U.S. DEPARTMENT OF THE INTERIOR MISCELLANEOUS FIELD STUDIES MAP MF–2362 U.S. GEOLOGICAL SURVEY Version 1.0

OVERVIEW 6.62-Ma tuff of Blacktail (age from Morgan and others, 1998) and DISCUSSION AND REGIONAL IMPLICATIONS Geological Society of America Abstracts with Programs, v. 113°45' MAP EXPLANATION overlying undated gravel deposits (Skipp, 1984; Skipp and others, 32, no. 5, p. A-12. 45°05' 113°30' Compilation of a 1:100,000-scale map of normal faults and 1979). Skipp (1984) assigned a Pliocene to early Pleistocene(?) The history of Tertiary extension in the Tendoy and Beaver- Janecke, S.U., VanDenburg, C.J., and Blankenau, J.J., 1998, age to these gravel deposits. Altogether the data suggest a latest head Mountains is complex, but detailed mapping and analysis Geometry, mechanisms and significance of extensional folds 113°15' extensional folds in southwest Montana and adjacent Idaho reveals Miocene to Pleistocene(?) age for the normal faults of set 5. from examples in the Rocky Mountain Basin and Range prov- Salmon a complex history of normal faulting that spanned at least the last show that the seemingly random array of normal faults represents ° 113 FAULTS, FOLDS, AND LINEAMENTS 50 m.y. and involved six or more generations of normal faults. The east-west-striking normal faults are parallel to the Centen- a fairly orderly sequence of more than six temporal and geometric ince, U.S.A.: Journal of Structural Geology, v. 20, no. 7, p.

45°05' The map is based on both published and unpublished mapping nial normal (Witkind, 1975) to the east of the map area and sets of normal faults (Janecke, 2000). Within each phase of nor- 841–856. Low-angle normal fault—Tics on hanging wall. Dashed where approximate; dotted where concealed and shows normal faults and extensional folds between the valley probably have a common origin (Janecke and others, 2000a). mal faulting a consistent extension direction prevails. Extension Janecke, S.U., VanDenburg, C.J., Blankenau, J.J., and basin Grasshopper Normal faults with east-west strikes are currently active in a 100- was northeast-southwest during phases 1, 3, and 6, whereas M’Gonigle, J.W., 2000c, Long-distance longitudinal transport Bloody Dick Creek fault of the Red Rock River of southwest Montana and the Lemhi and to 150-km-wide belt west of the Yellowstone caldera, in a region northwest-southeast extension characterized phases 2 and 4. The of gravel across the Cordilleran thrust belt of Montana and Normal fault that cuts out or reactivates a —Fault places older rocks on younger rocks. Birch Creek valleys of eastern Idaho between latitudes 45°05' N. directly east of the map area (Janecke and others, 2000a). Far- newly identified fifth phase of extension accommodated north- Idaho: Geology, v. 28, no. 5, p. 439–442. basin Sawteeth on hanging wall. Dashed where approximate; dotted where concealed and 44°15' N. in the Tendoy and Beaverhead Mountains. Some Salmon of the unpublished mapping has been compiled in Lonn and oth- ther to the west and northwest, east-west-striking normal faults are south extension during latest Miocene to Pleistocene(?) time. Kiilsgaard, T.H., Fisher, F.S., and Bennett, E.H., 1986, The no longer active. Instead, the active normal faults strike northwest The largest normal faults to form in the region accommodated Trans-Challis fault system and associated precious metal de- Muddy-Grasshopper ers (2000). Many traces of the normal faults parallel the generally Lemhi Moderately or steeply dipping normal fault—Bar and ball on downthrown side; opposed arrows show northwest to north-northwest structural grain of the preexisting (set 6) and appear to record “typical” northeast-southwest Basin northeast-southwest extension during phases 1, 3, and 6. These posits, Idaho: Economic Geology, v. 81, no. 3, p. 721–724. oblique-slip movement. Dashed where approximate; dotted where concealed basin Sevier fold and thrust belt and dip west-southwest, but northeast- and Range extension. The east-west-striking normal faults proba- three phases also appear to be the most protracted phases of ex- Landis, C.A., Jr., 1963, Geology of the Graphite Mountain–Tepee and east-striking normal faults are also prominent. Northeast- bly reflect transient stress reorientation in the vicinity of the Yel- tension. Phase 3, which produced among the largest faults in the Mountain area Montana-Idaho: University Park, Pa., Pennsyl- Pass Anticline—Arrow indicates plunge direction. Dashed where approximate; dotted where concealed lowstone hot spot due to subsidence toward the growing eastern area, persisted from the waning phases of Challis volcanism (about vania State University M.S. thesis, 153 p. detachment striking normal faults are subparallel to the traces of southeast-di- fault ? rected thrusts that shortened the foreland during the Laramide or- Snake River Plain. The inactive east-west-striking normal faults in 45 Ma) into early Miocene time (about 20 Ma) (VanDenburg and Lonn, J.D., Skipp, Betty, Ruppel, E.T., Janecke, S.U., Perry, Syncline—Arrow indicates plunge direction. Dashed where approximate; dotted where concealed ogeny. It is unlikely that the northeast-striking normal faults reac- the Tendoy and Beaverhead Mountains may have formed when others, 1998; Janecke and others, 1999), possibly until about 15 W.J., Jr., Sears, J.W., Bartholomew, M.J., Stickney, M.C., tivated fabrics in the underlying Precambrian basement, as has the Yellowstone hot spot was southwest of its current position (Ja- Ma (S.U. Janecke and M. Perkins, unpublished data). Fritz, W.J., Hurlow, H.A., and Thomas, R.C., 2000, Prelimi-

Arm necke and others, 2000a). The overall parallelism between the largely southwest-dipping nary geologic map of the Lima 30' x 60' quadrangle, south- fault (M.G.F.) —Arrow on steep limb of the fold been documented elsewhere in southwestern Montana (Schmidt and others, 1984), because exposures of basement rocks in the normal faults of sets 1, 3, and 6 and the preexisting contractional west Montana: Montana Bureau of Mines and Geology Open- stead fault Fault set 4. Early to middle Miocene northwest-dipping structures of the Sevier belt (Janecke and others, 1999) suggests Lineament—Dashed where uncertain map area exhibit north-northwest- to northwest-striking deforma- File Report MBMG 408, scale 1:100,000, includes pam- ° normal faults (shown in magenta on the map) 45 tional fabrics (Lowell, 1965; M’Gonigle, 1993, 1994; M’Gonigle that all three sets are due to gravitational collapse of the Sevier phlet. and Hait, 1997; M’Gonigle and others, 1991). The largest nor- orogenic belt. Royse and others (1975) and Constenius (1996) Lowell, W.R., 1965, Geologic map of the Bannack-Grayling area Normal faults of fault set 4 strike northeast, at a high angle to mal faults in the area are southwest-dipping normal faults that lo- documented the strong structural control of thrust belt structures Beaverhead County, Montana: U.S. Geological Survey Mis- AGES OF NORMAL FAULTS the overall northwest structural grain of the fold and thrust belt cally reactivate thrust faults (fig. 1). Normal faulting began before on younger normal faults in the Basin and Range province. Gravi- cellaneous Geologic Investigations Map I–433, scale and to most of the normal faults of the region, but parallel to the tational collapse of the fold and thrust belt cannot explain the ? middle Eocene Challis volcanism and continues today. The exten- 1:31,680, includes 6-page pamphlet. 