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  • MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE …Compilation of a 1:100,000-scale map of normal faults and extensional folds in southwest Montana and adjacent Idaho reveals a complex
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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 12 1927 North American datum SCALE 1:100 000 1 0 1 2 3 4 5 6 7 8 MILES 1 0 1 2 3 4 5 6 7 8 9 10 KILOMETERS CONTOUR INTERVAL 50 METERS (CONTOUR INTERVAL 40 METERS IN LEADORE QUADRANGLE) SUPPLEMENTARY CONTOUR INTERVAL 25 METERS IN DUBOIS AND LIMA QUADRANGLES NATIONAL GEODETIC VERTICAL DATUM OF 1929 113°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 show oblique-slip movement. Dashed where approximate; dotted where concealed Anticline—Arrow indicates plunge direction. Dashed where approximate; dotted where concealed Syncline—Arrow indicates plunge direction. Dashed where approximate; dotted where 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 constraints extensional event 6 Green Moderate Active or potentially active normal faults in the current tectonic regime. The age of initial slip is uncertain. 5 Brown Uncertain Many of the east-west striking normal faults cut the Paleogene normal faults (blue) and are cut by the active Basin and Range faults (green). The east-west normal faults may have formed during more than one episode of extension. All east-west normal faults are brown, 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 of this age. Nicholia School fault zone may have formed at this time. 3 Blue Major, the largest normal Paleogene normal faults—most are middle Eocene to Oligocene, faults in the region but some may have been active into early or middle Miocene time. formed at this time Cross-cutting relationships show that several systems of normal faults were active sequentially during this time (VanDenburg and others, 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 basin detachment Beaverhead fault Agency-Yearian fault Lemhi Pass fault Dan Patch fault Bloody Dick Creek fault Peterson Creek fault Reese Cr. flt. Reese Creek fault Agency-Yearian fault Goat Mountain fault Eightmile Creek fault Eightmile Creek fault Beaverhead fault Maiden Peak fault Muddy-Grasshopper fault (M.G.F.) Armstead 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 many additional small faults Grouse Canyon fault (west segment) Divide Creek fault more Grouse Canyon fault? M.G.F. M.G.F. M.G.F. M.G.F. Kate Creek fault Deadman fault Divide fault Beaverhead fault Lemhi fault Pass fault Canyon Rocky Mountain fault Middle Creek fault Little Bear fault Trail Wheetip fault D Scott Canyon fault Crooked Creek fault Middle Creek fault fault Black Canyon Medicine Lodge thrust Former Lima thrust system Creek fault Little Sheep Sage Creek fault (F.S.C.F.) Creek fault Deadman fault Nicholia West fault zone Side Deadman fault Cabin thrust thrust Lodge Medicine strand of Ellis Peak fault zone M.L.T. McKenzie Canyon McKenzie Canyon thrust mid basin faults Monument Hills fault zone Sage fault Grizzly Hill Phosphoria Klippe Creek Wild Cat fault (magenta and purple) (magenta and purple) Divide fault Little Little School fault zone thrust ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Janecke, photointerpretation Reinterpretation and photointerpretation of Sadler (1980), Skipp and Perry (unpublished mapping), McDowell (1992), Ryder and Scholten (1973), and Perry and others (1988) Perry and Skipp, unpublished mapping and Skipp and others (1979) Skipp (1984) Haller (1988) Scholten and