U.S. Department of the Interior

U.S. Geological Survey

Circular 1334

The Trans–Rocky Mountain Fault System—

A Fundamental Precambrian

Strike-Slip System
Cover photograph: Mountaintop view, Colorado. 

Courtesy of John J. Mosesso, NBII
The Trans–Rocky Mountain Fault System—

A Fundamental Precambrian 

Strike-Slip System

By P.K. Sims 

Circular 1334

U.S. Department of the Interior

U.S. Geological Survey
U.S. Department of the Interior

KEN SALAZAR, Secretary

U.S. Geological Survey

Suzette M. Kimball, Acting Director

U.S. Geological Survey, Reston, Virginia: 2009

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Suggested citation:

Sims, P.K, 2009, The Trans–Rocky Mountain Fault System—A fundamental Precambrian strike-slip system: U.S. 

Geological Survey Circular 1334, 13 p.
iii

Contents

Abstract...........................................................................................................................................................1

Introduction.....................................................................................................................................................1

Geologic Framework .....................................................................................................................................2

Trans–Rocky Mountain Fault System.........................................................................................................2

Magnetic Expression............................................................................................................................2

Geologic Characteristics.....................................................................................................................4

Mechanics of Deformation .................................................................................................................6

Regional Extent and Age .....................................................................................................................6

Dynamics................................................................................................................................................6

Rejuvenation..........................................................................................................................................7

Belt Supergroup Sedimentation................................................................................................8

Central Idaho Disturbance .........................................................................................................8

Late Neoproterozoic Transform Faulting .................................................................................8

Late Neoproterozoic and Cambrian–Ordovician Alkalic Plutonism....................................8

Mesozoic Sevier Thrust Faulting...............................................................................................8

Metamorphic Core Complexes..................................................................................................9

Summary and Discussion.............................................................................................................................9

Acknowledgments.........................................................................................................................................9

References Cited..........................................................................................................................................10

Figures

1. Map of the Rocky Mountain region showing relation of Precambrian-cored

uplifts to basement faults of the Trans–Rocky Mountain fault system................................3

2. Aeromagnetic map of southwestern Colorado, showing traces of major shear

zones of the Trans–Rocky Mountain fault system ..................................................................4

3. Map showing transcurrent faults in central Front Range, Colorado ..................................5

4. Rose diagram showing trend of the transcurrent faults in the Rocky

Mountain region............................................................................................................................6

5. Map showing central Idaho........................................................................................................7

Abstract

Recognition of a major Precambrian continental-scale, two-stage conjugate strike-slip 

fault system—here designated as the Trans–Rocky Mountain fault system—provides new 

insights into the architecture of the North American continent. The fault system consists chiefly 

of steep linear to curvilinear, en echelon, braided and branching ductile-brittle shears and 

faults, and local coeval en echelon folds of northwest strike, that cut indiscriminately across 

both Proterozoic and Archean cratonic elements. The fault system formed during late stages 

of two distinct tectonic episodes: Neoarchean and Paleoproterozoic orogenies at about 2.70 

and 1.70 billion years (Ga). In the Archean Superior province, the fault system formed (about 

2.70–2.65 Ga) during a late stage of the main deformation that involved oblique shortening 

(dextral transpression) across the region and progressed from crystal-plastic to ductile-brittle 

deformation. In Paleoproterozoic terranes, the fault system formed about 1.70 Ga, shortly following amalgamation of Paleoproterozoic and Archean terranes and the main Paleoproterozoic 

plastic-fabric-producing events in the protocontinent, chiefly during sinistral transpression. The 

postulated driving force for the fault system is subcontinental mantle deformation, the bottomdriven deformation of previous investigators. This model, based on seismic anisotropy, invokes 

mechanical coupling and subsequent shear between the lithosphere and the asthenosphere such 

that a major driving force for plate motion is deep-mantle flow.

Introduction

Transcurrent faults, chiefly northwest striking, are fundamental basement structures in the 

interior of the North American continent (Sims and others, 2008). These faults were developed 

during Neoarchean and Paleoproterozoic orogenesis. The faults are of particular significance 

because (1) they were significant structures in the early tectonic evolution of the continent, 

and (2) they localized much subsequent tectono-magmatic activity, particularly in the Western 

United States. The Rocky Mountain region provides a natural laboratory for study of the fault 

system because of abundant exposures of exhumed basement rocks and structural features as 

well as neofaults formed during reactivation. 