45° foreland uplifts to the east. These faults appear as widely spaced Fault set Color Magnitude of this Relative and absolute age constraints sion direction flipped by about 90° four times. northeast-southwest extension that is affecting the foreland farther Lucchitta, B.K., 1966, Structure of the Hawley Creek area, Idaho- fault zones. Faults of set 4 formed in early to middle Miocene Lemhi extensional event to the east, in the Ruby Mountains and adjacent areas. It is note- Montana: University Park, Pa., Pennsylvania State University ? Lemhi Pass fault time (for example, Fritz and Sears, 1993; Sears and Fritz, 1998), worthy that the same northeast-southwest extension direction per- Ph.D. dissertation, 235 p. fault INTRODUCTION and are relatively rare in the Tendoy and Beaverhead Mountains. Youngest 6 Green Moderate Active or potentially active normal faults in the current tectonic sists into the Ruby Range in the foreland east of the Sevier fold McBride, B.C., 1988, Geometry and kinematics of the central Similar normal faults to the northeast bound the southeast margin and thrust belt during the current tectonic regime (set 6) (Stickney regime. The age of initial slip is uncertain. Snowcrest Range—A Rocky Mountain foreland uplift in Pass Normal faults in southwest Montana and adjacent Idaho were of the Miocene Ruby and Beaverhead half and appear to and Bartholomew, 1987; Fritz and Sears, 1993). southwestern Montana: Kalamazoo, Mich., Western Michigan divided into distinct fault sets where cross-cutting relationships with reactivate southeast-vergent basement-cored thrust sheets of the Two sets of normal faults show an especially strong relation- University M.S. thesis, 267 p., 8 plates. 5 Brown Uncertain Many of the east-west striking normal faults cut the Paleogene datable features indicated the approximate age of slip across them. foreland (McBride, 1988; Fritz and Sears, 1993; Sears and Fritz, Agency-Yearian ship with preexisting contractional structures. Both the prevolcan- McDowell, R.J., 1992, Effects of synsedimentary basement tec- normal faults (blue) and are cut by the active Basin and Range faults Absolute and relative ages of normal faults are best constrained at 1998). Three northwest-dipping normal faults were included in ic normal faults of set 1 and the basin-forming normal faults of set tonics on fold-thrust belt geometry, southwestern Montana: (green). The east-west normal faults may have formed during more the sites of detailed studies in the southern Beaverhead Mountains this fault set: the Little Eightmile Creek fault (VanDenburg and Dan Patch fault ? Montana 3 are preferentially localized within structural culminations that Lexington, Ky., University of Kentucky Ph.D. dissertation, than one episode of extension. All east-west normal faults are (Skipp, 1984, 1985), the Horse Prairie basin (VanDenburg, 1997; others, 1998), and the newly identified Little Sheep Creek and formed during the Late Cretaceous to early Tertiary Sevier oroge- 328 p. VanDenburg and others, 1998), the southeast part of the Salmon Nicholia School fault zones. Further mapping is needed to con- brown, regardless of their age. ny (fig. 2). Janecke and others (2000c) briefly outlined the evi- M’Gonigle, J.W., 1993, Geologic map of the Medicine Lodge basin (Blankenau, 1999), and the Muddy Creek basin (Janecke firm the Miocene age of the latter two normal faults. Other north- dence for and positions of these structural culminations, and they Peak quadrangle, Beaverhead County, southwest Montana: 4 Magenta Minor Late early Miocene normal faults with northeast strikes. Little and others, 1999) (fig. 1). Faults of uncertain age are shown in east-striking normal fault zones that may have developed at this showed that the region between Salmon, Idaho, and the present M.G.F. ° U.S. Geological Survey Geologic Quadrangle Map GQ–1724, 112 45' black on the map. time include swarms of northeast-striking normal faults around El- Nicholia Creek basin contained numerous structural highs in Late ? Eightmile Creek and Little Sheep Creek faults are the main faults of scale 1:24,000. At least six geometric and temporally distinct sets of normal lis Peak in the western Tendoy Mountains (the Ellis Peak fault sys- Medicine this age. Nicholia School fault zone may have formed at this time. Cretaceous to early Tertiary time. The prevolcanic normal faults ——1994, Geologic map of the Deadman Pass quadrangle, Bea- ? faults are evident in the Tendoy and Beaverhead Mountains. Each tem), and spaced normal faults in the Beaverhead Mountains be- of set 1 bound the southwest margin of the combined Island verhead County, Montana, and Lemhi County, Idaho: U.S. set of normal faults, or group of related structures, is described as tween Lemhi Pass and Railroad Canyon. Butte–Carmen culminations (fig 2; Janecke and others, 2000c). 3 Blue Major, the largest normal Paleogene normal faults—most are middle Eocene to Oligocene, Geological Survey Quadrangle Map GQ–1753, scale McKenzie Canyon Monument Hills having formed during a distinct phase of extension, or time peri- The Little Sheep Creek normal fault southwest of Lima, Mon- Farther to the west, outside the map area, pre-volcanic normal Maiden 1:24,000. faults in the region but some may have been active into early or middle Miocene time. od. Fault sets are described from youngest (set 6) to oldest (set 1). tana, is here interpreted as a southwest continuation of the Sage faults in the northern Lemhi Range (Tysdal, 1996a, b; Tysdal and M’Gonigle, J.W., and Dalrymple, G.B., 1993, 40Ar/39Ar ages of formed at this time Cross-cutting relationships show that several systems of normal VanDenburg and others (1998) described five of these fault sets in thrust Creek normal fault that is exposed on the northwest flank of the Moye, 1996) extend the Hayden Creek culmination in a similar Challis volcanic rocks and the initiation of Tertiary sedimenta- faults were active sequentially during this time (VanDenburg and the Horse Prairie basin area and Blankenau (1999) detailed the Snowcrest Range (for example, McBride, 1988). Perry and others manner (fig. 2; Janecke and others, 2000c). The Hayden Creek Agency-Yearian ry basins in southwestern Montana: The Mountain Geologist, others, 1998; Blankenau, 1999; Janecke and others, 1999). multiple deformational events associated with the third event in (1988) had earlier interpreted the north-striking “Former Sage culmination is the next major culmination southwest of the Car- v. 30, no. 4, p. 112–118. Peak the southeast part of the Salmon basin. Sets 1–4 correspond Creek fault” as the southwest continuation of the Sage Creek fault men and Island Butte culminations (Janecke and others, 2000c). ——1996, 40Ar/39Ar ages of some Challis Volcanic Group rocks Beaverhead fault to sets 1–4 of VanDenburg and others (1998). Subsequent map- 2 Purple Minor Coeval with the Challis Volcanic Group, about 49.5 to about 45 Ma that is in the Snowcrest Range. The revised interpretation is pre- The detachment faults of set 3 further collapsed the preexisting and the initiation of Tertiary sedimentary basins in southwest- ping in the southern Nicholia Creek basin and adjacent areas re- ferred because the Little Sheep Creek fault is directly along strike culminations. The Salmon basin detachment fault collapsed the Armstead fault fault zone ern Montana: U.S. Geological Survey Bulletin 2132, 17 p., 1

