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geometric ince, U.S.A.: Journal of Structural Geology, v. 20, no. 7, p. 45°05' The map is based on both published and unpublished mapping nial normal fault (Witkind, 1975) to the east of the map area and sets of normal faults (Janecke, 2000). Within each phase of nor- 841–856. basin 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 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 thrust fault—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.) Monocline—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 Pass regime. The age of initial slip is uncertain. Snowcrest Range—A Rocky Mountain foreland uplift in Normal faults in southwest Montana and adjacent Idaho were of the Miocene Ruby and Beaverhead half grabens 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).