WARREN HAMILTON LATE CENOZOIC STRUCTURE OF WEST-CENTRAL IDAHO Abstract: The massive Salmon River Mountains low-grade metamorphic rocks intruded by semi- of interior Idaho are bounded on the west by a belt concordant stocks and small batholiths, products 30 miles wide of post-Miocene, west-tilted normal- largely of Late Jurassic (?) orogeny. fault blocks and west-dipping monoclines. The The Idaho batholith has been little deformed, belt is coincident with the western border zone of and its border-zone rocks of intermediate com- the middle Cretaceous Idaho batholith, as it ex- petence are broken by concordant structures. tends from the west edge of the massive interior of Young structures cut directly across the relatively the batholith to about the western limit of the incompetent rocks of the older orogen to the west. border zone of gneisses and schists. The mountains flanking the Snake River and West of this belt is the Columbia Plateau prov- Salmon River canyons are higher than those ince of irregular domal and anticlinal uplifts and farther away, suggesting that local isostatic uplift northwest-trending normal faults. These structures may be compensating for their erosion. are superimposed upon east- to northeast-trending, from the southeast lose their identity as they Introduction reach the granitic terrane; the westernmost of A belt of north-trending faults and warps the blocks striking from the southeast is the separates the massive mountains of interior Sawtooth Mountains (Fig. 1). Young fault Idaho from the irregular uplifts of the Columbia blocks are subparallel to but largely outside the Plateau of northeastern Oregon. Within this batholith on the east and west. belt the Miocene Columbia River Basalt thins The Salmon River Mountains, which include eastward on pre-Tertiary crystalline rocks. The that part of the mountains of central Idaho base of the basalt had low relief over broad shown in Figure 1, are largely of pre-Tertiary areas, and the basalt veneer makes post- rocks and lack the veneer of Columbia River Miocene structures easy to recognize. Struc- Basalt; hence their late Cenozoic structure tural data shown on Figure 1 in Oregon, and in must be inferred from the topography. Large Idaho near the Snake River Plain, were com- young fault blocks are certainly lacking: there piled from published sources (including Cohee, are no high scarps or troughlike valleys. 1962; Gilluly, 1937; Kirkham, 1931; Malde, Anderson (1934) and Capps (1941) thought 1959; Moore, 1937; Ross, 1938; Smith and that relatively straight canyons, trending Allen, 1941), whereas those in the rest of Idaho north to north-northeast in the western part are based chiefly on my own reconnaissance, of the mountains, follow late Cenozoic faults, augmented by information from Capps (1941), but these canyons have symmetrical cross Livingston (1923), Wagner (1945), and from sections, neither crest being consistently higher unpublished maps by Ralph S. Cannon, Jr., than the other; structural control by early and by John C. Reed. Tertiary structures is possible, and lateral dis- placements may have occurred, but there Salmon River Mountains cannot have been major late Pliocene or The mountains of central Idaho form an Quaternary vertical displacements along these uninterrupted mass that extends 300 miles hypothetical faults. Unquestioned, however, north to south and 100 miles east to west. are small faults of middle or late Cenozoic These mountains coincide approximately with age within the mountains (Capps, 1941). the Idaho batholith on all sides except the Within the Salmon River Mountains are south, where the batholith is concealed beneath broad areas of mountains with summits reaching the Snake River Plain. Late Cenozoic faults above 9000 feet, and also broad, equidimen- have broken the pre-Tertiary rocks into large sional basins with general levels near 5000 or blocks only beyond the limits of the batholith 6000 feet. These areas are so large that they to the west, north, and east. Fault blocks that must be compensated isostatically, so structural trend toward the batholith from the north and origin is indicated. The larger of these high and Geological Society of America Bulletin, v. 73, p. 511-516, 1 fig., April 1962 511 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/4/511/3432160/i0016-7606-73-4-511.pdf by guest on 02 October 2021 512 WARREN HAMILTON-LATE CENOZOIC STRUCTURE, WEST-CENTRAL IDAHO Figure 1. Late Cenozoic structure of northeastern Oregon and west-central Idaho. Geology in Oregon and in Idaho near Snake River Plain compiled largely from published sources; geology in remainder of Idaho based largely on reconnaissance by Warren Hamilton. low areas are interpreted as structural domes miles wide has compensated for lightening of and basins, respectively, on Figure 1. the crust by canyon erosion. The last mentioned The Salmon River flows across the mountains seems most likely. Erosion