? vealed the presence of an additional set of east-west striking nor- Oldest 1 Red Major? Older than the Challis Volcanic Group, pre 49.5 Ma and has the identical orientation as the Sage Creek fault to the Carmen culmination on the northeast side of the Leesburg syn- plate. mal faults (set 5) that formed in latest Miocene to Pleistocene(?) Spur northeast. The Sage Creek–Little Sheep Creek fault system prob- cline (fig. 2). The Muddy-Grasshopper and Maiden Peak normal ? M’Gonigle, J.W., and Hait, M.H., Jr., 1997, Geologic map of the Horse time before the currently active system of northwest-striking nor- ably reactivated northwest-dipping foreland thrusts of the Blacktail- ? ? Black Variable Faults of uncertain age faults collapsed the Maiden Peak culmination and its poorly char- Jeff Davis Peak quadrangle and eastern part of the Everson fault Lodge mal faults developed along the range fronts (Janecke and others, Snowcrest trend (McBride, 1988; Perry and others, 1988). The ? acterized continuation in the northern Tendoy Mountains (Hait Creek quadrangle, Beaverhead County, southwest Montana: ? Bell Canyon syncline fault 2000a; and unpublished mapping). The currently active normal origin of the other north-striking normal faults is less certain. It is and M’Gonigle, 1988; S.U. Janecke, unpublished mapping; Lonn U.S. Geological Survey Geologic Investigations Map I–2604, ? 114°00' 113°30' 113°00' 112°30' faults of set 6 correspond to faults formed during VanDenburg and unlikely that the northeast-striking normal faults reactivated fabrics and others, 2000), whereas the Deadman fault reversed the struc- Kissick fault ? M.G.F. scale 1:24,000. ? others’ (1998) phase 5. Big Hole in the underlying Precambrian basement, as has been documented tural relief produced by the Island Butte culmination (fig. 2). The M’Gonigle, J.W., Kirschbaum, M.A., and Weaver, J.N., 1991, basin 0 10 20 Extensional folds are very common in rift basins of southwest elsewhere in southwestern Montana (Schmidt and others, 1984), association between the largest normal faults and the culminations 45°15' Beaverhead Geologic map of the Hansen Ranch quadrangle, Beaverhead Kilometers Montana and eastern Idaho, and formed in association with the because exposures of basement rocks in the map area exhibit suggests that these normal faults were localized within these major Reese Creek fault fault County, southwest Montana: U.S. Geological Survey Geolog- BDF DILLON many normal faults that have extended the region since at least north-northwest- to northwest-striking deformational fabrics (Low- uplifts in the Cordilleran fold and thrust belt. Lesser extensional 113°45' SALMON ic Quadrangle Map GQ–1704, scale 1:24,000.

Eocene time (Janecke and others, 1998). Folds were not separat- Prairie Maiden ell, 1965; M’Gonigle 1993, 1994; M’Gonigle and Hait, 1997; Peterson Creek fault SBF strains characterize areas adjacent to the culminations. Morgan, L.A., Pierce, K.L., and McIntosh, W.C., 1998, The vol- Creek basin Grasshopper ed into distinct sets based on geometry and age because some M’Gonigle and others, 1991). Cross faults are those normal faults at a high angle to the over- (magenta and purple) M.G.F. Salmon Blacktail canic track of the Yellowstone hotspot—An update: Yellow- basin BTF folds formed over extended periods of time and many have uncer- all north-northwest structural grain of the major contractional and Cr. flt. Rocky stone Science, v. 5, p. 44. basin MPF ? tain origins (for example, VanDenburg and others, 1998; Blanke- Fault set 3. Late middle Eocene to early middle Miocene extensional features in the area. They developed during phases 2, Eightmile Pardee, J.T., 1950, Late Cenozoic block faulting in western Mon- Lemhi LPF nau, 1999; Janecke and others, 1998). Janecke and others west-southwest-dipping listric low-angle normal faults 4, and 5, in middle Eocene, late early to early middle Miocene, ° Mountains tana: Geological Society of America Bulletin, v. 61, no. 4, p. ? Peak ? 45 00' MGF (1998) illustrated the widely varying geometries of the extensional (shown in blue on the map) Reese Montana and latest Miocene to Pleistocene(?) time. The cross faults in the 359–406. fault Little Canyon fault ? folds and showed that many folds are oblique to the associated ? Horse Tendoy and Beaverhead Mountains are typically small-offset nor- Maiden PeakMedicine Perry, W.J., Jr., Haley, J.C., Nichols, D.J., Hammons, P.M., and ? DCFRange Spur ? AYF MontanaPrairie Sage normal fault. Some folds were active during sedimentation in the North-northwest-striking normal faults are among the largest in MF mal faults when compared with the structures that developed dur- Ponton, J.D., 1988, Interactions of Rocky Mountain foreland ? basin basin Creek rift basins and influenced the facies patterns in the basins (Janecke ? Mountain fault Idaho the region and include several major, middle Eocene to Miocene, ing phases 1, 3, and 6. The cross faults appear to record brief in- Goat BF Tendoy Red Rock Valley basin and Cordilleran thrust belt in Lima region, southwest Monta- Lodge and others, 1998, 1999; Blankenau, 1999). Other extension H low-angle normal faults of regional extent (detachment faults) (fig. terruptions in a protracted interval of northeast-southwest exten- na, in Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of ? ajorT basin folds deform preexisting synrift deposits. 1). From west to east, these are the Salmon basin, Agency-Yeari- sion (VanDenburg and others, 1998). Phase 2, for example, be- the Rocky Mountain foreland and the Cordilleran thrust belt: ? Rift basins formed during the third and sixth phase of exten- an, Lemhi Pass, Maiden Peak, Deadman, and Muddy-Grasshop- gan after 49.5 Ma and ended before 45 Ma (Janecke, 1992; Van- Geological Society of America Memoir 171, p. 267–290. ? 