Ramspott (1968) and Haller (1988) Modified from Skipp and others (1988) Janecke, reconnaissance and photointerpretation Janecke and Skipp, unpublished mapping Skipp (1988) Janecke, photointerpretation, aided by Kellogg, unpublished mapping and Scholten and others (1955) Janecke, photointerpretation and reconnaissance, aided by Kellogg (unpublished mapping), Landis (1963), and Ruppel (1993) Janecke and others (1999) and Janecke, unpublished mapping Haller (1988) Scholten and others (1955) and photointerpretation Modified from McDowell (1992) Dubois (1981) Janecke, photointerpretation W. Perry, unpublished mapping Reinterpreted from Lowell (1965) and Coryell and Spang (1988) M’Gonigle and Janecke, unpublished mapping M’Gonigle and others (1991) M’Gonigle and Hait (1997) Janceke, photo- interpretation and reconnaissance Janecke M’Gonigle (1993) M’Gonigle (1994) INDEX TO SOURCES OF DATA M’Gonigle and Dalrymple (1996) Janecke, unpublished mapping, aided by Dubois (1981, 1982) Janecke, M’Gonigle, and Perry, unpublished mapping Janecke, reconnaissance mapping and Coppinger (1974) VanDenburg and Janecke (see VanDenburg, 1997) and VanDenburg and others (1998) Janecke and VanDenburg, unpublished mapping and photointerpretations Reinterpreted from Staatz (1973) Staatz (1979) and Janecke, reconnaissance Lucchitta (1966) Modified from Ruppel (1968) and Haller (1988) Reinterpreted from Anderson (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 for descriptive purposes only and does not imply endorsement by the U.S. Geological Survey This map was produced on request, directly from digital files, on an electronic plotter For sale by U.S. Geological Survey Information Services Box 25286, Federal Center, Denver, CO 80225 1-888-ASK-USGS A PDF for this map is available at http://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 IDAHO By Susanne U. Janecke 1 , James J. Blankenau 1* , Colby J. VanDenburg 1** , and Bradley S. Van Gosen 2 2001 1 Department of Geology, Utah State University, Logan, UT 2 U.S. Geological Survey, Denver, CO Present address: * URS, Salt Lake City, UT ** Durango, CO U.S. DEPARTMENT OF THE INTERIOR U.S. GEOLOGICAL SURVEY MISCELLANEOUS FIELD STUDIES MAP MF–2362 Version 1.0 Leadore Lemhi Valley Railroad Canyon basin Prairie Horse Lemhi Pass Salmon basin Grasshopper basin Montana Medicine Lodge basin Maiden Peak Spur Ellis Peak Nicholia Creek basin Beaverhead Mountains Mud Creek Williams Creek Birch Creek Valley Bannack Pass Idaho Medicine Lodge Valley Lima Red Rock valley Tendoy Mountains Muddy Creek basin 45°N Patterson culmination MPC Yellowjacket Mine culmination Hayden Creek culmination Cobalt culmination Challis Salmon Ellis Tendoy Lemhi Carmen Lima Cobalt Middle Fork Salmon River Salmon River Lem Peak fault Beaverhead Divide fault MONTANA IDAHO Southern Lemhi Range S. Beaverhead Mountains MLT? MLT Lost River Range Brushy Gulch thrust Leesburg syncline f o r e la n d B lacktail-Snowcrest ? ? ? ? ? ? ? ? ? Grasshopper thrust Ermont thrust M c K e n z i e C a n y o n thrust MLT Tendoy Mountains N. Lemhi Range Salmon River Mtns. Mesozoic and Paleozoic sedimentary rocks Swauger and Gunsight Formations and equivalents Hoodoo Quartzite and Yellowjacket Formation Proterozoic to Archean basement 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, Bloody Dick Creek fault; MPF, Maiden Peak fault; MGF, Muddy- Grasshopper fault; MPC, Maiden Peak culmination; MLT, Medicine Lodge thrust. 