Relationship of the trans-Challis fault system in central Idaho to Eocene and Basin and Range extensions Earl H. Bennett Idaho Geological Survey, Room 332 Morrill Hall, University of Idaho, Moscow, Idaho 83843 ABSTRACT bonsville, Shoup, Leesburg, Parker Mountain and Yankee Fork (both The trans-Challis fault system (TCFS) is a new structural north of Stanley), Lowman, and Boise Basin. The mines and their rela- province in central Idaho mapped during the Challis CUSMAP tionship to the TCFS are discussed by Kiilsgaard et al. (1986). program conducted by the U.S. Geological Survey. The TCFS The TCFS is an extension of the Great Falls lineament described by contains numerous northeast-trending faults, several eruptive centers O'Neill and Lopez (1985). The Idaho porphyry belt (Olson, 1968), in for the Challis Volcanics, and many precious metal deposits. Major part localized along the TCFS, crosses central Idaho and continues into movement occurred in the fault system during Eocene extension that Montana (Hyndman et al., 1977). Basin and Range mountains in east- affected the area from the Snake River Plain north into British central Idaho, including the Beaverhead, Lemhi, and Lost River ranges, Columbia. appear to terminate at the TCFS. The TCFS, therefore, marks the transi- The eastern Snake River Plain is the southern boundary of tion from Eocene extensional features north of the TCFS to Eocene ex- Eocene extension, and in Idaho the Lewis and Clark line is the tensional features overprinted by younger Basin and Range structures northern boundary. The eastern Snake River Plain was "welded" extending to the eastern Snake River Plain south of the TCFS. together by passage of the plain over the Yellowstone hotspot. Younger Basin and Range faults were then able to extend across the FEATURES AND LIMITS OF EOCENE EXTENSION plain and now terminate at the next major Eocene crustal break IN THE PACIFIC NORTHWEST AND north of the plain, the TCFS. The TCFS is an extensional feature SOUTHERN BRITISH COLUMBIA equal in importance to the Lewis and Gark zone and the eastern The recognition of accreted terranes (Monger et al., 1982; Beck et Snake River Plain and should be considered in Tertiary al., 1980), extensional tectonics, and core complexes (Coney, 1980) has reconstructions for the Pacific Northwest. dramatically changed our concepts of the Tertiary geologic history of the Pacific Northwest. Major Eocene extensional features include volcanic fields, grabens, and other extensional fault structures, detachment features INTRODUCTION in some of the core complexes, and anorogenic plutonism (Fig. 1). A new structural province named the trans-Challis fault system Almost all the volcanic rocks of Eocene age in the northwestern (TCFS) was mapped during the Conterminous United States Mineral United States and southern British Columbia (the Challis arc of Vance, Appraisal Program (CUSMAP) conducted by the U.S. Geological Sur- 1979) are related to Eocene extension. Reviews of Eocene volcanism in vey from 1979 to 1983 in the Challis 2° map in central Idaho. The the northwestern United States and British Columbia have been given by TCFS is a major extensional feature of Eocene age and is key to under- Lipman et al. (1972) and Ewing (1980). standing the geology of central Idaho and the Pacific Northwest. Major Several metamorphic core complexes have been mapped in south- extensional features in the following discussion are shown in Figure 1. ern British Columbia, Washington, and northern Idaho (Fig. 1). Some of the core complexes probably formed in Cretaceous-Paleocene time and TRANS-CHALLIS FAULT SYSTEM were overprinted later in the Tertiary (Rhodes and Hyndman, 1984; The TCFS (Fig. 2) contains numerous northeast-trending normal Brown and Read, 1983; Okulitch, 1984). Price (1985) suggested that faults that cross the Challis 2° map diagonally from the southwest to the there are two types of core complexes: one related to Jurassic to Paleo- northeast corners (Kiilsgaard and Lewis, 1986; Bennett, 1984; Fisher et cene thrusting and a second related to Eocene extension. He also noted al., 1983). The system is at least 270 km long and 24 km wide in Idaho. that the Eocene event may in places overprint the older event. The Pur- Included are the Eightmile fault, faults forming the Panther Creek, Cus- cell trench is the eastern border of major Eocene extension in British ter, and Knapp Creek grabens, and many unnamed structures. The width Columbia, northeastern Washington, and northern Idaho. of the zone of deformation associated with some of these faults is impres- The mechanism of Eocene extension in British Columbia and sive (e.g., the Eightmile fault zone is over 500 m wide in some places). northeastern Washington has been related to ductile spreading between Several eruptive centers for the Challis volcanic field are localized major, en echelon, north-south strike-slip faults such as the Straight along the system (Mclntyre et al., 1982), including the Twin Peaks cal- Creek-Fraser faults and the Tintina fault-Rocky Mountain