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Snake Range Fault

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Journal Article

Rapid Miocene slip on the Snake Range–Deep Creek Range fault system, east-central Nevada Available to Purchase

Elizabeth L. Miller, Trevor A. Dumitru, Roderick W. Brown, Phillip B. Gans
Journal: GSA Bulletin
Published: 01 June 1999
GSA Bulletin (1999) 111 (6): 886–905.
DOI: https://doi.org/10.1130/0016-7606(1999)111<0886:RMSOTS>2.3.CO;2
..., a classic Basin and Range metamorphic core complex detachment fault in east-central Nevada. Here, the fission-track method provides a particularly effective tool for dating faulting where bracketing or crosscutting relations are not available. These new data suggest that the Snake Range décollement forms...
View PDF for article title, Rapid Miocene slip on the <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span>–Deep Creek <span class="search-highlight">Range</span> <span class="search-highlight">fault</span> system, east-central Nevada
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Journal Article

Thrust fault geometry of the Snake River Range, Idaho and Wyoming Available to Purchase

NICHOLAS B. WOODWARD
Journal: GSA Bulletin
Published: 01 February 1986
GSA Bulletin (1986) 97 (2): 178–193.
DOI: https://doi.org/10.1130/0016-7606(1986)97<178:TFGOTS>2.0.CO;2
...NICHOLAS B. WOODWARD Abstract The Snake River Range is located at the north end of the Absaroka fault system in the Idaho-Wyoming-Utah thrust belt, and it adjoins the Teton Mountains. Nine imbricate sheets of the Absaroka system form a shingled array of overlapping thrusts. Most fault nomenclature...
View PDF for article title, Thrust <span class="search-highlight">fault</span> geometry of the <span class="search-highlight">Snake</span> River <span class="search-highlight">Range</span>, Idaho and Wyoming
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Journal Article

Audiomagnetotelluric characterization of range-front faults, Snake Range, Nevada Available to Purchase

Theodore H. Asch, Donald S. Sweetkind
Journal: Geophysics
Published: 06 January 2011
Geophysics (2011) 76 (1): B1–B7.
DOI: https://doi.org/10.1190/1.3511358
...Theodore H. Asch; Donald S. Sweetkind Abstract Two controlled-source audio-magnetotelluric (CSAMT) profiles were collected on the eastern flank of the Snake Range in eastern Nevada across geologically complex terrain to investigate the suspected presence of faults along the range front...
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View PDF for article title, Audiomagnetotelluric characterization of <span class="search-highlight">range</span>-front <span class="search-highlight">faults</span>, <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span>, Nevada
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Image
Figure 1. Simplified geologic map of Robinson district. Inset: ER— Egan Range, SCR—Schell Creek Range, SR—Snake Range. Fault abbreviations: A—Alpha, BP—Ball Park, D—Dit, E—Eureka, EL—Elijah, FE—Footwall East, FW—Footwall West, HG—High Grade, K— Keystone, KB—Kaibab, L—Liberty, PK—Pilot Knob, Q—Queen. Modified from Brokaw and Heidrick (1966), Brokaw (1967), Brokaw and Barosh (1968), and Brokaw et al. (1973)

Figure 1. Simplified geologic map of Robinson district. Inset: ER— Egan Ran... Available to Purchase

Published: 01 June 2001
Figure 1. Simplified geologic map of Robinson district. Inset: ER— Egan Range, SCR—Schell Creek Range, SR—Snake Range. Fault abbreviations: A—Alpha, BP—Ball Park, D—Dit, E—Eureka, EL—Elijah, FE—Footwall East, FW—Footwall West, HG—High Grade, K— Keystone, KB—Kaibab, L—Liberty, PK—Pilot Knob, Q—Queen
Image
Figure 1. Two extensional structures with low-angle décollements. A: Interpretative north- south cross section of western part of Gulf of Corinth (Rigo et al., 1996). Aigion and Helike are active high-angle faults. Décollement fault beneath gulf is suggested by microseismicity analysis and low-angle fault planes (15°–30°) determined from microearthquake focal mechanisms. B: Interpretive east-west cross section of northern Snake Range fault system (Miller et al., 1999). Décollement surface is at surface level on topographic high and limits top of ductile deformation. Uniform fission-track cooling ages of 17 ± 3 Ma are found below décollement