20 KILOMETERS SCALE 0 BDCF I r o n L a k e t hr u s t Yac Yac Yac Yac Yac Yac Yac Modern rivers Porphyritic rapakivi granite Ordovician Beaverhead pluton Salmon basin detachment fault Island Butte culmination. MPF MGF Divide fault Deadman fault Divide con- glomerate 44°30'N u plift Tendoy thrust Cabin thrust Study area shown on map Hawley Creek thrust Divide conglomerate Carmen culmination Leadore Proterozoic to Archean rocks 114°W 113°W Figure 2. Subcrop map of the Idaho-Montana fold-and-thrust belt showing the locations of structural culminations (green labels) and normal faults that collapse them (blue and red). Map shows age of bedrock beneath middle Eocene Challis Volcanic Group and was constructed by using principles outlined in Rodgers and Janecke (1992). Complied from 80 sources in the data repository of Janecke and others (2000c). Normal faults of set 1 are red and faults of set 3 are blue. Notice the association between these large normal faults and structural culminations of the fold and thrust belt. The study area shown on the map is outlined in magenta. HajorT DCF Montana Medicine Lodge basin DILLON DELL LIMA LEADORE SALMON Sage Creek basin Muddy Creek basin 0 10 20 Kilometers basin Salmon Beaverhead Mountains Tendoy Mountains Blacktail Mountains Lemhi Range Montana Idaho Maiden Peak Spur 113°30' 113°00' 112°30' 114°00' 45°15' 44°45' 44°30' SBF MGF MPF BF Montana Idaho Location of figure UDMGF DF LPF 45°00' BTF RRF BDF Reservoir Mountain range Quaternary-Miocene basin fill Early Miocene-Eocene basin fill Mountains basin basin Prairie Horse Grasshopper Big Hole basin Medicine Lodge basin Idaho MF Nicholia basin Creek Upper Lemhi Valley Birch Creek Valley Red Rock Valley AYF BF 44°15' Faults—Bar and ball on downthrown side of steeper normal fault. Rectangle on 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. REFERENCES CITED [ indicates a source of data used to construct the map] Anders, M.H., Geissman, J.W., Piety, L.M., and Sullivan, J.T., 1989, Parabolic distribution of circumeastern Snake River Plain seismicity and latest Quaternary faulting—Migratory pat- tern and association with the Yellowstone hotspot: Journal of Geophysical Research, v. 94, no. B2, p. 1589–1621. Anderson, A.L., 1961, Geology and mineral resources of the Lemhi quadrangle, Lemhi County [Idaho]: Idaho Bureau of Mines and Geology Pamphlet 124, 111 p. Bartholomew, M.J., 1989, Road Log No. 2, The Red Rock fault and complexly deformed structures in the Tendoy and Four Eyes Canyon thrust sheets—Examples of Late Cenozoic and Late Mesozoic deformation in southwestern Montana, in Sears, J.W., ed., Structure, stratigraphy, and economic geolo- gy of the Dillon area, Tobacco Root Geological Society, 14 th Annual Field Conference, July 20–22, 1989: Northwest Ge- ology, v. 18, p. 21–35. Blankenau, J.C., 1999, Cenozoic structure and stratigraphy of the Southeast Salmon basin, east-central Idaho: Logan, Utah, Utah State University M.S. thesis, 143 p., 2 plates. Constenius, K.N., 1996, Late Paleogene extensional collapse of the Cordilleran foreland fold and thrust belt: Geological Soci- ety of America Bulletin, v. 108, no. 1, p. 20–39. Coppinger, W.W., 1974, Stratigraphic and structural study of the Belt Supergroup and associated rocks in a portion of the Bea- verhead Mountains, southwest Montana and east-central Ida- ho: Oxford, Ohio, Miami University Ph.D. dissertation, 224 p. Coryell, J.J., and Spang, J.H., 1988, Structural geology of the Armstead anticline area, Beaverhead County, Montana, in Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt: Geo- logical Society of America Memoir 171, p. 217–227. Crone, A.J., and Haller, K.M., 1991, Segmentation and the co- seismic