Preliminary Precambrian Basement 

Structure Map of the Continental United 

States – An Interpretation of Geologic and 

Aeromagnetic Data 

By Paul K. Sims, Richard W. Saltus, and Eric D. Anderson 

Any use of trade, firm, or product names is for descriptive purposes only and does not 

imply endorsement by the U.S. Government 

Open-File Report 2005-1029 

U.S. Department of the Interior 

U.S. Geological Survey 
Preliminary Precambrian Basement Structure Map 

of the Continental United States—an Interpretation 

of Geologic and Aeromagnetic Data 

By P.K. Sims, R.W. Saltus, and E.D. Anderson 

Introduction 

The Precambrian basement rocks of the continental United States are largely covered by 

younger sedimentary and volcanic rocks, and the availability of updated aeromagnetic data 

(NAMAG, 2002) provides a means to infer major regional basement structures and tie together 

the scattered, but locally abundant, geologic information. 

Precambrian basement structures in the continental United States have strongly 

influenced later Proterozoic and Phanerozoic tectonism within the continent, and there is a 

growing awareness of the utility of these structures in deciphering major younger tectonic and 

related episodes. Interest in the role of basement structures in the evolution of continents has 

been recently stimulated, particularly by publications of the Geological Society of London 

(Holdsworth and others, 1998; Miller and others, 2001). These publications, as well as others, 

stress the importance of reactivation of basement structures in guiding the subsequent evolution 

of continents. Knowledge of basement structures is an important key to understanding the 

geology of continental interiors. 

Previous studies of the basement tectonics of the continental United States have 

delineated the gross structural fabric (Baars and others, 1995; Marshak and Paulsen, 1996). In a 

significant contribution, Baars and others (1995) recognized the continental-scale orthogonal 

pattern and the basic similarity of the structures to the San Andreas fault system in California. 

2 
They, therefore, postulated an overall transpressional stress mechanism by analogy. Marshak 

and Paulsen (1996), on the other hand, suggested an intracratonic extensional mechanism for the 

deformation. 

This map and text are an outgrowth of preparation of Precambrian basement geologic 

maps of several states in the western part of the United States, utilizing updated aeromagnetic 

data (NAMAG, 2003) as an interpretive tool: Wyoming, Sims and others, 2001b; Colorado, Sims 

and others, 2001a; Montana, Sims and others 2004; Idaho, Sims and others, in press; and 

Arizona (Sims, 2004, unpub. data). In the western Cordillera, in particular, as contrasted to 

interior sectors of the continent—which have been relatively stable since the end of the 

Precambrian—later episodic deformation focused on basement zones of weakness strongly 

influenced structures formed during the late Paleozoic Ancestral Rocky Mountain orogenesis, the 

Late Cretaceous Laramide and Sevier orogenies, and Cenozoic extensional tectonics (Sims, 

submitted). In addition, long-lived basement structures localized ore-bearing igneous activity, 

such as in the Colorado mineral belt (Tweto and Sims, 1963; Wilson and Sims, 2003), the central 

Idaho-Montana mineral belt (O’Neill and others, 2002), and the New Mexico structural zone 

(Sims and others, 2002). 

Two fundamental continent-scale basement fracture systems, that formed in late 

Paleoproterozoic- and early Mesoproterozoic-time and thatoverlap spatially, primarily have 

influenced later tectogenesis. These structures abruptly formed after consolidation of the protocontinent (1.76 to 1.70 Ga), and probably represent initial lithospheric fragmentation that 

eventually led to continental separation in latest Proterozoic time, that is, the breakup of 

postulated Rodinia. The two fracture systems generally are northwest-striking transcurrent 

faults, formed by continent-scale transpressional deformation, and northeast-striking ductile 

shears. 

An exception to a Paleoproterozoic age for initial shearing in the United States continent 

is the Vermilion fault in northern Minnesota (VF, pl. 1). This fault apparently is the 

3 
southernmost structure of a regional system of dextral strike-slip shear zones within the Superior 

province (SP, fig. 10; Card and Poulsen, 1998, fig. 2.6) that was formed in Late Archean time 

(Hudleston and Bauer, 1995). 