44°45' Mountains ? M.G.F. Mountains DELL sion in the Tendoy-Beaverhead region (fig. 1). Basins that devel- per low-angle normal faults (M’Gonigle and Dalrymple, 1993, Denburg and others, 1998). Phase 4 is best dated east of the ? Upper Muddy Pierce, K.L., and Morgan, L.A., 1992, The track of the Yellow- Lemhi RRF oped during phase 3 include the Eocene to Oligocene Salmon, DF Creek 1996; VanDenburg and others, 1998; Blankenau, 1999; Janecke Tendoy Mountains as a late early to early middle Miocene event Valley stone hot spot—Volcanism, faulting, and uplift, in Link, P.K., LEADORE basin Horse Prairie, Grasshopper, Montana Medicine Lodge, Muddy and others, 1999). The Divide fault, which may also have formed (Sears and others, 1995; Sears and Fritz, 1998). In the Horse UDMGF Mountains Kuntz, M.A., and Platt, L.B., eds., Regional geology of east- Creek, and Nicholia Creek rift basins (Fields and others, 1985; Ja- Eightmile Creek fault ? Montana LIMA during this phase of extension (Karl Kellogg, written commun., Prairie rift basin, VanDenburg and others (1998) showed that the ern Idaho and western Wyoming: Geological Society of ? Nicholia necke and others, 1998, 1999) (fig. 1). Deposition in the Horse 2000), is distinctly steeper than these other normal faults (Lucchit- northwest-dipping Little Eightmile Creek normal fault was active BF America Memoir 179, p. 1–53. (magenta and purple) Prairie half locally continued into the early Miocene (Fields Little McKenzie Canyon Location Creek ta, 1966). West-southwest-dipping normal faults of set 3 are the during a short interval around 17 Ma, in late early Miocene time. Reynolds, M.W., 1979, Character and extent of Basin-Range Idaho of figure and others, 1985; VanDenburg, 1997; VanDenburg and others, fault thrust basin main basin-forming normal faults in the study area (fig. 1). The Phase 5 may represent a transient change in the extension direc- fault D M.G.F. ° faulting, western Montana and east-central Idaho, in New- 44 30' 1998). The Red Rock, Birch Creek, upper Lemhi, and Idaho Muddy-Grasshopper normal fault is interpreted as the main break- tion as this part of North America migrated past the Yellowstone Kate Creek fault man, G.W., and Goode, H.D., eds., Basin and Range Sympo- Medicine Lodge valleys formed primarily during phase 6 (fig. 1). away fault for Eocene to Oligocene extension within a north- hotspot (Janecke and others, 2000a). ? sium and Great Basin Field Conference: Denver, Colo., Rocky Minor localized sediment related to phase 4 is preserved southwest Mountain range Idaho trending rift zone (Janecke, 1994; Janecke and others, 1999). The tectonic significance of these three “cross” phases of ex- Birch ? Mountain Association of Geologists, p. 185–193. Creek Medicine of Lima in the hanging wall of the Little Sheep Creek normal fault The Armstead normal fault in its footwall (Lowell, 1965; Coryell tension is incompletely understood. Phase 2, during Challis vol- Valley Lodge (Ryder and Scholten, 1973; Betty Skipp, unpublished mapping) Rodgers, D.W., and Janecke, S.U., 1992, Tertiary paleogeo- basin and Spang, 1988) is probably a smaller localized normal fault. canism, may be related to arc-parallel extension during northeast- ? ? Quaternary-Miocene basin fill and in the hanging wall of the Little Eightmile Creek fault (Van- graphic maps of the western Idaho–Wyoming– Peak fault zone Cross-cutting relationships with syntectonic basin-fill deposits show directed subduction of the Farallon plate beneath North America Grizzly Hill Montana thrust belt, in Link, P.K., Kuntz, M.A., and Platt, Denburg, 1997; VanDenburg and others, 1998). Farther to the that several generally west-southwest-dipping low-angle normal from 50 to 45 Ma (Janecke, 1992). Phase 4 was a time of gravi- 44°15' L.B., eds., Regional geology of eastern Idaho and western 44°45' northeast, however, major half grabens formed during phase 4 faults were active sequentially during this major pulse of normal tational collapse of the northeast-trending basement cored uplifts ? Ellis Early Miocene-Eocene basin fill Wyoming: Geological Society of America Memoir 179, p. Faults—Bar and ball on downthrown side of (Fritz and Sears, 1993; Sears and Fritz, 1998). faulting (Blankenau, 1999; VanDenburg, 1997; VanDenburg and of the southwest Montana foreland province (Sears and others, 83–94. Phosphoria Klippe Peak steeper normal fault. Rectangle on hang- others, 1998). As much as 2.5 km of syntectonic deposits are 1995). These uplifts first formed in Cretaceous time and may ing wall of low-angle normal fault Fault set 6. Miocene(?) to Holocene southwest- and Royse, F., Jr., Warner, M.A., and Reese, D.L., 1975, Thrust belt ? Reservoir still preserved in the hanging walls of these normal faults despite have been active into early Tertiary time (Scholten and others, Medicine Set 1 Set 3 Set 6 northeast-dipping normal faults (shown in green on the structural geometry and related stratigraphic problems, 113°30' ? strand of Ellis 44°45' subsequent uplift and exhumation by tributaries of the Missouri 1955; Ryder and Scholten, 1973; Perry and others, 1988; Mc- fault Wyoming-Idaho-northern Utah, in Bolyard, D.W., ed., Deep ? ? Red Rock valley map) and Salmon Rivers systems (Janecke and others, 1999; Blanke- Bride, 1988; Janecke and others, 2000b). It is unclear why the Creek drilling frontiers of the central Rocky Mountains: Denver, Co- Wild Cat Lodge nau, 1999; VanDenburg and others, 1998). subsequent collapse along northwest-dipping normal faults occur- Kate Creek fault lo., Rocky Mountain Association of Geologists, p. 41–54. Figure 1. Map displaying the geographic setting of the study area (magenta out- Fault set 6 includes normal faults that are known or suspected Faults of sets 3 and 6 might be mistaken for one another be- red more than 30 m.y. after the end of shortening during a rela- Ruppel, E.T., 1968, Geologic map of the Leadore quadrangle, line), some of the larger normal faults in the region, and locations of rift ba- to be active in the current tectonic regime. The Tendoy and Bea- cause they have similar orientations. Most of the older normal tively short time period. Sears (1995) proposed that initiation of Cabin thrust thrust Lemhi County, Idaho: U.S. Geological Survey Geologic Beaverhead fault Divide fault sins. Basins with predominantly Paleogene basin fill are distinguished from verhead Mountains lie within the Rocky Mountain Basin and faults of set 3, however, strike in a more northerly direction, and the Yellowstone hot spot about 16.5 Ma may have triggered this Muddy Quadrangle Map GQ–733, scale 1:62,500. basins with mostly Miocene to Quaternary basin fill. The age of the basin fill Range Province and the Intermountain seismic belt, along the unpublished seismic data show that faults of set 3 are significantly event. Phase 5 produced small east-west-striking normal faults M.L.T. ——1978, Medicine Lodge thrust system, east-central Idaho and Railroad Canyon Red Rock fault in the southern Big Hole basin is poorly known. The Sage Creek basin was northern arm of the Yellowstone seismic parabola (Scott and oth- shallower than the active normal faults of set 6. The presence of over a broad geographic region. Flexure both along and toward