The basic concept of transcurrent faulting in the Rocky Mountains and implications for 

basement control were conceived during early tectonic analyses in the region (Thom, 1923; 

Chamberlin, 1940, 1945; Sales, 1968; Stone, 1969). The tectonic models of Sales (1968) and 

Stone (1969), based chiefly on geologic field studies, were prescient in that they related the 

faults to a regional shear couple associated with northeast-southwest shortening. Chamberlin 

(1940, p. 698) proposed a geometric analogy with structures associated with the San Andreas 

fault system in California. Baars and others (1995) also recognized the geometric similarity of 

the Trans–Rocky Mountain fault system to the San Andreas fault system. The work of Stone 

(1969), in particular, established wrench faulting qualitatively as the causal mechanism for the 

transcurrent fault system in the Rocky Mountain region. 

The Trans–Rocky Mountain Fault System—

A Fundamental Precambrian 

Strike-Slip System
2 The Trans–Rocky Mountain Fault System—A Fundamental Precambrian Strike-Slip System

This paper is an outgrowth of preparation of regional 

geologic maps of the Precambrian basement in the Western 

United States (for Wyoming and Colorado, Sims and others, 

2001; for Montana, Sims and others, 2002, 2004; for Idaho, 

Sims and others, 2005) followed by preparation of a Precambrian structure map of the continental United States (Sims 

and others, 2008). The basement mapping relied heavily on 

interpretation of regional aeromagnetic-anomaly maps (North 

American Magnetic Anomaly Group, 2002). The purpose of 

this paper is to describe general aspects of the fault system, 

including its characteristics and regional extent, with emphasis on the Rocky Mountains, and to provide a background 

needed for more rigorous future kinematic and geochronologic 

studies. I propose that the deformation resulted from subcontinental mantle deformation, in a manner discussed by Tikoff 

and others (2002, 2004) as bottom-driven deformation.

Geologic Framework

The Rocky Mountains are spectacular ranges in the western part of the continental United States (fig. 1) that characteristically have Precambrian cores (Reed and others, 1987). The 

Precambrian rocks form a continuous basement that is covered 

in intervening basins by sedimentary rocks of Mesoproterozoic, Neoproterozoic, Paleozoic, and Late Cretaceous–Tertiary 

age, and volcanic rocks of Tertiary and Quaternary age. These 

basement rocks are intruded at places by late Mesozoic and 

Tertiary granitic intrusions. The basement rocks were deformed 

by regional tectonic events in Archean and Paleoproterozoic 

times. The basement and overlying rocks have been deformed 

subsequently in the Western United States, principally by a late 

Paleozoic orogeny equated with development of the Ancestral 

Rocky Mountains (Tweto, 1975; Hamilton, 1981, 1988; Perry 

and others, 1992; Ye and others, 1996), Cretaceous–Eocene 

(Laramide) orogeny (Tweto, 1975, 1980a), and differential 

uplift during the Neogene (Taylor, 1975; Tweto, 1980b), followed by extensive normal faulting (basin-and-range structure) 

along the western and southern parts of the Rocky Mountain 

region (fig. 1; Tweto, 1979). The Sevier orogen (Armstrong, 

1968), or Cordilleran thrust belt (Powers, 1983), which 

temporally and spatially overlaps with the Laramide orogen, 

encroached from a western hinterland (Dickinson, 2001) and 

disturbed the western margin of the Northern Rocky Mountain 

Province (Kulik and Schmidt, 1988). 

The Precambrian basement terranes in the region consist 

of the Archean Wyoming province (Houston and others, 1993) 

and three accretionary Paleoproterozoic ocean-arc terranes 

(fig. 1). The oldest of these, the Trans–Montana orogen (Sims 

and others, 2002, 2004), coincides approximately with the 

previously designated Great Falls tectonic zone of O’Neill 

and Lopez (1985). It was sutured to the northwest margin of 

the craton at ca. 1.85 Ga as indicated by the age of oceanic 

arc rocks exposed in the Little Belt Mountains in Montana 

(fig. 1) (Mueller and others, 2002). The Trans–Hudson orogen 

(Peterman and Sims, 1993)—not shown in figure 1—was 

attached to the eastern margin (circa 1.8 Ga) of the Wyoming 

province craton; and finally the Colorado province (Bickford 

and others, 1986; Sims and Stein, 2003) was added to the 

southern margin during the interval 1.78 to 1.70 Ga. Following 

stabilization of the protocontinent, rocks within the Colorado province (fig. 1) were deformed by intraplate tectonism, 

designated the Berthoud orogeny (circa 1.7–1.4 Ga; Sims and 

Stein, 2003), which is expressed by northeast-striking ductile 

shear zones accompanied by regional thermal metamorphism 

(Selverstone and others, 1997, 2000) and younger, mainly 

alkali-calcic plutonism (1.5–1.35 Ga; Anderson and Cullers, 

1999; Sims and others, 2001). The Berthoud orogeny was followed abruptly by renewed continent-scale strike-slip faulting 

in Paleoproterozoic terranes (Sims and Stein, 2003).