The tectonic model presented here for evolution of the continent after its consolidation 

accords with new geodynamic models for North and South American plate motions, motivated 

by seismic anisotropy (Silver, 1996; Russo and Silver, 1996; Dueker and others, 2001), that 

invoke mechanical coupling and subsequent shear between the lithosphere and asthenosphere 

such that deep mantle flow is considered a major driving force for plate movement. 

Principal Basement Structures 

The principal regional Precambrian basement structures shown on plate 1 are: (1) suture 

zones formed during amalgamation of the continent (Laurentia) in Paleoproterozoic time, (2) a 

wide belt of partitioned northeast-striking ductile shear zones (~1.7-1.4 Ga) that extends across 

the width of the continent into Canada, and (3) an even broader zone of conjugate strike-slip 

shears (~1.7 Ga), the principal displacement zones of which strike west-northwest to northwest. 

Each of these basement structures was reactivated episodically during Mesoproterozoic and later 

times, and accordingly they provide the building blocks for subsequent tectonic evolution of the 

interior of the continent. 

Except for the Dillon suture zone (pl. 1) of the Trans-Montana orogen (Sims and others, 

2004), suture zones generally have had a secondary role in subsequent evolution of the continent. 

The Trans-Montana suture zone controlled the localization of late Cretaceous igneous-related 

mineral deposits in the central Idaho-Montana mineral belt (O’Neill and others, 2002). Also, the 

zone influenced sedimentary facies patterns in the Paleozoic. 

The northeast-striking ductile shears profoundly influenced the localization of ~1.4 Ga, 

predominantly alkali-calcic intrusions of the Transcontinental Proterozoic provinces (Sims, 

2004, unpublished data; c.f. pl. 1 and Ferguson and others, 2004, fig. 9). The intrusions coincide 

4 
spatially with the regional shear system, and at a detailed scale, were intruded within the 

preexisting shears (Nyman and others, 1994; Aleinikoff and others, 1993; Ferguson and others, 

2004). 

The strike-slip fault system fragmented the continental crust into fault bounded, but 

coherent, blocks of varying dimensions, the boundaries of which were favorably oriented to later 

stress fields to be repeatedly reactivated. These structures are distributed throughout the 

continent, affecting both Archean and Proterozoic provinces, and arguably are the most 

important of the basement fault systems. These structures are discussed in the context of the 

regional geology in following sections. 

Magnetic Map Interpretation 

The recently compiled Magnetic Anomaly Map of North America (NAMAG, 2002) is an 

appropriate basemap for Precambrian structural interpretation. Regional-scale aeromagnetic 

anomaly patterns reflect the distribution of magnetic minerals, primarily magnetite and other 

minerals in the magnetite-ulvöspinel solid-solution series, throughout the entire crust (Reynolds 

and others, 1990). The relative concentration of these magnetic minerals is an indicator of 

geologically important factors including original lithology, metamorphic grade, and degree of 

geochemical alteration. Generally speaking, igneous rocks are much more magnetic than 

sedimentary rocks; mafic igneous rocks are more magnetic than felsic igneous rocks; magnetism 

may be either created or destroyed during metamorphism of sedimentary rocks; and 

magnetization of igneous rocks generally is diminished by metamorphism and metasomatism 

(Reynolds and others, 1990). Structural juxtaposition of rocks with contrasting magnetization 

can produce particularly sharp anomaly boundaries. Measured aeromagnetic anomalies are a 

complex integration of the effect of these crustal magnetic sources, most of which lie within the 

Precambrian basement. 

5 
 Quantitative interpretation of regional-scale aeromagnetic anomalies is problematic 

because, in general, insufficient auxiliary information is available to overcome the inherent 

nonuniqueness of potential field inversion (for example, Blakely, 1995). Ideally, quantitative 

interpretations would be constrained by magnetic property measurements of all lithologies 

involved and independent depth information from seismic or other geophysical methods. 

