sets 3 and 6 might be mistaken for one another be-cause 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 as-signed 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 extend-ed the Beaverhead Mountains during middle Eocene Challis vol-canism. 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 re-gions to the west near the core of the Challis volcanic field (Kiils-gaard and others, 1986; Janecke, 1992) and northwest in the Bit-terroot 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 Ten-doy and Beaverhead Mountains and are exposed northeast of Lea-dore, Idaho, on both sides of the continental divide. Ruppel (1968), Staatz (1973, 1979), and Lucchitta (1966) originally map-ped these structures as thrust faults and Skipp (1988) noted that they were in fact normal faults. Critical, but localized, overlap rela-tionships 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-wall-flat geometry, and consistently place upper Paleozoic rocks on Middle Proterozoic Belt Supergroup rocks (VanDenburg and oth-ers, 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) incor-rectly named this Goat-Cat low-angle normal fault system the Di-vide Creek fault, based on an erroneous correlation with the Di-vide Creek fault of Skipp (1984, 1985) in the southern Beaver-head Mountains. Subsequent mapping (S.U. Janecke, unpublish-ed) 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 indi-cate 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 reactivat-ed as a normal fault (M’Gonigle, 1993, 1994).
DISCUSSION AND REGIONAL IMPLICATIONS
The history of Tertiary extension in the Tendoy and Beaver-head 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 nor-mal 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 north-south 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 ex-tension. 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. Gravi-tational 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 note-worthy that the same northeast-southwest extension direction per-sists 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 relation-ship with preexisting contractional structures. Both the prevolcan-ic 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 oroge-ny (fig. 2). Janecke and others (2000c) briefly outlined the evi-dence 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 Car-men 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 syn-cline (fig. 2). The Muddy-Grasshopper and Maiden Peak normal faults collapsed the Maiden Peak culmination and its poorly char-acterized 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 struc-tural 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 over-all 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 nor-mal faults when compared with the structures that developed dur-ing phases 1, 3, and 6. The cross faults appear to record brief in-terruptions in a protracted interval of northeast-southwest exten-sion (VanDenburg and others, 1998). Phase 2, for example, be-gan after 49.5 Ma and ended before 45 Ma (Janecke, 1992; Van-Denburg 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 direc-tion as this part of North America migrated past the Yellowstone hotspot (Janecke and others, 2000a).
The tectonic significance of these three “cross” phases of ex-tension is incompletely understood. Phase 2, during Challis vol-canism, may be related to arc-parallel extension during northeast-directed subduction of the Farallon plate beneath North America from 50 to 45 Ma (Janecke, 1992). Phase 4 was a time of gravi-tational 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; Mc-Bride, 1988; Janecke and others, 2000b). It is unclear why the subsequent collapse along northwest-dipping normal faults occur-red more than 30 m.y. after the end of shortening during a rela-tively 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 over-all pattern can be viewed as a more than 50-m.y. history of largely northeast-southwest extension, which was punctuated by brief in-tervals of fairly minor northwest-southeast and north-south exten-sion.
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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