Trans–Rocky Mountain Fault System

The Trans–Rocky Mountain fault system is a systematic 

array of strike-slip shears and faults (fig. 1) of Precambrian 

ancestry. The system consists chiefly of northwest-striking 

strike-slip faults, commonly with horizontal displacements 

of a few to several tens of kilometers (km) (for example, 

Lewis and Clark line, Wallace and others, 1990; Clearwater 

shear zone, Sims and others, 2005; J.M. O’Neill, 2007, written commun.), and conjugate north-striking (transfer) faults. 

Repeated rejuvenation of the faults mainly involved reactivation of discrete preexisting structures, but also involved the 

development of neoformed offshoots of the preexisting faults, 

as demonstrated by Bump (2003) for the Laramide orogeny in 

the Rocky Mountain region.

Magnetic Expression

Individual structures within the Trans–Rocky Mountain 

fault system are commonly expressed on magnetic-anomaly 

maps (North American Magnetic Anomaly Group, 2002) by 

linear negative anomalies, as shown in figure 2, an area typical of the Rocky Mountain region. The shear zones and faults 

are conspicuous where they transect basement rocks with 

strong positive magnetism, such as Archean and Proterozoic 

calc-alkaline granitic plutons, but are less well defined in 

metamorphosed volcanic and sedimentary country rocks with 

neutral magnetism. The negative expression mainly results 

from the destruction of magnetite caused by ductile-brittle 

shearing and attendant alteration by circulating fluids during 

retrograde metamorphism (Finn and Sims, 2005). The magnetic 

expression of the faults indicates that they are deep seated, 

fundamental fracture zones that transect the basement rocks.

Interpretation of magnetic features is highly subjective, 

but existing geologic mapping in several areas in the region, 

such as that shown in figure 3, provides reliable ground truth 

for most areas. Aeromagnetic-anomaly data provide a means 

to extrapolate known geology into covered regions. 
Trans–Rocky Mountain Fault System 3

( ( ( ( ( ( ( (

( ( (

( ( (

(

( ( (

(

( (

( ( (

( (

( ( ( (

( (

(

( ( ( ( (

(

(

(

(

( (

(

LC

SJSZ

SR

GFSZ

DSZ

MMZ

B

115°W

115°W

110°W

110°W 105°W

105°W

100°W

35°N 35°N

40°N 40°N

45°N 45°N

Beartooth

Mtns

Bighorn

Mtns

Front Range

Needle

Mtns

SandiaManzano

Mtns

TRANS-MONTANA

OROGEN

Fold-and-Thrust belt

SW Montana

re-entrant

Sevier

thrust

front

Sevier

thrust

front

Wind River

 Mtns

Uinta Mtns

Cheyenne Belt

Little

Belt Mtns

MONTANA

SOUTH

DAKOTA

COLORADO

NEW MEXICO

IDAHO

WYOMING PROVINCE

OROGEN

WYOMING

Rio Grande rift

Fig. 2 outline

Fig. 3 outline

SRS

0.706 Sr

CSZ

Fig. 5

outline

Sevier

thrust

front

COLORADO

PLATEAU

ARIZONA

UTAH

Eastern limit of

Basin and

Range

extension

Tusas

Mtns

Sawatch

Range

COLORADO

( ( ( ( ( ( ( ( ( ( (

(

( (

( ( ( ( ( ( ( (

( (

(

( ( ( ( ( (

, ,

, , , , , ,

Figure 1. Rocky Mountain region showing relation of Precambrian-cored uplifts to basement faults of the Trans–Rocky 

Mountain fault system. Small inset map shows location of Rocky Mountain region. B, Bitterroot mylonite zone; CSZ, 

Clearwater shear zone; DSZ, Dillon shear zone (suture); GFSZ, Great Falls shear zone; MMZ, Madison mylonite zone; 

SRS, Salmon River suture zone; SJSZ, St. Joe shear zone; LC, Lewis and Clark line; SR, Snake River tectonic zone. 