Lacking these constraints, useful qualitative interpretation is possible. Building on early 

pioneering work by Zietz and others (1969), Hinze and Zietz (1985) examined an analog 

compilation of magnetic data for the conterminous United States and discussed many proposed 

correlations with regional geologic features. In particular, they pointed out numerous regional 

magnetic-anomaly provinces that characterize the Precambrian basement. These, in turn, are cut 

by very intense, generally linear anomaly zones that represent mafic extrusive and shallow 

intrusive bodies within the Midcontinent rift, southern Oklahoma aulacogen, and the Reelfoot rift 

(Hinze and Zietz, 1985). Kane and Godson (1989) used the same analog compilation, together 

with a gravity map, to define a set of geophysical “anomaly-trend zones,” bounded by linear 

geophysical gradients, for the conterminous United States. Within their magnetic anomaly 

zones, they recognized and identified preferred anomaly orientation directions. In particular, 

they pointed out the correspondence of northwest trends in the Midcontinent with the “puzzling” 

trends previously identified by Bickford and others (1986). The interpretations in this report 

follow this general strategy, but the greater resolution of the new digital compilation (NAMAG, 

2002), allows for more detailed identification of zones and trends. 

Examination of the conterminous United States portion of the North American magnetic 

map (NAMAG, 2002), reveals a large number of linear features that fall into three broad 

categories: (1) boundaries of zones with common anomaly patterns, (2) continuous or 

discontinuous sets of steep anomaly gradients, or (3) narrow magnetic troughs. In all three cases, 

these linear features are postulated here to correspond with faults or fractures in the Precambrian 

basement. Category (1) is analogous to the zone boundaries defined by Kane and Godson (1989) 

6 
and, indeed, follows or parallels many of their features. These features typically are Precambrian 

province boundaries, orogen margins, or rift edges. Category (2) features generally fall within 

the broad zones and represent structures within the broader zones. Category (3) has not been 

previously emphasized in regional interpretation of the conterminous United States. Within this 

category, basement shear zones, in particular, are readily identified because magnetite in the 

host rocks is apparently destroyed during ductile shearing and later alteration by hydrothermal 

or circulating surficial waters (Finn and Sims, in press). Accordingly, these basement structures 

are expressed by negative magnetic anomalies, particularly evident in terranes dominated by 

rocks of high magnetic susceptibly, such as most Precambrian granitic plutons. 

Identification of significant geophysical lineaments and boundaries remains a subjective 

process, particularly at a continental scale. Various mathematical techniques exist for 

geophysical edge detection and enhancement (for example, Blakely and Simpson, 1986; Cordell 

and McCafferty, 1989), but ultimately it falls to the interpreter to edit and interpret these results. 

In addition to the conterminous United States studies referenced above, other examples of 

regional geophysical domain mapping reveal a variety of interpretive styles and techniques (for 

example, Cordell and Grauch, 1985; Cady, 1989; Phillips, 1990; Saltus and others, 1997). In this 

study, extensive prior knowledge of Precambrian geology by one of us plays a key role in 

determining the significance of the selected aeromagnetic features. 

Structure Map Preparation 

The Precambrian structure map portrays major regional structural features inherent to the 

basement, that is, structures known and presumed to have formed during Archean and 

Proterozoic times. The structures include major suture zones formed during Paleoproterozoic 

accretionary tectonism and regional shear zones and faults initiated in late Paleoproterozoic-early 

Mesoproterozoic time. 

7 
Knowledge of the accretionary history of the continent has accrued from geologic 

mapping and related studies, particularly geochronology, over the past century and has been 

summarized in the Decade of North American Geology volume on the Precambrian (Reed and 

others, 1993). Although fundamental shear zones in the basement rocks have been recognized in 

some areas since the pioneering studies of Billingsley and Locke (1941) and Mayo (1958), their 

extent and significance in the geologic evolution of the continent have not been fully 

appreciated, nor has the timing of the initial faulting been known until recently (Sims, 2002). 

Earlier compilation of known basement faults in the mid-continent region (for example, 

Kisvarsanyi, 1984; Sims, 1985; Sims and others, 1991) revealed persistent abundant northwestand northeast-striking faults, but their age was only known at the time to be pre-1.50 Ga, the time 

of abundant rhyolite extrusion in southeast Missouri (see Sims and others, 1987, for discussion). 