M.G.F. southwestern Montana: U.S. Geological Survey Professional fault east of the zone of active rifting during the Paleogene (Janecke, 1994). Ab- ers, 1985; Anders and others, 1989; Smith and Arabasz, 1991; fault scarps, the cross-cutting relationships with other faults of the northeast-trending axis of the eastern Snake River Plain may Paper 1031, 23 p. breviations are: AYF, Agency-Yearian fault; BF, Beaverhead fault; BDF, Pierce and Morgan, 1992; Wernicke, 1992). Pardee (1950) and known age, the age of the basin-fill deposits in the hanging walls have produced small bending-related normal faults in the vicinity ——1993, Preliminary geologic map of the Tepee Mountain Bloody Dick Creek fault; BTF, Blacktail fault; DF, Deadman fault; LPF, Lemhi Reynolds (1979) first documented some of these active normal of the faults (VanDenburg, 1997; VanDenburg and others, 1998; of the Yellowstone hot spot that nucleated the normal faults of set fault quadrangle, Montana: Montana Bureau of Mines and Geology Deadman fault Pass fault; MF, Monument Hills fault zone; MGF, Muddy-Grasshopper detach- faults in southwest Montana. Extension in this area is currently Janecke and others, 1999; Blankenau, 1999), the presence of 5 (Janecke and others, 2000a). The north-dipping Centennial Open-File Report 317, scale 1:24,000, includes 8-page pam- ment fault; MPF, Maiden Peak fault; RRF, Red Rocks fault; SBF, Salmon ba- being accommodated by range-front faults that strike approximate- pediments, and the flow directions of modern streams adjacent to and eastsoutheast-dipping Teton normal faults are examples of Canyon ° phlet. ? sin detachment fault; and UDMGF, upper detachment fault of the Muddy- ly N. 35–50 W. (Crone and Haller, 1991) due to approximately the normal faults were all considered when the faults were as- such faults that are forming around the current position of the hot ° Ryder, R.T., and Scholten, Robert, 1973, Syntectonic conglomer- Creek Grasshopper detachment fault system. N. 45 E. extension (Stickney and Bartholomew, 1987). Normal signed to a particular set. Minor reactivation of parts of some spot (Hamilton, 1960; Hamilton and Myers, 1966). The older Tendoy ates in southwestern Montana—Their nature, origin, and tec- Mountain faults of set 6 in the northern half of the map area have very con- faults of set 3 might have occurred during phase 6. east-west-striking normal faults in the Tendoy and Beaverhead ? tonic significance: Geological Society of America Bulletin, v. Leadore sistent northwest strikes. Most of the active range-front faults dip Mountains may have formed when the hot spot was southwest of 84, p. 773–796. to the southwest, but northeast of a synclinal accommodation zone Fault set 2. Middle Eocene northwest-dipping normal it current position at the northeast edge of the Heise volcanic field Rocky Sadler, R.K., 1980, Structure and stratigraphy of the Little Sheep near the crest of the Tendoy Mountains most of the active range faults (shown in purple on the map) (Janecke and others, 2000a). Middle Creek area, Beaverhead County, Montana: Corvallis, Oreg., Lemhi Valley Little Bear front faults dip to the northeast (Stewart and others, 1998; Stew- Although the extension direction changed five times in the Oregon State University, M.S. thesis, 294 p. art, 1998; Sears and Fritz, 1998, plate 1). The southwest-dipping Tendoy and Beaverhead Mountains during the Cenozoic, the over- ? basin Widely scattered, small northeast-striking normal faults extend- Schmidt, C., Sheedlo, Mark, Werkema, Michael, 1984, Control of Monument Hills normal fault zone, at the northeast edge of the all pattern can be viewed as a more than 50-m.y. history of largely Creek fault ed the Beaverhead Mountains during middle Eocene Challis vol- range-boundary normal faults by earlier structures, southwest- map area, is an exception to this rule. The Monument Hills fault northeast-southwest extension, which was punctuated by brief in- canism. Overlap relationships and along-strike changes in the ern Montana: Geological Society of America Abstracts with zone may be antithetic to the Red Rock fault. tervals of fairly minor northwest-southeast and north-south exten- Beaverhead fault Mountains Tertiary stratigraphy in the footwall and hanging walls of these Programs, v. 16, no. 4, p. 253. M.G.F. Offset Quaternary deposits along the Red Rock, Monument northeast-striking normal faults show that they were active during sion. Hills, and Beaverhead normal faults show that these normal faults Scholten, Robert, Keenmon, K.A., and Kupsch, W.O., 1955, Ge- or slightly after middle Eocene Challis magmatism (M’Gonigle and ology of the Lima region, southwestern Montana and adja- are young and active (Haller, 1988, 1990; Bartholomew, 1989; others, 1991; VanDenburg and others, 1998). Some faults of this ACKNOWLEDGMENTS Crone and Haller, 1991). Some other normal faults lack fault cent Idaho: Geological Society of America Bulletin, v. 66, no. Divide fault set may have been reactivated during phase 4. This tectonic 4, p. 345–404, scale 1:130,000. scarps, yet they cut all older structures and are parallel to the Betty Skipp, William Perry, John M’Gonigle, and Karl Kellogg, event was a relatively minor one in this region, in contrast to re- Scholten, Robert, and Ramspott, L.D., 1968, mecha- known faults of set 6; these faults—the Kissick, Kate Creek, Rocky each of the U.S. Geological Survey (USGS) in Denver, kindly Lima gions to the west near the core of the Challis volcanic field (Kiils- nisms indicated by structural framework of central Beaverhead Creek fault Canyon, and Bloody Dick Creek faults (Coppinger, 1974; Dubois, gaard and others, 1986; Janecke, 1992) and northwest in the Bit- shared some of their unpublished mapping. Mapping was partially ° Deadman fault Mountains, Idaho-Montana: Geological Society of America 113 15' Sage 1982; VanDenburg, 1997; S.U. Janecke, unpublished mapping; terroot metamorphic core complex (Foster and Fanning, 1997). supported by a grant from the National Science Foundation (EAR fault Special Paper 104, 70 p., 6 plates, scale 1:62,500. Lonn and others, 2000)—were also included in this group. 9317395) and two contracts with the USGS. Personnel of the Scott, W.E., Pierce, K.L., and Hait, M.H., Jr., 1985, Quaternary Fault set 1. Pre-middle Eocene low-angle normal faults Dillon office of the USDA Forest Service provided aerial photos. M.G.F. Fault set 5. Miocene to Pleistocene(?) north- and south- tectonic setting of the 1983 Borah Peak earthquake, central with original southwest dips (shown in red on the map) Trail 112°30' dipping normal faults (shown in brown on the map) Idaho: Bulletin of the Seismological Society of America, v. REFERENCES CITED 75, no. 4, p. 1053–1066. Beaverhead Normal faults of set 1 are the oldest normal faults in the Ten- [ indicates a source of data used to construct the map] Lima thrust A newly identified set of consistently east-west-striking normal Sears, J.W., 1995, Middle Miocene rift system in SW Monta- doy and Beaverhead Mountains and are exposed northeast of Lea- na—Implications for the initial outbreak of the Yellowstone system faults displace all but the youngest range-front normal faults. The dore, Idaho, on both sides of the continental divide. Ruppel Anders, M.H., Geissman, J.W., Piety, L.M., and Sullivan, J.T., mid basin faults east-west faults dip both north and south and typically display hotspot, in Thomas, R.C., ed., Geologic history of the Dillon Mountains F. C. . S F. (1968), Staatz (1973, 1979), and Lucchitta (1966) originally map- 1989, Parabolic distribution of circumeastern Snake River area, southwestern Montana: Northwest Geology, v. 25, p. Wheetip small offsets. Most of these normal faults