0.706 Sr line (red) essentially follows the Salmon River suture. Map compiled by author.
4 The Trans–Rocky Mountain Fault System—A Fundamental Precambrian Strike-Slip System

39 o

38o

37 o

o

107 105 o 106 o104 o 108 o109 o

o

107 105 o 106 o104 o 108 o109 o

39 o

38o

37 o

Front Range

G

I

C

Sawatch Range

Needle 

Mtns

Wet

Mtns

Pikes

Peak

batholith

Homestake

-470 -275 -185 -105 -40 25 115 220 475

nT

25 0 25 50

km

~~~

~~~~~~~

~

0 25 50 KILOMETERS

Geologic Characteristics

The Trans–Rocky Mountain fault system (fig. 1) is a 

complex array of braided and branching faults, commonly en 

echelon, with locally accompanying en echelon folds. Initial 

structures formed chiefly under ductile (mylonite) conditions, 

as indicated by exposures of deeply exhumed Precambrian 

basement rocks (Tweto, 1980c). Commonly, mylonitic textures 

are partly degraded by structures indicative of more brittle 

conditions of deformation caused by reactivation under lower 

temperature-pressure conditions or higher strain rate at higher 

crustal levels.

Examples of typical known strike-slip faults include the 

Cimarron–Red Rocks shear zone in the Gunnison River area 

(C, fig. 2) and the Gore and Ilse faults (G and I respectively, 

fig. 2) in Colorado, as described by Tweto (1980c), and the 

Clearwater shear zone in Idaho (Sims and others, 2005). In 

Montana, abundant northwest-striking transcurrent faults with 

predominant sinistral displacements have been identified in 

the mountain ranges within the southwestern part of the State 

(Schmidt and Garihan, 1983; Ruppel, 1982). The Lewis and 

Clark fault is a major segment of the fault system in Montana; it records predominant dextral strike-slip displacement, 

apparently resulting chiefly from Late Cretaceous reactivation 

(Wallace and others, 1990).

The transcurrent fault system has been mapped across a 

broad area in the well-exposed east-central part of the Colorado Front Range (Gable, 2000) (fig. 3). In this area, steeply 

dipping folds on northwest to east-west axes predated the 

faulting and presumably formed by regional north-south 

shortening, accompanying sinistral transpression. The transcurrent faults are several kilometers long, wavy, braided 

or anastamosing and branching northwest-striking sinistral 

strike-slip fault arrays and accompanying conjugate north- to 

Figure 2. Aeromagnetic map of southwestern Colorado, showing traces of major shear zones of the Trans–Rocky Mountain fault 

system. Solid line, fault known from geologic mapping, dash line fault inferred from magnetic data. C, Cimarron–Red Rocks shear zone; 

G, Gore fault; I, Ilse fault; H, Homestake shear zone. 
Trans–Rocky Mountain Fault System 5

northeast-striking dextral faults. The sinistral faults have horizontal displacements of as much as 1.5 km (Junction Ranch 

fault; fig. 3); the north-striking dextral faults have smaller 

horizontal displacements. For example, detailed mapping of 

a major north-northeast-striking fault (Dory Hill fault) within 

the western part of the region of figure 3 (Central City district; 

Sims and others, 1963) shows horizontal dextral displacement 

of Paleoproterozoic rock units of about 100 m; steeply dipping 

IDAHO

SHEAR ZONE

SPRINGS – RALSTON

105°22'30" 105°15'

39°45'

39°30'

39°52'30"

40°

Boulder

Golden

Mountain

front Junction Ranch fault

Floyd Hill fault

Dory Hill fault

Figure 3. Transcurrent faults in central Front Range, Colorado adapted from Gable 

(2000). The faults are dominantly sinistral; they transect and are younger than the 

Idaho Springs–Ralston shear zone. The north-striking faults are dextral faults having 

small displacements. Northeast-trending folds formed during deformation (Berthoud 

orogeny) accompanying Idaho Springs–Ralston shear zone. Northwest-trending, 

en-echelon folds formed during deformation accompanying the sinistral strikeslip faulting. 

Tertiary porphyritic dikes are offset 

about 30 m. In 3-D, the apparent difference in offset may be due to vertical 

displacement of a dipping fault or Tertiary dike, suggesting that the fault was 

reactivated subsequent to emplacement 

of the Tertiary dikes. The main sinistral 

strike-slip faults in the east-central Front 

Range generally are relatively narrow, 

but the fracture pattern indicates that 

the entire region has undergone similar 

transpressional shear. Throughout most 

of the region (fig. 3), the interiors of 

fault-bounded blocks are not appreciably 

deformed, for example, older foliations and folds are virtually undisturbed 

(Gable, 2000). The northwest-striking 

faults shown in figure 3 are reflected by 

the fine-scale northwest-southeast magnetic fabric in the area north of the Pikes 

Peak batholith, shown in figure 2. 