Northeast-trending fundamental basement shear zones have been known for many decades to be 

present in the Colorado Front Range (Moench and others, 1962) and ranges to the west in the 

Rocky Mountains (Tweto and Sims, 1963). Later studies (Karlstrom and others, 1997) have 

demonstrated their existence in New Mexico and adjacent Arizona. Sims and others (2002) have 

demonstrated the influence of these northeast-trending basement shear zones on localization of 

epigenetic mineral deposits in New Mexico. Northwest-striking shear zones, however, were not 

recognized as fundamental basement structures of regional extent until recently (Sims, 2002). 

Recognition of these structures as a continent-wide integrated strike-slip fault system was largely 

dependent on the availability of updated aeromagnetic anomaly data (NAMAG, 2002). The 

magnetic data provided a means to tie together known geologic observations and extend this 

knowledge into the vast areas covered by younger rocks. Accordingly, the Precambrian structure 

map combines known geology gained from studies of exposed areas and geophysical 

interpretation of aeromagnetic anomaly data. We attempt to quantify relative uncertainties in 

mapping of the basement structures, but this is somewhat objective because aeromagnetic 

8 
anomaly data commonly are a major tool in preparation of geologic maps of basement rocks at 

small and large scales. 

Summary of Precambrian Basement Geology 

The Precambrian basement in continental United States is composed essentially of a 

northern tract of Archean cratonic elements and enclosing pre-1.80 Ga Proterozoic magmatic 

terranes—which have been designated as the Hudsonian craton (fig. 1a; Van Schmus and others, 

1993)—and a southern tract of diverse igneous and sedimentary rocks (1.80-1.65 Ga), known as 

the Transcontinental Proterozoic provinces (fig. 1b). The Hudsonian craton was consolidated 

during the Penokean (Sims, 1996) and Trans-Hudson (Sims and others, 1991; Peterman and 

Sims, 1993) orogenies. It formed the foreland against which 1.8ʊ1.7 Ga magmatic arc rocks of 

the Central Plains orogen (Sims and Peterman, 1986) were accreted (fig. 1a). 

The volcanic rocks of the Central Plains orogen are partly overlain by extensive rhyolite 

and hypabyssal granite extruded during the interval 1.76 to 1.65 Ga (fig. 1b). 

Penecontemporaneous quartz arenite formed locally on the rhyolite successions and locally 

overlapped Penokean orogenic rocks in the Lake Superior region (pl. 1; Sims, 1996; Medaris and 

others, 2003). Together, these rocks compose the 1.76ʊ1.65 Ga rhyolite-quartz arenite belt of 

figure 1b. In Mesoproterozoic time (1.5 to 1.35 Ga), voluminous rhyolite was erupted across a 

broad region in the Midcontinent, and predominantly A-type intrusions were emplaced in a 

lengthy belt extending from southern California northeastward through eastern Canada as far 

north as Labrador (Sims, 2004, unpub. data). In Mesoproterozoic time, clastic sedimentary rocks 

(for example, Belt Supergroup (Winston, 1986) and Apache Supergroup (Wrucke, 1989) were 

deposited nearly contemporaneously in intracontinental structural basins in the western United 

States. Younger, Neoproterozoic clastic sedimentary successions were deposited along both 

continental margins in rift-related basins (Stewart, 1972; Thomas, 1977). 

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The western margin of the continent is sharply defined (figs. 1a, 1b); exotic late 

Paleozoic and Triassic rocks were accreted to the rifted margin in mid-Cretaceous time. The 

eastern and southern margins, however, are less well defined because mid-Proterozoic and older 

continental rocks are overridden by thrust faults along the ~1.1 Ga Grenville and Llano fronts. 

Intracontinental Proterozoic faults of northwest trend (Sims, submitted) strongly influenced 

sculpting of the continental margins during the Grenville and Cretaceous orogenies. Preexisting 

transcurrent faults were reactivated as transform faults to facilitate rifting. 