behavior of Basin and Range normal faults—Examples from east-central Idaho and southwestern Montana, U.S.A.: Journal of Structural Geology, v. 13, no. 2, p. 151–164. Dubois, D.P., 1981, Geology and tectonic implications of the Deer Canyon area, Tendoy Range, Montana: University Park, Pa., Pennsylvania State University M.S. thesis, 87 p. ——1982, Tectonic framework of basement thrust terrane, north- ern Tendoy Range, southwest Montana, in Powers, R.B., ed., Geologic studies of the Cordilleran thrust belt, volume I: Den- ver, Colo., Rocky Mountain Association of Geologists, p. 145–158. Fields, R.W., Tabrum, A.R., Rasmussen, D.L., and Nichols, Ralph, 1985, Cenozoic rocks of the intermontane basins of western Montana and eastern Idaho, in Flores, R.M., and Kaplan, S.S., eds., Cenozoic paleogeography of the west-central Unit- ed States, Rocky Mountain Paleogeography Symposium 3: Denver, Colo., The Rocky Mountain Section, Society of Eco- nomic Paleontologists and Mineralogists, p. 9–36. Foster, D.A., and Fanning, C.M., 1997, Geochronology of the northern Idaho batholith and the Bitterroot metamorphic core complex—Magmatism preceding and contemporaneous with extension: Geological Society of America Bulletin, v. 109, no. 4, p. 379–394. Fritz, W.J., and Sears, J.W., 1993, Tectonics of the Yellowstone hotspot wake in southwestern Montana: Geology, v. 21, no. 5, p. 427–430. 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Janecke, S.U., Skipp, B., and Perry, W.P., Jr., 2000b, Revised thrust fault patterns in the Tendoy Mountains of SW MT: Geological Society of America Abstracts with Programs, v. 32, no. 5, p. A-12. Janecke, S.U., VanDenburg, C.J., and Blankenau, J.J., 1998, Geometry, mechanisms and significance of extensional folds from examples in the Rocky Mountain Basin and Range prov- ince, U.S.A.: Journal of Structural Geology, v. 20, no. 7, p. 841–856. Janecke, S.U., VanDenburg, C.J., Blankenau, J.J., and M’Gonigle, J.W., 2000c, Long-distance longitudinal transport of gravel across the Cordilleran thrust belt of Montana and Idaho: Geology, v. 28, no. 5, p. 439–442. Kiilsgaard, T.H., Fisher, F.S., and Bennett, E.H., 1986, The Trans-Challis fault system and associated precious metal de- posits, Idaho: Economic Geology, v. 81, no. 3, p. 721–724. Landis, C.A., Jr., 1963, Geology of the Graphite Mountain–Tepee Mountain area Montana-Idaho: University Park, Pa., Pennsyl- vania State University M.S. thesis, 153 p. Lonn, J.D., Skipp, Betty, Ruppel, E.T., Janecke, S.U., Perry, W.J., Jr., Sears, J.W., Bartholomew, M.J., Stickney, M.C., Fritz, W.J., Hurlow, H.A., and Thomas, R.C., 2000, Prelimi- nary geologic map of the Lima 30' x 60' quadrangle, south- west Montana: Montana Bureau of Mines and Geology Open- File Report MBMG 408, scale 1:100,000, includes pam- phlet. Lowell, W.R., 1965, Geologic map of the Bannack-Grayling area Beaverhead County, Montana: U.S. Geological Survey Mis- cellaneous Geologic Investigations Map I–433, scale 1:31,680, includes 6-page pamphlet. Lucchitta, B.K., 1966, Structure of the Hawley Creek area, Idaho- Montana: University Park, Pa., Pennsylvania State University Ph.D. dissertation, 235 p. McBride, B.C., 1988, Geometry and kinematics of the central Snowcrest Range—A Rocky Mountain foreland uplift in southwestern Montana: Kalamazoo, Mich., Western Michigan University M.S. thesis, 267 p., 8 plates. McDowell, R.J., 1992, Effects of synsedimentary basement tec- tonics on fold-thrust belt geometry, southwestern Montana: Lexington, Ky., University of Kentucky Ph.D. dissertation, 328 p. M’Gonigle, J.W., 