A rose diagram plot of strike-slip 

fault orientations in the Rocky Mountain 

region (fig. 4), determined from regional 

Precambrian basement geologic maps 

of Colorado (Sims and others, 2001), 

Wyoming (Sims and others, 2001), 

Montana (Sims and others, 2004) and 

Idaho (Sims and others, 2005), indicates 

that the faults predominantly strike 

N. 30°–50° W. Principal displacement 

zones (fig. 1); for example, Lewis and 

Clark, Clearwater, and St. Jo shear zones 

in Montana, and Cimarron–Red Rocks 

and nearby Uncompahgre shear zone 

in Colorado (designated as principal 

components of the Snake River–Wichita 

fault zone; Sims and others, 2001), 

strike N. 55°–70° W. A lesser number 

of faults strike N. 30° W. to N. 20° E. 

and N. 70°–90° W. Strike-slip faults in 

the northwest sector of the rose diagram 

chiefly have both known or inferred 

sinistral and dextral displacements. 

Some major faults in central Colorado 

(for example, the Gore and Ilse shear 

zones, fig. 2) strike north-northwest.

The principal faults in the Trans–

Rocky Mountain fault system commonly are expressed 

topographically by northwest-trending linear deep valleys (see 

Thelin and Pike, 1991) that cut across and displace the older 

structural grain, as for example the Lewis and Clark lineament in Montana (Smith, 1965). The combined Gore–Ilse fault 

complex in Colorado also comprises a prominent north- to 

northwest-trending topographic corridor between the Front 

Range and Sawatch Range (fig. 2).
6 The Trans–Rocky Mountain Fault System—A Fundamental Precambrian Strike-Slip System

Mechanics of Deformation

The Trans–Rocky Mountain fault system is interpreted 

to have resulted from transpressional tectonics, as defined 

by Dewey and others (1998). Earlier northwest- to eaststriking folds were followed by ductile shears. Transpression and transtension deviate from simple shear (Sylvester, 

1988) because of a component of, respectively, shortening or 

extension orthogonal to the deformation zone. Such threedimensional noncoaxial strains principally develop in response 

to obliquely convergent or divergent relative motions across 

crustal deformation zones.

Evidence for a conjugate relationship of the sinistral 

northwest and dextral north-south faults is given by the 

repeated field observations of the consistent geometry and 

opposing displacement directions of the two structures.

Regional Extent and Age

The Trans–Rocky Mountain fault system has been interpreted to be part of a continent-scale strike-slip fault system 

(Sims and others, 2008). This interpretation is based on the 

kinematic similarity and orientation of Late Archean shear 

zones in the southern part of the Archean Superior province 

(Hudleston and others, 1988) and those within Paleoproterozoic rocks in the Rocky Mountain region (Sims and 

others, 2008).

The transcurrent faults are interpreted to have formed 

during two distinct major orogenic events in Neoarchean and 

Paleoproterozoic times. They formed in each orogeny during 

the transition from crystal-plastic to ductile-brittle deformation, for example, following development of the main internal 

structures in the rocks—foliation and folds. Hudleston and 

others (1988) and Bauer and Hudleston (1995) have demonstrated that the fault system in the southern Archean Superior 

province formed late in major dextral transpression during the 

approximate interval 2.7–2.6 Ga. A west- to northwest-striking 

transcurrent fault exposed in the Ferris Mountains in Wyoming, within the Archean Wyoming province, and interpreted 

here to be an Archean component of the Trans–Rocky Mountain fault system, has been dated at about 2.7 Ga (Bowers and 

Chamberlain, 1993)—the shear zone cuts a 2.732 Ga granitic 

pluton and is crosscut by a circa 2.71 Ga pluton.

The age of the Paleoproterozoic transcurrent faults 

is less well known than that of the Neoarchean faults. The 

transcurrent faults in the central Front Range (fig. 3) transect 

the Boulder Creek batholith and therefore are younger than 

1.725 Ga (Premo and Fanning, 2000). In Arizona, Karlstrom 

and Bowring (1988) present evidence that faults interpreted 

by me to be of this system are about contemporaneous with 

emplacement of the 1.7 Ga Crazy Basin Quartz Monzonite. 

Minimum age for the faults in Colorado is given by the Iron 

Dike (1.36 Ga) dated by Braddock and Peterman (1989). The 

Neoarchean faults initiated as dextral faults and the Paleoproterozoic faults formed as sinistral faults. 

Dynamics

 The protracted Precambrian faulting and the coherent pattern of the structures throughout the interior of the 

North American plate suggest a common causal mechanism: 

continent-scale transpression resulting from upper mantle 

deformation that involved oblique shortening and progressed 

temporally from crystal-plastic through ductile, to brittle 

deformation. This mechanism is the same as the bottom-driven 

deformation of Tikoff and others (2002, 2004). 