Precambrian Basement Architecture 

The Hudsonian craton consists of two Archean nuclei, the Superior and Wyoming 

provinces, that are circumscribed by Paleoproterozoic (pre-1.80 Ga) magmatic accretionary 

terranes (fig. 1a). The Trans-Hudson orogen (THO, fig. 1a) separates the two Archean nuclei, 

and the approximately coeval Penokean (PO) and Trans-Montana (TMO; Sims and others, 2004) 

orogens bound their outer margins. The Penokean orogen is juxtaposed with the southeast 

margin of the Superior province, and the Trans-Montana orogen is juxtaposed with the northwest 

margin of the Wyoming province. Accordingly, the Archean nuclei were isolated largely from 

later continental-margin tectonism by an outer rind of magmatic arc rocks capable of absorbing 

much of the subsequent strain. This can partly account for the scarce evidence of post-1.80 Ga 

distributed compressional deformation in Archean province rocks. 

During the interval 1.78ʊ1.72, Ga magmatic ocean-arc rocks were accreted to the 

southern margin of the Hudsonian craton, extending southward from the Cheyenne (suture) belt 

(Karlstrom and Houston, 1984) in southern Wyoming, at least into northern New Mexico, across 

a distance of ~1,000 km. These rocks have been assigned to the Colorado province in the 

southern Rocky Mountains (Bickford and others, 1986), the Central Plains orogen in the 

Midcontinent (fig. 1a; Sims and Peterman, 1986), and the Yavapai province in Arizona 

(Karlstrom and Bowring, 1988). The provinces are combined in figure 1a under the Central 

10 
­

Plains orogen. Despite intensive studies, details of assembly of the rocks remain uncertain. 

Available data, however, suggest that accretion was piecemeal (Sims and Stein, 2003) and 

apparently was younger to the south, as suggested by the distribution of comagmatic calc

alkaline plutons (Reed and others, 1987). Seismic reflection data across the Park Range in 

northern Colorado (Morozova and others, 2002) suggest the presence of a possible suture zone 

(Farwell Mountain-Lester Mountain) within Colorado province rocks, about 50 km south of the 

Cheyenne belt. This zone separates predominantly (1.79ʊ1.77 Ga) arc rocks (Green Mountain 

arc) to the north from 1.76ʊ1.72 Ga arc and derivative sedimentary rocks (Rawah block) to the 

south (Tyson and others, 2002). At the same latitude in the Colorado Front Range, to the east, 

Selverstone and others (2000) have recognized an important northeast-striking Paleoproterozoic 

(~1.7 Ga) transcurrent boundary within the Colorado province. Rocks to the south of these 

tectonic boundaries have been dated in the age range 1.76ʊ1.72 Ga. Combined, these age data 

indicate the probable presence of at least two distinct, accretionary arcs in Coloradoʊ the Green 

Mountain and Rawah arcs. Rocks assigned to the Yavapai province in Arizona also are mainly 

1.76-1.72 Ga in age (Karlstrom and Bowring, 1988). Ocean-arc rocks in the range 1.76ʊ1.72 

have been recognized from drill cores in the Midcontinent (Van Schmus and others, 1993). 

These rocks have been assigned to the Central Plains orogen (Sims and Peterman, 1986), which 

we redefine here to consist only of ocean-arc rocks formed in the approximate range 1.78 to 

1.72 Ga. Rocks of the Colorado and Yavapai provinces are intensely deformed and 

metamorphosed generally to amphibolite grade. Consolidation and stabilization of the protocontinent at ~ 1.72 Ga were followed abruptly by intracontinent shearing of continental-scale 

(Sims, 2002); these fundamental basement structures guided later Proterozoic tectono-magmatic 

activity, discussed as follows. 

The intracontinental shearing in late Paleoproterozoic time initiated alkali-calcic 

magmatism along the southeastern margin of the Paleoproterozoic accretionary terranes of the 

Central Plains orogen and areas to the west. In the southwestern United States, rhyolitic rocks, 

11 
mainly ash-flow tuffs, were erupted across a broad region extending from central Arizona into 

the Midcontinent, and essentially penecontemporaneous quartz-arenite successions were formed 

in structural basins of more limited extent (Sims, 2004, unpublished data). These quartz-arenite 

deposits include the Mazatzal province in southeastern Arizona (Karlstrom and Bowring, 1988), 

the Vadito and Hondo Groups in northern New Mexico (Read and others, 1999), and the 

Uncompahgre Formation (Barker, 1969) in southwestern Colorado. In the northern 

Midcontinent region, rhyolite eruption started earlier (1.76 Ga) in southern Wisconsin (Sims, 