Figure 1. Two extensional structures with low-angle décollements. A: Inter... Available to Purchase

Published: 01 May 2001
-angle fault planes (15°–30°) determined from microearthquake focal mechanisms. B: Interpretive east-west cross section of northern Snake Range fault system ( Miller et al., 1999 ). Décollement surface is at surface level on topographic high and limits top of ductile deformation. Uniform fission-track
Journal Article

Stable isotope and modeling studies of fluid-rock interaction associated with the Snake Range and Mormon Peak detachment faults, Nevada Available to Purchase

Steven Losh
Journal: GSA Bulletin
Published: 01 March 1997
GSA Bulletin (1997) 109 (3): 300–323.
DOI: https://doi.org/10.1130/0016-7606(1997)109<0300:SIAMSO>2.3.CO;2
...Steven Losh Abstract Stable isotope and fluid-inclusion data were obtained from rocks from traverses within and above the Snake Range and Mormon Peak detachments in Nevada in order to evaluate fluid sources and the nature of fluid flow associated with detachment faults during faulting...
View PDF for article title, Stable isotope and modeling studies of fluid-rock interaction associated with the <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span> and Mormon Peak detachment <span class="search-highlight">faults</span>, Nevada
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Journal Article

Tertiary extensional faulting and evolving ductile-brittle transition zones in the northern Snake Range and vicinity: New insights from seismic data Available to Purchase

P. B. Gans, E. L. Miller, J. McCarthy, M. L. Ouldcott
Journal: Geology
Published: 01 March 1985
Geology (1985) 13 (3): 189–193.
DOI: https://doi.org/10.1130/0091-7613(1985)13<189:TEFAED>2.0.CO;2
...P. B. Gans; E. L. Miller; J. McCarthy; M. L. Ouldcott Abstract New geologic mapping and high-resolution seismic data from east-central Nevada shed light on the evolution of the northern Snake Range decollement (NSRD) and its relation to the present basins and ranges. We suggest that the NSRD...
View PDF for article title, Tertiary extensional <span class="search-highlight">faulting</span> and evolving ductile-brittle transition zones in the northern <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span> and vicinity: New insights from seismic data
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Image
Kinematic model of Jurassic–Cretaceous crustal shortening and Tertiary unroofing (after Lewis et al., 1999). MP—Mississippian through Permian strata; OD—Ordovician through Devonian strata; C—Cambrian strata; Cpm—Prospect Mountain Quartzite; pC—Precambrian; NC—Negro Creek section; HC—Hendry’s Creek section. (A) West-directed thrust system responsible for Jurassic–Cretaceous tectonic burial, with Water Canyon anticline in the southern Deep Creek Range located at the top of the thrust zone (e.g., Nelson, 1966; Rodgers, 1987). Note the initial structural position of Hendry’s Creek required for the formation of the staurolite-garnet schist unit (Lewis et al., 1999; Gébelin et al., 2011). (B) Early Eocene collapse of thickened crust, inducing exhumation, flattening, and 40Ar/39Ar resetting in the metamorphic units (including Negro Creek). Black dashed lines represent the initial trajectory of the northern Snake Range detachment (NSRD) and associated normal fault system. (C) Initial stages of low-angle detachment faulting (northern Snake Range detachment) and propagation of normal faults within the hanging wall. Normal faults and tension fractures facilitate downward transport of meteoric fluids to the active northern Snake Range detachment footwall. (D) Oligocene–Miocene extension showing the evolution of a rolling hinge with Negro Creek and Hendry’s Creek remaining in the footwall of the developing northern Snake Range detachment. Intensively developed east-dipping normal faults and tension fractures permit circulation of meteoric fluids to the northern Snake Range detachment footwall mylonites (Hendry’s Creek) with high heat flow as driving force for hydrothermal fluid circulation in this part of the northern Snake Range detachment. Note the presence of basins that form directly above the zone of active faulting and connect fluids from the surface to depth. In contrast, along the western flank of the range (Negro Creek), the rolling hinge detachment system induces the rotation and translation of blocks preventing the fluid connection between Earth’s surface and the northern Snake Range detachment footwall.