Field studies within the continental basement indicate 

that the primary stress directions through later Precambrian 

time have been orthogonal, involving predominant northnorthwest–south-southeast shortening during Neoarchean 

time, as indicated by the kinematics in the southern Superior 

province (Hudleston and others, 1988), and predominant 

northeast-southwest shortening during major orogeny in late 

Paleoproterozoic time.

Teleseismic images of the North America subcontinental upper mantle (Van der Lee and Nolet, 1997; Dueker and 

others, 2001) provide mirror images of fundamental orthogonal Archean and Paleoproterozoic basement shear zones and 

faults (Sims and others, 2005): (1) northeast-striking ductile 

shear zones, which formed (Berthoud orogeny) in Mesoproterozoic (1.42 Ga) time, and (2) northwest-striking strike-slip 

ductile-brittle faults, which formed during the intervals 2.7 and 

1.7 Ga. This coherence of solid lithosphere structure and 

mantle structure suggests that the upper-mantle structure was 

established during Neoarchean time (see also Karlstrom and 

Humphreys, 1998, for a similar old interpretation), and has 

persisted at least through the Paleoproterozoic. The forces that 

formed the northwest-striking structures are also perpendicular 

to the present-day southwestward (absolute) motion of the 

North America plate (Schutt and Humphreys, 2001).

80

70

60

50

40

30

20 10 0

20

w E

N

N = 155

Figure 4. Rose diagram showing strike of the transcurrent 

faults in the Rocky Mountain region. Compiled from existing 

Precambrian basement geologic maps by Sims. See text for 

discussion. 
Trans–Rocky Mountain Fault System 7

Rejuvenation

The basement fault system had a major role in the subsequent tectonic evolution of the continent interior. In the Rocky 

Mountain region, in particular, reactivation of the faults and 

local reworking of intervening blocks episodically since early 

Mesoproterozoic time have strongly influenced sedimentary 

patterns, intracratonic magmatism, and regional structural features, such as Mesozoic thrust faults (Sims and others, 2008).

The Trans–Rocky Mountain fault system has been reactivated repeatedly since its inception. The rejuvenation chiefly 

involved reactivation of the primary shear zones, but also

involved local crustal reworking, as discussed by Holdsworth 

and others (2001), resulting in prograde metamorphism and/or 

magmatism within fault-bounded blocks. The rejuvenation 

is exemplified by the geology along and adjacent to the 

Clearwater shear zone in Idaho (fig. 5), an area that has been 

mapped in moderate detail (Evans and Green, 2003; O’Neill, 

written commun., 2008).

In the Clearwater shear zone, geologic mapping indicates several distinct and separate tectono-magmatic episodes, 

ranging in age from about 1.5 Ga to at least the Tertiary, which 

can be directly or indirectly related to rejuvenation of the 

transcurrent fault system. This area is symbolic of the rejuvenation throughout at least the western part of the continent.

Figure 5. Map of central Idaho compiled by Karen Lund. This region illustrates many of the rejuvenated 

structures related to the Trans–Rocky Mountain fault system. B, Bitterroot mylonite zone; BG–RR, Brushy 

Gulch–Red River thrust fault; Cz, Clearwater shear zone.

Salmon River suture

46°

45°30'

116° 115° 114°

45°

IDAHO

MONTANA

0 50 KILOMETERS

Columbia River basalt Group, valley fill

Magmatic rocks

Allochthonous island arc rocks

Windermere Supergroup

Metasedimentary rocks of Gospel-Hump

– Plutons (650 Ma)

– Plutons (490 Ma)

1380 granites

Lemhi Group (gneiss and schist)

Lemhi Group (eastern facies) virtually unmet

Yellowjacket Formation

Mostly but not entirely Belt Supergroup

EXPLANATION

QT

TK

Z

Y

Y

Y

Y

Y

Z

Oe

CZ

B

BG-RR

GFSZ

Big Creek shear zone

x

x

x

x

x

x

x

x
8 The Trans–Rocky Mountain Fault System—A Fundamental Precambrian Strike-Slip System

Belt Supergroup Sedimentation

The oldest recognized event apparently related to reactivation of the northern part of the Trans–Rocky Mountain 

strike-slip fault system took place before and during deposition of the 1.47–1.40 Ga Belt Supergroup (Evans and others, 

2000). The Clearwater shear zone (fig. 5) effectively separates 

depocenters between the Belt Supergroup and the combined 

Lemhi Group and Yellowjacket Formation to the south, suggesting that the Clearwater shear zone was active tectonically 

during deposition of the Mesoproterozoic sedimentary rocks 

and that it probably constituted a topographic barrier between 

the two basins. This observation supports the hypothesis of 

Winston (1986) that the Belt Supergroup and correlative rocks 

were deposited in isolated cratonic basins within the continent. 