trench (Price, dera. Many of the faults in the TCFS served as conduits for rhyolite 1979, 1984; Harms and Price, 1983). Gabrielse (1985) showed that the plugs and dikes that are the youngest members of the Challis Volcanics. large-scale movements along the Rocky Mountain trench continued into The faults not only guided the rhyolites but also locally delineated major the Eocene. grabens that contain younger Challis units. In central Idaho, the eastern part of the Bitterroot lobe of the Idaho Many epithermal gold deposits and other types of mineralization batholith has many of the features of a core complex (Hyndman, 1980; are localized along the TCFS. These include mines in the vicinity of Gib- Reid, 1984). Structures in pre-Belt rocks such as the Boehls Butte Forma- GEOLOGY, v. 14, p. 481-484, June 1986 481 KAMLOOPS Figure 1. Trans-Challis fault system and other selected geologic fea- tures in Pacific North- west and southern British Columbia, Can- ada. Modified from Tipper etal. (1981); strontium data from Armstrong (1979) and Armstrong et al. (1977). Volcanics: 1—McAbee Basin; 2—Tranquille Basin; 3—Monte Lake Volcanics; 4—Torada graben; 5—Republic graben; 6—Kettle graben; 7—Clarno Vol- canics; 8—Challis Vol- canics; 9—Challis Volcanics in Owyhee County. Core com- plexes: A—Shuswap Complex; B—Valhalla gneiss dome and Pass- more gneiss dome; C— Kcitie yueiss dome; D—Okanogan gneiss dome; E—Selkirk igne- ous complex (Kaniksu batholith); F—Spokane dome; 6—Boehls Butte Formation; H—Pioneer Myellow- ' STONE—3 Mountain core complex; I—House Mountain metamorphic complex. X—Chilly Buttes; Borah Peak earthquake, Oc- tober 28,1983. Dash- dot line = boundary of Basin and Range prov- ince in Oregon. southern Atlanta lobe of the batholith and terminated against the next major crustal break to the north, the TCFS. The Lewis and Clark zone is the northern boundary of Eocene ex- tension in Idaho. Rehrig and Reynolds (1981), Seyfert (1984), and Sher- iff et al. (1984) have all suggested that the Lewis and Clark zone is a transform fault between the Newport fault or the Purcell trench and the Leesburg Bitterroot front. The zone is bordered to the south by the St. Joe fault °Salmon and contains the Hope, Osburn, and other faults. All of these faults have right-lateral movement. This movement probably accommodated northwest to east-west extension that produced detachment surfaces in core complexes and other extensional features south of the line. Panther Creek graben Note that an extension of the St. Joe and Hope faults intersects the TCFS-Great Falls lineament just west of the Boulder batholith. The St. Joe eastern Snake River Plain systems probably mark platelet boundaries in the North American plate. BASIN AND RANGE PROVINCE IN IDAHO The Beaverhead, Lemhi, and Lost River ranges in eastern Idaho formed by Basin and Range faulting as described by Skipp (1985). Rup- pel (1982) noted that some of these faults may be reactivated older struc- tures. Similar faults extend throughout the southern part of the Atlanta lobe of the Idaho batholith (Bennett, 1980a). These faults include the 44° - Wood River-Sawtooth fault, the Montezuma fault, the Deer Park fault, the Trinity Mountain fault, the Willow Creek fault, and the Boise Front fault. Remnants of older, northeast-trending faults probably related to Eocene extension have been mapped in fault blocks between the northwest-trending faults. All of the major northwest-trending structures Figure 2. Major geologic features of trans-Challis fault system in cen- from the Beaverhead Range to the Boise Front fault terminate against the tral Idaho. Modified from Kiilsgaard et al. (1986). TCFS (Fig. 1). South of the Snake River Plain, the Grouse Creek and Albion ranges have characteristics of core complexes (Compton and Todd, tion and anorthosites south of the St. Joe fault, discussed by Hietanen 1983) but are perhaps related to Basin and Range extension rather than (1984), may be related to extension, according to Seyfert (1984). Eocene extension. The Pioneer Mountain core complex (Wust, 1985) Eocene plutonic rocks associated with extension in the Idaho batho- and the House Mountain Complex (Jacob, 1985) have some extensional lith consist of bimodal tonalite and granite plutons, each type having as- features that also may be related to Basin and Range extension. sociated hypabyssal dike swarms and volcanic rocks. The granites have characteristics of A-type or anorogenic granites that are generally asso- SUMMARY ciated with rifting and extension (Bennett and Knowles, 1986). The Ter- Two areas that have undergone massive Eocene extension, the tiary granite and tonalite intrusions are widespread between the Snake southern British Columbia-northeastern Washington area and the central River Plain and the St. Joe fault (Bennett, 1980b). Idaho platelet, are underlain by continental crust where extension is well Although the TCFS is the most obvious northeast structural zone expressed by the detachment zones of core complexes, anorogenic plu- related to extension, there are two others.