have steep to moderate M.G.F. ped these structures as thrust faults and Skipp (1988) noted that Plain seismicity and latest Quaternary faulting—Migratory pat- 43–46. dips but some faults are listric in the subsurface and produce roll- they were in fact normal faults. Critical, but localized, overlap rela- tern and association with the Yellowstone hotspot: Journal of over folds. The east-west normal faults are most numerous in the Sears, J.W., and Fritz, W.J., 1998, Cenozoic tilt domains in south- ? tionships show that these low-angle normal faults formed before Geophysical Research, v. 94, no. B2, p. 1589–1621. southern part of the map area, but some occur as far north as the western Montana—Interference among three generations of Nicholia middle Eocene Challis volcanism (VanDenburg and others, 1998). Anderson, A.L., 1961, Geology and mineral resources of the extensional fault systems, in Faulds, J.E., and Stewart, J.H., Montana Medicine Lodge basin (M’Gonigle and others, 1991; Du- These normal faults are potentially the largest faults in the region, Lemhi quadrangle, Lemhi County [Idaho]: Idaho Bureau of eds., Accommodation zones and transfer zones—The region- 113°45' bois, 1982) and Polaris, Montana (Zimbelman, 1984). Few east- because they omit a large stratigraphic section in a hanging-wall- Mines and Geology Pamphlet 124, 111 p. 113°30' Creek fault al segmentation of the Basin and Range province: Geological 113°15' west striking normal faults were documented in the study area pri- flat geometry, and consistently place upper Paleozoic rocks on Bartholomew, M.J., 1989, Road Log No. 2, The Red Rock fault 113° or to this study (for example, Skipp and others, 1979), in part be- Middle Proterozoic Belt Supergroup rocks (VanDenburg and oth- and complexly deformed structures in the Tendoy and Four Society of America Special Paper 323, p. 241–247. cause faults of this orientation are typically small and discontinu- ers, 1998). The Goat Mountain, Grizzly Hill, Phosphoria Klippe, Eyes Canyon thrust sheets—Examples of Late Cenozoic and Sears, J.W., Hurlow, Hugh, Fritz, W.J., and Thomas, R.C., 1995, Coryell and ReinterpretedSpang (1988) from ? ous. The large number of normal faults with this orientation, Late Cenozoic disruption of Miocene grabens on the shoulder M West and Wild Cat Creek faults are probably offset segments of a once Late Mesozoic deformation in southwestern Montana, in Lowell (1965) and ’ Creek however, shows that they are an important feature of the region. of the Yellowstone hotspot track in southwest Montana—Field Mapping incomplete unpublishedGonigle mapping and Janecke, continuous normal fault. The continuity of the Middle Proterozoic Sears, J.W., ed., Structure, stratigraphy, and economic geolo- There are more south-dipping than north-dipping normal faults, th guide from Lima to Alder, Montana, in Mogk, D.W., ed., Janecke, reconnaissance mapping Little Sheep footwall and upper Paleozoic hanging wall adjacent to the younger gy of the Dillon area, Tobacco Root Geological Society, 14 but both dip directions are present. Divide normal fault of Lucchitta (1966) suggests that the Divide Annual Field Conference, July 20–22, 1989: Northwest Ge- Field guide to geologic excursions in southwest Montana, and Coppinger (1974) ? ? The relative age of the east-west-striking normal faults is in- fault probably cuts out the southeast continuation of the Goat ology, v. 18, p. 21–35. Rocky Mountain section, GSA Annual Meeting, Bozeman, completely known and faults of this orientation may have been ac- Mont., May 16–21, 1995: Northwest Geology, v. 24, p. Side Mountain–Grizzly Hill–Phosphoria Klippe–Wild Cat Creek fault Blankenau, J.C., 1999, Cenozoic structure and stratigraphy of the 45° tive during more than one deformational event. The east-west- system (Goat-Cat fault system). Southeast Salmon basin, east-central Idaho: Logan, Utah, 201–219. Blankenau and Janecke basin ? striking faults clearly postdate and offset normal faults formed dur- VanDenburg (1997) and VanDenburg and others (1998) incor- Utah State University M.S. thesis, 143 p., 2 plates. Skipp, Betty, 1984, Geologic map and cross sections of the Italian (see Blankenuau, 1999) Sage Creek fault (F.S.C.F.) ing phase 3, and possibly some faults formed during phase 4. rectly named this Goat-Cat low-angle normal fault system the Di- Constenius, K.N., 1996, Late Paleogene extensional collapse of Peak and Italian Peak Middle Roadless Areas, Beaverhead fault zone Faults of set 4 are relatively rare in the Tendoy and Beaverhead vide Creek fault, based on an erroneous correlation with the Di- the Cordilleran foreland fold and thrust belt: Geological Soci- County, Montana, and Clark and Lemhi Counties, Idaho: Former Mountains, and unequivocal cross-cutting relationships between vide Creek fault of Skipp (1984, 1985) in the southern Beaver- ety of America Bulletin, v. 108, no. 1, p. 20–39. U.S. Geological Survey Miscellaneous Field Studies Map W. Perry, fault sets 5 and 4 have not been observed. Northwest-striking VanDenburg and Janecke head Mountains. Subsequent mapping (S.U. Janecke, unpublish- Coppinger, W.W., 1974, Stratigraphic and structural study of the MF–1061–B, scale 1:62,500. unpublished 112°45' normal faults of set 6 appear to displace some of the east-west- (see VanDenburg, 1997) ed) shows that the Divide Creek fault is too young to correlate Belt Supergroup and associated rocks in a portion of the Bea- ——1985, Contraction and extension faults in the southern Bea- mapping striking normal faults. For example, the southwest-dipping verhead Mountains, Idaho and Montana: U.S. Geological Sur- and M’Gonigle and Hait M’Gonigle and others Deadman fault with these pre-middle Eocene normal faults because it cuts the verhead Mountains, southwest Montana and east-central Ida- Crooked Creek normal fault of set 6 displaces the eastern and vey Open-File Report 85–545, 170 p., 3 plates. VanDenburg and (1997) (1991) 44°30' middle Eocene Challis Volcanic Group. The Divide Creek fault ho: Oxford, Ohio, Miami University Ph.D. dissertation, 224 Janecke, western parts of the south-dipping Grouse Creek normal fault (set (sensu strictu) is now included in fault set 5. p. ——1988, Cordilleran thrust belt and faulted foreland in the Bea- others (1998) 5) in the southern part of the Beaverhead Mountains (see map; M’Gonigle, ? Presently, the Goat Mountain, Grizzly Hill, Phosphoria Klippe, Coryell, J.J., and Spang, J.H., 1988, Structural geology of the verhead Mountains, Idaho and Montana, in Schmidt, C.J., School fault zone Skipp, 1984). These data suggest that some (or most?) of the and Perry, photointerpretation Medicine Lodge 44°30' and Wild Cat Creek normal faults dip gently south or southwest Armstead anticline area, Beaverhead County, Montana, in and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain unpublished Janecke, east-west-striking normal faults are older than the currently active foreland and the Cordilleran thrust belt: Geological Society of Staatz (1979) ? (Lucchitta, 1966; VanDenburg 1997). The Goat Mountain fault Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of the ? northwest-striking range-front normal faults. Elsewhere, faults of and mapping ? thrust segment, north of Little Eightmile Creek, restores to a gentle Rocky Mountain foreland and the Cordilleran thrust belt: Geo- America Memoir 171, p. 237–266. ? set 6 appear to reactivate segments of some east-west-striking Skipp, Betty, Hassemer, J.R., Kulik, D.M., Sawatzky, D.L., Lesz- Janecke, reconnaissance Bannack southwest dip after the effects of subsequent tilting are removed logical Society of America Memoir 171, p. 217–227. Reinterpreted from Nicholia normal faults. This could explain the pronounced east-west jog in (VanDenburg and others, 1998). These normal faults thus indi- Crone, A.J., and Haller, K.M., 1991, Segmentation and the co- cykowski, A.M., and Winters, R.A., 1988, Mineral resources Janecke, ? Anderson (1961) Dubois Pass Black the Beaverhead fault north of Leadore, Idaho, and the presence of cate that northeast-southwest extension had begun in the region seismic behavior of Basin and Range normal of the Eighteenmile Wilderness Study Area, Lemhi County, Janecke unpublished (1981) fault scarps on south-dipping faults in the Williams Creek to Mud Idaho: U.S. Geological Survey Bulletin 1718–B, p. B1–B21. Scholten and others (1955) ? ? prior to middle Eocene volcanism (VanDenburg, 1997). Some of faults—Examples from east-central Idaho and southwestern mapping, aided ? Creek area in the southwest part of the map (Haller, 1988, 1990; the normal slip on the enigmatic south-dipping fault D of Montana, U.S.A.: Journal of Structural Geology, v. 13, no. Skipp, Betty, Prostka, H.J., and Schleicher, D.L., 1979, Prelimi- by Dubois (1981, Modified and photointerpretation ? Skipp and others, 1988). ’ M’Gonigle ? M’Gonigle (1993, 1994) may also date from this period. Fault D 2, p. 151–164. nary geologic map of the Edie Ranch quadrangle, Clark M Gonigle 1982) Offset latest Cenozoic rocks indicate a young age for the east- (1993) from Divide has been interpreted as a depositional contact (Skipp, 1988), as a Dubois, D.P., 1981, Geology and tectonic implications of the County, Idaho, and Beaverhead County, Montana: U.S. Geo- (1994) ? Janecke McDowell ? Canyon? ? west-striking normal faults. Near the continental divide around logical Survey Open-File Report 79–845, scale 1:62,500. M’Gonigle and Creek thrust fault (Ruppel, 1978), and as a thrust that was later reactivat- Deer Canyon area, Tendoy Range, Montana: University Park, and (1992) Middle Bannack Pass the east-west striking normal faults displace the ed as a normal fault (M’Gonigle, 1993, 1994). Pa., Pennsylvania State University M.S. thesis, 87 p. Smith, R.B., and Arabasz, W.J., 1991, Seismicity of the Inter- Reinterpreted Janceke, Dalrymple fault VanDenburg, ——1982, Tectonic framework of basement thrust terrane, north- mountain seismic belt, in Slemmons, D.B., Engdahl, E.R., Zo- from photo- (1996) unpublished mapping fault ern Tendoy Range, southwest Montana, in Powers, R.B., ed., back, M.D., and Blackwell, D.D., eds., Neotectonics of North Staatz (1973) interpretation and Geologic studies of the Cordilleran thrust belt, volume I: Den- America: Boulder, Colo., Geological Society of America, Dec- and Creek photointerpretations ? ver, Colo., Rocky Mountain Association of Geologists, p. ade Map Volume to Accompany the Neotectonic Maps, Part 44°45' reconnaissance fault 145–158. of the Continent-Scale Maps of North America, p. 185–228. Janecke, photointerpretation and Fields, R.W., Tabrum, A.R., Rasmussen, D.L., and Nichols, Ralph, Staatz, M.H., 1973, Geologic map of the Goat Mountain quad- ? rangle, Lemhi County, Idaho, and Beaverhead County, Mon- Modified from reconnaissance, aided by 1985, Cenozoic rocks of the intermontane basins of western Kellogg (unpublished mapping), Montana and eastern Idaho, in Flores, R.M., and Kaplan, tana: U.S. Geological Survey Geologic Quadrangle Map Ruppel (1968) and Lonn and others ° ° Idaho 114 W 113 W S.S., eds., Cenozoic paleogeography of the west-central Unit- GQ–1097, scale 1:24,000. Haller (1988) Landis (1963), and Haller ed States, Rocky Mountain Paleogeography Symposium 3: ——1979, Geology and mineral resources of the Lemhi Pass tho- Ruppel (1993) Janecke and (1988) (2000) Syncline rium district, Idaho and Montana: U.S. Geological Survey photointerpretation, others (1999) and Lower Tertiary and Upper Denver, Colo., The Rocky Mountain Section, Society of Eco- unpublishedaided mappingby Kellogg, Professional Paper 1049–A, 90 p., 2 plates. Janecke, Yac Cretaceous Beaverhead Group nomic Paleontologists and Mineralogists, p. 9–36. Janecke, Anticline Brushy Gulch thrust and Scholten and Medicine Foster, D.A., and Fanning, C.M., 1997, Geochronology of the Stewart, J.H., 1998, Regional characteristics, tilt domains, and unpublished mapping Mud Creek Lucchitta (1966) others (1955) Thrust fault Idaho and Pioneer batholiths northern Idaho batholith and the Bitterroot metamorphic core extensional history of the late Cenozoic Basin and Range complex—Magmatism preceding and contemporaneous with province, western North America, in Faulds, J.E., and Stew- Low-angle normal fault Ordovician Beaverhead MONTANA extension: Geological Society of America Bulletin, v. 109, no. art, J.H., eds., Accommodation zones and transfer IDAHO pluton Normal fault 4, p. 379–394. zones—The regional segmentation of the Basin and Range

112°30' province: Geological Society of America Special Paper 323, Lodge Mesozoic and Paleozoic Fritz, W.J., and Sears, J.W., 1993, Tectonics of the Yellowstone many of bedding sedimentary rocks hotspot wake in southwestern Montana: Geology, v. 21, no. p. 47–74. Janecke and Skipp, ? additional Stewart, J.H., Anderson, R.E., Aranda-Gomez, J.J., Beard, L.S., unpublished mapping more Modern rivers Middle Proterozoic rocks 5, p. 427–430. Reinterpretation and small faults Hait, M.H., Jr., and M’Gonigle, J.W., 1988, Implications of Ceno- Billingsley, G.H., Cather, S.M., Dilles, J.H., Dokka, R.K., ? Grouse Canyon fault? Salmon River Mtns. ? Beaverhead Divide fault ? ? ? Creek Porphyritic rapakivi granite photointerpretation of Crooked Creek Leesburg syncline t Faulds, J.E., Ferrari, L., Grose, T.L.T., Henry, C.D., Janecke, Skipp (1988) Yac s zoic extension to interpretation of Tertiary basin paleogeogra- Salmon basin detachment fault u Sadler (1980), Perry and Skipp, r Ermont thrust phy and thrust plate paleotectonics: Geological Society of S.U., Miller, D.M., Richard, S.M., Rowley, P.D., Roldan-Quin- h Swauger and Gunsight Valley ? t Skipp and Perry ? ? Carmen r America Abstracts with Programs, v. 20, no. 7, p. A–108. tana, J., Scott, R.B., Sears, J.W., and Williams, V.S., 1998, unpublished mapping Carmen culmination e Formations and equivalents p