O’Neill and others (written commun., 2008) have confirmed 

this observation and extended the fault by field mapping 

several kilometers to the southeast; they named the larger fault 

the Great Divide Megashear. The position of the western margin of the continent, as interpreted from geologic and initial 

strontium data (0.706 Sr line; fig. 1), is considered to approximate the position of the continental margin relative to the Belt 

basin in Mesoproterozoic time (Sims and others, 2005).

Central Idaho Disturbance

A tectono-magmatic event of local extent, previously 

designated as the Central Idaho disturbance (Sims and others, 

2005) and earlier as the Salmon River arch (Armstrong, 1975), 

is confined to a 70-km-wide belt adjacent to the Clearwater 

shear zone. The disturbed belt lies between the Clearwater 

zone and the Big Creek fault zone to the south (fig. 5) and 

extends northward between the Atlanta and Bitterroot lobes 

of the Cretaceous Idaho batholith. It consists principally 

of migmatitic gneiss and schist that are intruded by diabase, gabbro-diorite, and megacrystic granite. The tectonomagmatic belt is now exposed in the upper plate of the Mesozoic Brushy Gulch–Red River thrust (fig. 5). The belt has been 

the subject of controversy for more than a quarter of a century. 

The argument revolves around the question of whether or 

not some of the metamorphic rocks in the belt are pre-Belt 

Supergroup in age, as proposed earlier by Armstrong (1975). 

Sims and others (2005) suggested that at least some of the 

metamorphic rocks could be about 1.7 Ga in age and grossly 

equivalent with 1.7 Ga juvenile crust inferred to underlie the 

Mesoproterozoic sedimentary rocks in the region. The central 

Idaho disturbance is similar in age and, probably, origin to the 

East Kootenay orogen in adjacent British Columbia, Canada 

(Leech, 1962; McMechan and Price, 1982). 

Late Neoproterozoic Transform Faulting

During the rift-drift breakup of Rodinia in late Neoproterozoic time (Stewart, 1972), the Clearwater zone is interpreted to have had a major role in sculpting the shape of the 

western continent margin, in a manner suggested by Vauchez 

and others (1997). The 0.706 Sr isotope line coincides closely 

with the Clearwater zone (fig. 1) and a north-trending shear 

zone to the south, which during the Mesozoic Sevier orogeny 

probably localized the Salmon River suture zone (Lund and 

Snee, 1988; Sims and others, 2005). Reactivation of these and 

related strands of the Trans–Rocky Mountain fault system on 

the continental margin (Sims and others, 2005) can account for 

the zig-zag pattern of the Pacific rifted margin.

Late Neoproterozoic and Cambrian–Ordovician 

Alkalic Plutonism

Two apparently discrete episodes of alkalic plutonism are 

recognized within the central Idaho disturbed belt, between the 

Clearwater and Big Creek shear zones (fig. 5)—an older episode dated at 650 Ma and a younger episode of 490 Ma (Karen 

Lund, unpub. data). These bodies intrude mainly amphibolitegrade gneisses inferred to have formed principally from Lemhi 

Group and Yellowjacket Formation protoliths (Lund and 

others, 2004). They are aligned northwest-southeast, suggesting that they were intruded along preexisting shears of the 

Trans–Rocky Mountain fault system. These alkalic plutons are 

comparable in age, composition, and orientation to the Iron 

Hill and other similar alkalic plutons of early Paleozoic age 

localized along the Cimarron–Red Rocks shear zone in southwestern Colorado (Sims and others, 2001).

Mesozoic Sevier Thrust Faulting

Many of the major east-vergent Sevier thrust faults strike 

northwest-southeast, subparallel to the strike of the Trans–

Rocky Mountain fault system (fig. 5). Faults spatially associated with the Clearwater zone include the newly defined eastcentral Idaho thrust belt (Lund and others, 2003b; Tysdal and 

others, 2003). These thrust faults are segmented by inherited 

north- and northeast-striking faults that apparently acted as 

tear faults during Sevier thrust faulting (Tysdal, 2002; Lund, 

2004; Lund and others, 2003; Tysdal and others, 2003). The 

ancient basement structures are interpreted to have focused 

strain during northeast-southwest directed Mesozoic Sevier 

deformation, resulting in neoformed thrust faults.
Acknowledgments 9

Metamorphic Core Complexes

The Precambrian basement strike-slip fault system 

actively participated in the development of Eocene metamorphic core complexes in the northwestern United States 