Kinematic model of Jurassic–Cretaceous crustal shortening and Tertiary unro... Available to Purchase

Published: 01 January 2015
the initial trajectory of the northern Snake Range detachment (NSRD) and associated normal fault system. (C) Initial stages of low-angle detachment faulting (northern Snake Range detachment) and propagation of normal faults within the hanging wall. Normal faults and tension fractures facilitate downward
Image
(A) Composite cross section across the Duck Creek, Schell Creek, and Northern Snake Ranges, consisting of B-B′ combined with F-F′ (lines of section shown on Fig. 1), which were joined at the longitude on B-B′ that matched the elevation of the Northern Snake Range décollement (NSRD) at the western edge of F-F′ (see footnote 1). The top cross section shows the present-day geometry, and the bottom cross section shows the geometry after retrodeformation of post–Schell Creek Range detachment system (SCRDS) faults (see footnotes and captions of Figs. 5 and 10). Bold italic numbers are referenced to footnotes. (B) Composite B-B′–F-F′ cross section (shown at a smaller size than A), showing a geometry that is restored for displacement on the linked Schell Creek Range detachment system–Northern Snake Range décollement fault system (SCRDS-NSRD; top cross section) and the post-SCRDS-NSRD geometry (bottom cross section; note that the geometry shown predates Neogene extension along high-angle Basin and Range normal faults). The restored section was drafted following published interpretations indicating that rocks in the Northern Snake Range décollement footwall restore to pre-extensional stratigraphic depths of ~7–12 km (Gans and Miller, 1983; Miller et al., 1983; Gans et al., 1985); we acknowledge that other pre-extensional geometries are possible (see text for discussion). Red numbers 1–4 on the restored section are displacement markers, and their corresponding positions in the footwall (F) and hanging wall (H) of the linked Schell Creek Range detachment system–Northern Snake Range décollement fault system are shown on the deformed cross section. The magnitude of isostatic uplift in rocks that restore to the west of the Duck Creek Range is schematic. COCORP—Consortium for Continental Reflection Profiling.

(A) Composite cross section across the Duck Creek, Schell Creek, and Northe... Open Access

Published: 12 May 2022
are referenced to footnotes. (B) Composite B-B′–F-F′ cross section (shown at a smaller size than A), showing a geometry that is restored for displacement on the linked Schell Creek Range detachment system–Northern Snake Range décollement fault system (SCRDS-NSRD; top cross section) and the post-SCRDS-NSRD
Image
(A) Map of the Basin and Range Province in Nevada and western Utah (modified from Long, 2019), showing the locations of metamorphic core complexes (SR—Snake Range; REH—Ruby–East Humboldt; RAG—Raft River–Albion–Grouse Creek). (B) Generalized geologic map of the Duck Creek, Schell Creek, Northern Snake, and southern Snake Ranges (modified from Lee et al., 2017). The map pattern of the basal fault of the Schell Creek Range detachment system (SCRDS) is outlined in red, and the map pattern of the Northern Snake Range décollement (NSRD) is outlined in purple. R.—Range, Mts.—Mountains. (C) Geologic map of the Schell Creek and Duck Creek Ranges, compiled from the mapping of Young (1960), Dechert (1967), Drewes (1967), Hose and Blake (1976), Gans et al. (1985), Walker et al. (1992), and this study. The locations of the lines of section for A-A′, B-B′, C-C′, D-D′, and E-E′ are shown, as well as the map areas for Figures 6 and 8. Individual Schell Creek Range detachment faults discussed in the text (A1–A4, B1–B5, C1–C6) are labeled along with strike and dip symbols defining representative bedding attitudes (from Young, 1960; Drewes, 1967) and attitudes of Schell Creek Range detachment faults determined from three-point problems (supporting data in Table S3 [see text footnote 1]).

(A) Map of the Basin and Range Province in Nevada and western Utah (modifie... Open Access

Published: 12 May 2022
, Northern Snake, and southern Snake Ranges (modified from Lee et al., 2017 ). The map pattern of the basal fault of the Schell Creek Range detachment system (SCRDS) is outlined in red, and the map pattern of the Northern Snake Range décollement (NSRD) is outlined in purple. R.—Range, Mts.—Mountains. (C
Image
Simplified geologic map of the Snake Range, east-central Nevada, showing the location of the study area and Great Basin National Park (green solid lines). Map units: 1, intrusive rocks; 2, Neoproterozoic and Lower Cambrian siliciclastic rocks; 3, Paleozoic carbonate rocks; 4, Cenozoic primarily Miocene continental sedimentary rocks; 5, coarse-grained, mostly Pliocene, alluvium; 6, Quaternary alluvium; 7, playa deposits. The heavy solid black line is the mapped trace of Snake Range detachment, a low-angle normal fault; thin solid black lines are mapped traces of major normal faults; the thick dashed line is the inferred trace of a buried major normal fault; and thin dotted lines are the traces of mapped Quaternary faults. Thin brown lines are roads; blue lines are perennial streams. Geology modified from National Park Service 2007 and Miller et al. 1995. (Figure 1 from “Audiomagnetotelluric characterization of range-front faults, Snake Range, Nevada” by Asch and Sweetkind.)