Middle Fork Salmon River ? p Map showing Cenozoic tilt domains and associated structural Scholten and Ramspott (1968) (unpublished mapping), and fault Haller, K.M., 1988, Segmentation of the Lemhi and Beaverhead ? o Salmon h Yac Apple Creek Formation s MGF faults, east-central Idaho, and Red Rock fault, southwest Mon- features, western North America, in Faulds, J.E., and Stew- McDowell (1992), Janecke, s BDCF Grouse Canyon fault a Modified from Yac r tana, during the late Quaternary: Boulder, Colo., University of art, J.H., eds., Accommodation zones and transfer Ryder and Scholten (west segment) MPF Hoodoo Quartzite and photointerpretation Cobalt G ? Cobalt culmination zones—The regional segmentation of the Basin and Range and Haller (1988) Skipp and others Williams Iro Yellowjacket Formation Colorado M.S. thesis, 141 p., 10 plates. Janecke, reconnaissance (1973), and Perry n L (1988) ake ——1990, Late Quaternary movement on basin-bounding normal province: Geological Society of America Special Paper 323, t ? Proterozoic to Archean and photointerpretation and others (1988) Yac h ? ° r faults north of the Snake River Plain, east-central Idaho, in plate 1. 45 N u Yac N. Lemhi rocks ? s Range Tendoy Stickney, M.C., and Bartholomew, M.J., 1987, Seismicity and late 44°30' Grouse Canyon t Proterozoic to Roberts, Sheila, ed., Geologic field tours of western Wyoming fault MPC n cKe zie and parts of adjacent Idaho, Montana and Utah: Geologic Quaternary faulting of the northern Basin and Range prov- (east segment) Yellowjacket M C Archean Scott Mine culmination a n Survey of Wyoming Public Information Circular No. 29, p. ince, Montana and Idaho: Bulletin of the Seismological Soci- y basement Canyon o MLT? Birch Creek Yac Lemhi n 41–54. ety of America, v. 77, no. 5, p. 1602–1625. t h t r Tysdal, R.G., 1996a, Geologic map of adjacent areas in the Hay- u s e Hamilton, Warren, 1960, Late Cenozoic tectonics and volcanism Hayden Creek culmination s r t c fault w of the Yellowstone region, Wyoming, Montana, and Idaho, in den Creek and Mogg Mountain quadrangles, Lemhi County, o n S ift Campau, D.E., and Anisgard, H.W., eds., West Yellow- Idaho: U.S. Geological Survey Miscellaneous Investigations f il- l o a up r t th Series Map I–2563, scale 1:24,000. Divide fault e ck stone—Earthquake area, Billings Geological Society 11 An- ? Cabin l la Yac Ellis Lem Peak fault a B ——1996b, Geologic map of the Lem Peak quadrangle, Lemhi ? ? Patterson culmination MLT Tendoy Mountains nual Field Conference, September 7–10, 1960: Billings Geo- Valley n ? Island Butte culmination. d Divide logical Society, p. 92–105. County, Idaho: U.S. Geological Survey Geologic Quadrangle Leadore thrust Lima conglomerate Hamilton, Warren, and Myers, W.B., 1966, Cenozoic tectonics of Map GQ–1777, scale 1:24,000. Tysdal, R.G., and Moye, Falma, 1996, Geologic map of the Alli- Tendoy thrust the western United States: American Geophysical Union, Re- Deadman fault son Creek quadrangle, Lemhi County, Idaho: U.S. Geological Hawley Creek thrust views of Geophysics, v. 4, no. 4, p. 509–549. Challis Janecke, S.U., 1992, Kinematics and timing of three superposed Survey Geologic Quadrangle Map GQ–1778, scale 1:24,000. Haller Skipp and others (1979) 44°30'N extensional systems, east-central Idaho—Evidence for an Eo- VanDenburg, C.J., 1997, Cenozoic tectonic and paleogeographic (1988) Abbreviations: BDCF, Bloody Lost River Range evolution of the Horse Prairie half graben, southwest Monta- Dick Creek fault; MPF, Maiden Divide con- cene tectonic transition: Tectonics, v. 11, no. 6, p. Study area MLT glomerate na: Logan, Utah, Utah Sate University M.S. thesis, 152 p., 2 Peak fault; MGF, Muddy- 1121–1138. Skipp (1984) Southernshown Lemhi on plates. Grasshopper fault; MPC, S. Beaverhead ——1994, Sedimentation and paleogeography of an Eocene to Range map Maiden Peak culmination; Mountains Oligocene rift zone, Idaho and Montana: Geological Society VanDenburg, C.J., Janecke, S.U., and McIntosh, W.C., 1998, MLT, Medicine Lodge thrust. of America Bulletin, v. 106, p. 1083–1095. Three-dimensional strain produced by >50 My of episodic ex- ——2000, Normal faults galore in the Tendoy and Beaverhead tension, Horse Prairie basin area, SW Montana, U.S.A.: 44°15' Salmon River Mountains of SW Montana and eastern Idaho: Geological So- Journal of Structural Geology, v. 20, no. 12, p. 1747–1767. Wernicke, Brian, 1992, Cenozoic of the 113° ciety of America Abstracts with Programs, v. 32, no. 5, p. U.S. Cordillera, in Burchfiel, B.C., Lipman, P.W., and Zo- 112°45' 44°15' SCALE A–13. back, M.L., eds., The Cordilleran Orogen—Conterminous 112°30' Janecke, S.U., McIntosh, W., and Good, S., 1999, Testing models 0 20 KILOMETERS of rift basins—Structure and stratigraphy of an Eocene-Oligo- U.S.: Boulder, Colo., Geological Society of America, The Ge- cene supra-detachment basin, Muddy Creek half graben, ology of North America, v. G–3, p. 553–581. 44°15' Base from U.S. Geological Survey, 1:100,000, SCALE 1:100 000 Figure 2. Subcrop map of the Idaho-Montana fold-and-thrust belt showing the locations of structural culminations (green labels) and normal faults that southwest Montana: Basin Research, v. 11, no. 2, p. Witkind, I.J., 1975, Geology of a strip along the Centennial fault, 1 0 1 2 3 4 5 6 7 8 MILES collapse them (blue and red). Map shows age of bedrock beneath middle Eocene Challis Volcanic Group and was constructed by using principles 143–167. southwestern Montana and adjacent Idaho: U.S. Geological Borah Peak, 1989; Dillon, 1983; Dubois, 1983; MONTANA Leadore, 1980; Lima, 1987; Salmon, 1981. outlined in Rodgers and Janecke (1992). Complied from 80 sources in the data repository of Janecke and others (2000c). Normal faults of set 1 Janecke, S.U., Perkins, M.E., and Smith, R.B., 2000a, Normal Survey Miscellaneous Investigations Series Map I–890, scale 1 0 1 2 3 Projection: Universal Transverse Mercator, zone 12 4 5 6 7 8 9 10 KILOMETERS 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. faults patterns around the Yellowstone hot spot—A new mod- 1:62,500. INDEX TO SOURCES OF DATA 1927 North American datum The study area shown on the map is outlined in magenta. el: Geological Society of America Abstracts with Programs, v. Zimbelman, D.R., 1984, Geology of the Polaris 1SE quadrangle, CONTOUR INTERVAL 50 METERS 32, no. 7, p. A–45. Beaverhead County, Montana: Boulder, Colo., University of (CONTOUR INTERVAL 40 METERS IN LEADORE QUADRANGLE) IDAHO Janecke, S.U., Skipp, B., and Perry, W.P., Jr., 2000b, Revised Colorado M.S. thesis, 158 p. SUPPLEMENTARY CONTOUR INTERVAL25 METERS IN DUBOIS AND LIMA QUADRANGLES NATIONAL GEODETIC VERTICAL DATUM OF1929 thrust fault patterns in the Tendoy Mountains of SW MT:

Manuscript approved for publication April 6, 2001

Any use of trade names in this publication is for descriptive purposes only and does not imply MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE TENDOY MOUNTAINS AND BEAVERHEAD RANGE, SOUTHWEST MONTANA AND EASTERN IDAHO endorsement by the U.S. Geological Survey

This map was produced on request, directly from By digital files, on an electronic plotter 1Department of Geology, Utah State University, Logan, UT Susanne U. Janecke1, James J. Blankenau1*, Colby J. VanDenburg1**, and Bradley S. Van Gosen2 For sale by U.S. Geological Survey Information Services 2U.S. Geological Survey, Denver, CO Box 25286, Federal Center, Denver, CO 80225 1-888-ASK-USGS Present address: 2001 * URS, Salt Lake City, UT A PDF for this map is available at **Durango, CO http://geology.cr.usgs.gov/greenwood-pubs.html