(Sims and others, 2005). Notably, the Bitterroot core complex, 

which includes the Bitterroot lobe of the Idaho batholith, is 

bounded by conjugate faults interpreted herein to be components of the Trans–Rocky Mountain fault system (fig. 5) that 

were reactivated during formation of the core complex. The 

eastern boundary, the north-striking Bitterroot mylonite zone 

(Hyndman, 1980; Foster and Fanning, 1997), coincides with a 

sharp magnetic linear low that extends from east-central Idaho 

northward through western Montana nearly to the Canada border (Sims and others, 2004); it is interpreted as a reactivated 

fault of the system. The sinistral Clearwater shear zone forms 

the southern margin of the complex. These structures formed 

zones of weakness, the reactivation of which probably localized the core complex.

Summary and Discussion

The Trans–Rocky Mountain fault system, previously 

overlooked as a significant component of the architecture of 

the North American continent, is a fundamental continentscale Precambrian basement structural system. Abundant 

evidence indicates that the fault system formed during the 

Neoarchean and late Paleoproterozoic orogenies; it segmented 

the continent into mostly coherent blocks of different dimensions that have been rejuvenated repeatedly since Mesoproterozoic time. Reactivation of these intracratonic structures 

controlled many aspects of subsequent tectonism, sedimentation, and magmatism. Knowledge of these structures enhances 

our understanding of the early crustal evolution and the subsequent tectonic history of the North American continent. 

The fault system consists predominantly of northwestoriented, steeply dipping strike-slip ductile shears and faults 

and conjugate sets of northerly striking transfer faults. The 

fault system was initiated in Neoarchean time during late 

stages of dextral transpression (orogenesis) that progressed 

from crystal-plastic to brittle deformation. The Neoarchean 

faults are best known from structural studies in the southern 

part of the Archean Superior province, where they transect 

regional folds with east-west axes in the Archean rocks 

(Hudleston and others, 1988, and references therein). Faults of 

the Neoarchean age also are recognized on the Archean Wyoming province and a major fault of this system has been dated 

at about 2.7 Ga (Bowers and Chamberlain, 1993). Regional 

deformation under transpressional conditions resumed at about 

1.70 Ga, after accretion of 1.8–1.72 Ga magmatic arc rocks in 

the Colorado province. 

The driving force for the development of the Trans–

Rocky Mountain fault system is attributed here to subcontinental mantle deformation, a mechanism discussed by several 

investigators (for example, Silver, 1996; Tikoff and others, 

2004). This mechanism can account for inception of the 

continent-scale Trans–Rocky Mountain fault system and the 

repeated tectonism along the same tectonic axis. The model 

presented here accords with new geodynamic models for plate 

motions, based on seismic anisotropy beneath the North and 

South American continents (Russo and Silver, 1996; Silver, 

1996; Tikoff and others, 2002, 2004), that invoke mechanical 

coupling and subsequent shear between the lithosphere and the 

asthenosphere such that a major driving force for plate motion 

is deep-mantle flow. The model of subcontinental mantle 

deformation previously has been proposed to explain the 

Andes Cordillera in South America, and by analogy, the U.S. 

Rocky Mountains (Russo and Silver, 1996). The inadequacy of 

plate tectonic theory to explain major deformation within the 

continents has been pointed out previously by Molnar (1988).

An implication of this tectonic thesis is that subcontinental mantle deformation (or bottom-driven deformation; Tikoff 

and others, 2004) must be taken into account as a major, if 

not principal, cause of late Precambrian deformation (since 

2.7 Ga), at least in the North American craton, and perhaps 

on other cratons. This inference is supported by the regional 

extent and structural similarity of the Precambrian shear zones 

and their structural coincidence with upper-mantle teleseismisity, as well as the general absence of diagnostic features of 

modern plate tectonics, such as ophiolites. It is widely recognized (Silver, 1996; Karlstrom and Humphreys, 1998; Savage, 

1999) that in older continental regions seismic anisotropy in 

the lithosphere has been frozen since Precambrian time.

Acknowledgments

This manuscript is based on regional geologic studies 

of the Precambrian basement rocks in the continental United 

States, with emphasis on the Rocky Mountain region. Definition of the fault system is based on extrapolation from known 

basement structures determined by geologic mapping and 

interpretation of new aeromagnetic anomaly data. Reviews by 

Robert L. Bauer and A.W. Stokes improved an earlier version 

of the manuscript.
10 The Trans–Rocky Mountain Fault System—A Fundamental Precambrian Strike-Slip System

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