Simplified geologic map of the Snake Range, east-central Nevada, showing th... Available to Purchase

Published: 01 March 2011
, Cenozoic primarily Miocene continental sedimentary rocks; 5, coarse-grained, mostly Pliocene, alluvium; 6, Quaternary alluvium; 7, playa deposits. The heavy solid black line is the mapped trace of Snake Range detachment, a low-angle normal fault; thin solid black lines are mapped traces of major normal
Image
Cross-section B-B′, showing the modern geometry. Schell Creek Range detachment system (SCRDS) faults B1–B5 are labeled, and bold italic numbers are referenced to footnotes. The subsurface geometry for Spring Valley and the western part of the Northern Snake Range is from the interpreted seismic cross section of Gans et al. (1985, their fig. 2). Translucent areas above the modern surface represent eroded rock. (B) Cross-section B-B′ after retrodeformation of displacement on post–Schell Creek Range detachment system high-angle normal faults and 15° of post–Schell Creek Range detachment system westward tilting (see footnote 2). Stratigraphic cutoff angles are labeled for Schell Creek Range detachment system faults. NSRD—Northern Snake Range décollement.

Cross-section B-B′, showing the modern geometry. Schell Creek Range detachm... Open Access

Published: 12 May 2022
Figure 5. Cross-section B-B′, showing the modern geometry. Schell Creek Range detachment system (SCRDS) faults B1–B5 are labeled, and bold italic numbers are referenced to footnotes. The subsurface geometry for Spring Valley and the western part of the Northern Snake Range is from the interpreted
Journal Article

Core complex mechanics: From the Gulf of Corinth to the Snake Range Available to Purchase

Jean Chéry
Journal: Geology
Published: 01 May 2001
Geology (2001) 29 (5): 439–442.
DOI: https://doi.org/10.1130/0091-7613(2001)029<0439:CCMFTG>2.0.CO;2
...-angle fault planes (15°–30°) determined from microearthquake focal mechanisms. B: Interpretive east-west cross section of northern Snake Range fault system ( Miller et al., 1999 ). Décollement surface is at surface level on topographic high and limits top of ductile deformation. Uniform fission-track...
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View PDF for article title, Core complex mechanics: From the Gulf of Corinth to the <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span>
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(a) The Snake River plain is bounded to the north and southeast by the basin-and-range block-faulted mountains and to the southwest and northeast by the rhyolitic and basaltic Owyhee and Yellowstone plateaus (after Kuntz et al., 1992). (b) Nine rift zones (dark, elongated features on the eastern Snake River plain) trend northwest-southeast. Light circular areas represent the locations of underlying, subsurface rhyolitic calderas that influence features in the rift zones. Rift zones appear as extensions of the basin-and-range normal faults (Embree and McBroome, 1982; Kuntz et al., 1992).

(a) The Snake River plain is bounded to the north and southeast by the basi... Available to Purchase

Published: 31 October 2007
Figure 1. (a) The Snake River plain is bounded to the north and southeast by the basin-and-range block-faulted mountains and to the southwest and northeast by the rhyolitic and basaltic Owyhee and Yellowstone plateaus (after Kuntz et al., 1992 ). (b) Nine rift zones (dark, elongated features
Image
—F2 quartz-muscovite tectonic accumulations in higher part of marble in Kern Mountains and Snake Range. a. Small wedge structures. Microscopic faults repeat lens. Dolomite aligned along fault and in large crystal at edge of accumulation. Undulatory extinction adjacent to faults. Rodding structure at edge of accumulation and microscopic faults become parallel with quartz-calcite contact at edge of accumulation. b. Parasitic folds. c. b-pencillike structures. Almond-shaped bodies elongate east-west commonly show this microscopic pattern. d. Fold apex. Orientation of 0001 into tectonite fabric completed prior to folding. Structures are typical in both eastern Kern and northern Snake Ranges.