1993, Geologic map of the Medicine Lodge Peak quadrangle, Beaverhead County, southwest Montana: U.S. Geological Survey Geologic Quadrangle Map GQ–1724, scale 1:24,000. ——1994, Geologic map of the Deadman Pass quadrangle, Bea- verhead County, Montana, and Lemhi County, Idaho: U.S. Geological Survey Quadrangle Map GQ–1753, scale 1:24,000. M’Gonigle, J.W., and Dalrymple, G.B., 1993, 40 Ar/ 39 Ar ages of Challis volcanic rocks and the initiation of Tertiary sedimenta- ry basins in southwestern Montana: The Mountain Geologist, v. 30, no. 4, p. 112–118. ——1996, 40 Ar/ 39 Ar ages of some Challis Volcanic Group rocks and the initiation of Tertiary sedimentary basins in southwest- ern Montana: U.S. Geological Survey Bulletin 2132, 17 p., 1 plate. M’Gonigle, J.W., and Hait, M.H., Jr., 1997, Geologic map of the Jeff Davis Peak quadrangle and eastern part of the Everson Creek quadrangle, Beaverhead County, southwest Montana: U.S. Geological Survey Geologic Investigations Map I–2604, scale 1:24,000. M’Gonigle, J.W., Kirschbaum, M.A., and Weaver, J.N., 1991, Geologic map of the Hansen Ranch quadrangle, Beaverhead County, southwest Montana: U.S. Geological Survey Geolog- ic Quadrangle Map GQ–1704, scale 1:24,000. Morgan, L.A., Pierce, K.L., and McIntosh, W.C., 1998, The vol- canic track of the Yellowstone hotspot—An update: Yellow- stone Science, v. 5, p. 44. Pardee, J.T., 1950, Late Cenozoic block faulting in western Mon- tana: Geological Society of America Bulletin, v. 61, no. 4, p. 359–406. Perry, W.J., Jr., Haley, J.C., Nichols, D.J., Hammons, P.M., and Ponton, J.D., 1988, Interactions of Rocky Mountain foreland and Cordilleran thrust belt in Lima region, southwest Monta- na, in Schmidt, C.J., and Perry, W.J., Jr., eds., Interaction of the Rocky Mountain foreland and the Cordilleran thrust belt: Geological Society of America Memoir 171, p. 267–290. Pierce, K.L., and Morgan, L.A., 1992, The track of the Yellow- stone hot spot—Volcanism, faulting, and uplift, in Link, P.K., Kuntz, M.A., and Platt, L.B., eds., Regional geology of east- ern Idaho and western Wyoming: Geological Society of America Memoir 179, p. 1–53. Reynolds, M.W., 1979, Character and extent of Basin-Range faulting, western Montana and east-central Idaho, in New- man, G.W., and Goode, H.D., eds., Basin and Range Sympo- sium and Great Basin Field Conference: Denver, Colo., Rocky Mountain Association of Geologists, p. 185–193. Rodgers, D.W., and Janecke, S.U., 1992, Tertiary paleogeo- graphic maps of the western Idaho–Wyoming– Montana thrust belt, in Link, P.K., Kuntz, M.A., and Platt, L.B., eds., Regional geology of eastern Idaho and western Wyoming: Geological Society of America Memoir 179, p. 83–94. Royse, F., Jr., Warner, M.A., and Reese, D.L., 1975, Thrust belt structural geometry and related stratigraphic problems, Wyoming-Idaho-northern Utah, in Bolyard, D.W., ed., Deep drilling frontiers of the central Rocky Mountains: Denver, Co- lo., Rocky Mountain Association of Geologists, p. 41–54. 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Page 1: MAP OF NORMAL FAULTS AND EXTENSIONAL FOLDS IN THE …Compilation of a 1:100,000-scale map of normal faults and extensional folds in southwest Montana and adjacent Idaho reveals a complex

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

?

?

?

??

??

?

??

?

?

?

?

? ?

?

??

??

??

??

?

?

?

?

?

??

? ??

?

?

?

?

?

??

? ?? ?

?

?

?

?

?

??

??

???

?

?

?

?

?

?

?

?

?

?

??

??

??

?

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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