of the canyon has in a narrow canyon 4000-6000 feet deep. The removed several cubic miles of rock per mile of peaks and ridges 3-8 miles from the river on canyon length, and about the same volume of both sides generally stand about 1000 feet flanking peaks now stands above the regional higher than do summits 5 to 15 miles farther level. The close correspondence between back. (This feature is shown strikingly by the regional altitude and gravity intensity in most pressed-relief 1:250,000 maps of the Army places (for example, Mabey, 1960) shows that Map Service.) Possible explanations for this isostatic balance is generally achieved. Iso- relationship are: entrenchment along the crest static compensation for erosion certainly takes of a curving ridge; control of the river course place; whether such compensation can be by faults and folds which have continued to discerned against a background of other factors rise along it; control by hydrologic factors such is unproved. as grade, load, and runoff of master and tributary streams, complicated by regional Western Idaho Fault Belt uplift or other variables; or that isostatic up- The west margin of the Salmon River lift of a belt centered on the river and about 30 Mountains is a north-trending belt about 30 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/4/511/3432160/i0016-7606-73-4-511.pdf by guest on 02 October 2021 SHORT NOTES 513 EXPLANATION Sedimentary deposits and volcanic rocks Alluvium and glacial deposits of Quaternary age north of 44°40', and east of 116° 15 ; alluvium of Quaternary age and volcanic rocks of Pliocene age in southwest Columbia River Basalt Loco/// Includes associated sediments , and also other volcanic rocks of middle Cenozoic age Pre-Tertiary crystalline rocks Mostly Idaho batholith (Cretaceous) and its border zone east of 116° 20] and rocks meta- morphosed and intruded by stocks during Jurassic orogeny west of I'16°'20. Locally includes younger rocks Normal fault of late Cenozoic age Dashed where uncertain. Ticks on downthrown side Axis of anticline Axis of syncline Showing direction of plunge. Dashed where uncertain \ \ Flexure at top of monocline Flexure at bose of monocline Showing direction of plunge. Dashed where uncertain Dome Basin Dashed where uncertain. Larger symbol indicates larger structure Contact Dashed where uncertain 10 20 30 40 50 MILES miles wide of normal faults and monoclines and gentle dip slopes characterize the basaltic ter- subordinate other folds. Some of the structures rane. in this belt were described by Kirkham (1931), The faults offset the Miocene Columbia Anderson (1934), and Capps (1941). Most of River Basalt. Creep of possible tectonic origin the fault blocks are tilted westward and have occurred along two faults in 1927 and 1928 imposing east-facing scarps, the largest nearly (Kirkham and Johnson, 1929). Most of the 3000 feet high; the monoclines also dip west- faults are marked by impressive scarps, vari- ward. The general altitude, interrupted by one ably modified by erosion. A number of scarps fault after another, decreases irregularly west- which I saw first on topographic maps were ward across the belt, and the level west of the confirmed by field investigation to be faults belt is little more than 3000 feet south of which offset the basalt. latitude 45°00' and north of 45°45'; between The western Idaho fault belt is broadly con- these latitudes are the high Seven Devils gruent with the north-trending border zone Mountains and a relatively high plateau to the of gneisses and schists (Hamilton, unpublished west. The average throw on each of the long data; D. L. Schmidt, 1958, U.S. Geol. Survey faults is about 1000 or 1500 feet. Structural Open File Rept. 456) west of the Idaho batho- relief is much increased locally by open folds lith of middle Cretaceous age (Jaffe etal., 1959). with dips to 35° and is as great as 7000 feet in The western limit of the continuous massive the vicinity of Riggins, but dips are less than granodiorite and quartz monzonite of the 10° over most of the region, and long, regular, interior of the batholith lies within a few miles Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/73/4/511/3432160/i0016-7606-73-4-511.pdf by guest on 02 October 2021 514 WARREN HAMILTON-LATE CENOZOIC STRUCTURE, WEST-CENTRAL IDAHO on either side of the 116th meridian, and this is also the eastern limit of the fault belt. West Columbia Plateau Province of the massive granitic rocks is a zone about West of the western Idaho fault belt is the 20 miles wide of gneisses in which the propor- Columbia Plateau province of irregular uplifts tion of intrusive granitic material (gneissic in northeastern Oregon and the adjacent part quartz diorite, granodiorite, quartz monzonite, of Idaho. Both late Cenozoic and pre-Tertiary and trondhjemite) decreases westward as the structures here are quite different from those proportion of metamorphic gneiss increases.