—F 2 quartz-muscovite tectonic accumulations in higher part of marble in K... Available to Purchase

Published: 01 February 1969
Fig. 16. —F 2 quartz-muscovite tectonic accumulations in higher part of marble in Kern Mountains and Snake Range. a . Small wedge structures. Microscopic faults repeat lens. Dolomite aligned along fault and in large crystal at edge of accumulation. Undulatory extinction adjacent to faults
Journal Article

The low-angle breakaway system for the Northern Snake Range décollement in the Schell Creek and Duck Creek Ranges, eastern Nevada, USA: Implications for displacement magnitude Open Access

Sean P. Long, Jeffrey Lee, Nolan R. Blackford
Journal: Geosphere
Published: 12 May 2022
Geosphere (2022) 18 (4): 1194–1222.
DOI: https://doi.org/10.1130/GES02482.1
... are referenced to footnotes. (B) Composite B-B′–F-F′ cross section (shown at a smaller size than A), showing a geometry that is restored for displacement on the linked Schell Creek Range detachment system–Northern Snake Range décollement fault system (SCRDS-NSRD; top cross section) and the post-SCRDS-NSRD...
FIGURES
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View PDF for article title, The low-angle breakaway system for the Northern <span class="search-highlight">Snake</span> <span class="search-highlight">Range</span> décollement in the Schell Creek and Duck Creek <span class="search-highlight">Ranges</span>, eastern Nevada, USA: Implications for displacement magnitude
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Journal Article

Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the Snake River Plain in the Knoll Mountain–Ruby–East Humboldt Range region, northeastern Nevada and south-central Idaho Open Access

Phyllis Camilleri, Jack Deibert, Michael Perkins
Journal: Geosphere
Published: 25 September 2017
Geosphere (2017) 13 (6): 1901–1948.
DOI: https://doi.org/10.1130/GES01318.1
... streams currently drain north into the Snake River Plain. The Knoll and Thousand Springs basins form half-grabens that are filled with the ca. 16 Ma to ca. 8–5 Ma Humboldt Formation, which was deposited in alluvial, eolian, and lacustrine environments during slip along range-bounding faults and a series...
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View PDF for article title, Middle Miocene to Holocene tectonics, basin evolution, and paleogeography along the southern margin of the <span class="search-highlight">Snake</span> River Plain in the Knoll Mountain–Ruby–East Humboldt <span class="search-highlight">Range</span> region, northeastern Nevada and south-central Idaho
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Figure 3. Evolutionary model of core complex. A: Half graben, or Andersonian stage: stress orientation is not perturbed, because of high fault friction. B: Gulf of Corinth stage: elevated pore pressure Pp leads to low effective friction that forces σ1 to be fault parallel in footwall. Low-angle fault forms and is ready to act as décollement. C: Décollement propagation stage: upper crust is thinned above décollement by normal faulting. New high-angle faults control décollement propagation and help crustal exhumation. D: Snake Range stage: major and rapid horizontal extension (20–40 km in a few million years) raises isostatically and isothermally. Décollement develops as antiform that migrates toward shallow depths (0–4 km). Temperature reequilibrates to regional values after end of extension, resulting in uniform cooling ages below décollement, as observed in northern Snake Range

Figure 3. Evolutionary model of core complex. A: Half graben, or Andersonia... Available to Purchase

Published: 01 May 2001
-angle fault forms and is ready to act as décollement. C: Décollement propagation stage: upper crust is thinned above décollement by normal faulting. New high-angle faults control décollement propagation and help crustal exhumation. D: Snake Range stage: major and rapid horizontal extension (20–40 km
Journal Article

Structural Development of Northernmost Snake Range, Kern Mountains, and Deep Creek Range, Nevada and Utah Available to Purchase

Robert B. Nelson
Journal: AAPG Bulletin
Published: 01 May 1966
AAPG Bulletin (1966) 50 (5): 921–951.
DOI: https://doi.org/10.1306/5D25B605-16C1-11D7-8645000102C1865D
... in the northern Snake Range and Kern Mountains is the Snake Range thrust of Misch and Hazzard. A foliated marble with thick schistose members and boudinage zones occurs as the highest part of the autochthonous sequence. In the southern Deep Creek Range a reverse fault, whose southward movement is believed to have...
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Chapter 13: Geophysics of the Intermontane system Available to Purchase

Author(s)
L. C. Pakiser
Book: Geophysical Framework of the Continental United States
Publisher: Geological Society of America
Published: 01 January 1989
DOI: 10.1130/MEM172-p235
... Crustal thicknesses in the Intermontane system—Colorado Plateau, Basin and Range province, and High Lava Plains of the Cordilleran region—have been reliably determined by an extensive network of seismic-refraction profiles as about 30 km in the Basin and Range and 42 km in both the Snake River...
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Crustal thicknesses in the Intermontane system—Colorado Plateau, Basin and Range province, and High Lava Plains of the Cordilleran region—have been reliably determined by an extensive network of seismic-refraction profiles as about 30 km in the Basin and Range and 42 km in both the Snake River Plain and the Colorado Plateau. The upper crust (velocity, 5.2 to 6.3 km/sec) averages about 20 km in thickness in the Basin and Range, 10 km in the Snake River Plain, and 28 km in the Colorado Plateau. The velocity in the lower crust ranges from 6.4 to possibly 7.5 km/sec. Upper-mantle velocities of 7.4 to 8.0 km/sec have been reported, but the lower P n velocities are probably apparent downdip velocities. The true upper-mantle velocity throughout the Intermontane system is probably 7.8 to 7.9 km/sec. Moho depths from seismic-reflection studies are about the same as those from refraction studies in the Basin and Range, but they are 5 to 10 km deeper in the Colorado Plateau. The patterns of crustal reflections are distinctly different in the western and eastern parts of the Basin and Range. Geophysical studies indicate that the lithosphere is 50 to 65 km thick in the Basin and Range, 50 km in the Snake River Plain, and 90 to 100 km in the Colorado Plateau. Bouguer gravity values range from -50 mGal in southwestern Arizona to -250 mGal in the Uinta basin of northeastern Utah. Gravity correlates inversely with topography in the Basin and Range, indicating isostatic equilibrium, but the Snake River Plain is slightly undercompensated. Long-wavelength gravity anomalies indicate that much of the isostatic compensation results from a mass deficiency in the mantle, probably related to variations in the thickness of the lithosphere. Prominent magnetic-high anomalies in the Snake River Plain and north-central Nevada result from mafic rock formed in Miocene rifts. Heat flow is high throughout the Intermontane system, ranging from 1.5 heat-flow units (HFU) in central Nevada and the Colorado Plateau to more than 2.5 HFU in northwestern Nevada and the Snake River Plain. Electrical conductivity is anomalously high in the lower crust and upper mantle of the system. Seismicity is distributed in broad bands in the Nevada and intermountain seismic zones, and epicenters commonly do not coincide with mapped faults; the larger earthquakes (magnitude greater than 6.0), however, occur only on planar, normal faults with dips of 50 to 60°. The stress field is compatible with lateral extension in the Basin and Range. Examination of evidence for a “double Moho” in Utah suggests that the apparent velocities associated with the Moho propagation paths on which this model is based vary widely, indicating severe relief on the refractor surface; the evidence favors a single Moho, below which the velocity is 7.8 to 7.9 km/sec. Ray tracing and amplitude studies leave unresolved the question of whether a transitional layer of velocity 7.5 km/sec exists in the lower crust of the Basin and Range. Many geologic and geophysical characteristics are similar throughout the Intermontane system, but lithospheric models are distinctly different in the separate provinces, owing to the different geologic processes that have prevailed. The crust throughout the system prior to Cenozoic extension and volcanism probably consisted of an upper layer of silicic gneisses and schists overlying a lower crust of silicic to intermediate composition in the granulite facies. During extension, the crust of the Basin and Range was thinned and invaded by gabbroic magma from the mantle, resulting in a lower crust consisting of as much as two-thirds mafic material. During crustal rifting or extension in the Snake River Plain, the crust was extensively invaded by mafic magma from the mantle, and a silicic fraction was fused and erupted as rhyolite. The Snake River Plain crust is now predominantly mafic, and the 10-km-thick upper crust consists primarily of sedimentary and volcanic rocks. Resolution of remaining unsolved problems must await thorough reanalysis of available data and new field experiments based on coincident application of different geophysical methods, combined with geologic studies and deep drilling.