2004, unpublished data). This magmatism was followed in the same region by local deposition 

of quartz arenite of the Baraboo interval (Dott, 1983). The existence of mature weathering 

products between rhyolite and quartz arenite successions in the northern Midcontinent (Medaris 

and others, 2003) and northern New Mexico (Grambling and Williams, 1985) suggests extensive 

crustal stability at ~1.70 Ga within the region. The rhyolite and quartz arenite successions were 

deformed and weakly metamorphosed at 1.65ʊ1.63 Ga, probably during intracontinental 

tectonogenesis. The imposed deformational fabrics, within the rhyolite successions and 

contained folds, are chiefly oriented northeastward, but the quartz arenite successions occupy 

fault-bound basins oriented northwestward, which suggest deformation on northeast-southwest 

axes, as demonstrated for the Uncompahgre Formation in southwestern Colorado (Barker, 1969). 

The orthogonal northeast- and northwest-trending structures are interpreted to reflect partitioning 

of strain along kinematic fault boundaries, rather than being directly related to a regional stress 

field. This mode of transpression-transtension is interpreted as resulting from deformation as 

described by Dewey and others (1998). 

Continental fragmentation across the United States was diachronous during the apparent 

interval 1.76ʊ1.70 Ga. Fundamental, partitioned northeast-striking shear zones in this age 

range transect the Proterozoic provinces across a northeast-trending >500-km-wide belt 

extending from southern California into eastern Canada, and probably beyond. The northeaststriking shear zones have been interpreted as resulting from northwest-southeast shortening 

12 
(Nyman and others, 1994), probably resulting from regional transpression (Sims and Stein, 

2003). Approximately contemporaneous continental-scale strike-slip faulting, chiefly oriented 

northwestward, segmented the Hudsonian craton as well as the Proterozoic provinces to the 

south. These fundamental lithospheric structures provided first-order controls on later 

Proterozoic tectonism, which probably took place in an intracontinental setting. 

A second major episode of predominantly alkali-calcic A-type magmatism, commonly 

called the “Transcontinental anorogenic province” (Silver and others, 1977; Anderson, 1983) 

occurred in Mesoproterozoic (1.5ʊ1.35 Ga) time. It followed the same general path as the 

earlier (1.76ʊ1.65 Ga) rhyolitic magmatism (rhyolite-quartz arenite belt, fig. 1b), extending 

from southern California diagonally across the United States to Labrador. The felsic magmatism 

was not associated spatially with significant clastic sedimentation. The felsic magmatism 

consists of coeval and apparently cogenetic volcanic and mesozonal plutonic rocks that overlap 

spatially. The volcanic and associated hypabyssal rocks (fig. 1b) have been referred to (Van 

Schmus and others, 1993) as an Eastern granite-rhyolite province (1.47±30 Ga) and a southern 

granite-rhyolite province (1.37±30 Ga) (fig. 1b); they blanket much of the Midcontinent (Van 

Schmus and others, 1996), apparently mostly as a thin veneer. The mesozonal plutonic rocks 

form isolated or circumscribed intrusions that are widely spaced throughout the belt of 

Transcontinental Proterozoic provinces (Van Schmus and others, 1993); they lack possible 

associated volcanic rocks. Although generally granitic in composition, some plutons contain 

anorthosite (for example, Wolf River batholith in Wisconsin (Anderson and Cullers, 1978) and 

the Sherman batholith in southern Wyoming (Frost and others, 2002). A few plutons in 

Colorado are composed of foliated granodiorite (e.g., Mt. Evans batholith; Aleinikoff and others, 

1993). 

The designation of the mesozonal Mesoproterozoic plutons as anorogenic (Anderson, 

1983) has been disputed because some bodies are partly deformed (Nyman and others, 1994; 

Aleinikoff and others, 1993; Ferguson and others, 2004). The recognition that the shear zones 

13 
that host the intrusions are intracontinental structures is a key to understanding this tectonic 

enigma. These structures have partitioned strain imposed by a regional stress field during and 

subsequent to emplacement of the plutons, resulting in alternating compression and extension 

caused by transpressional-transtensional deformation. In this tectonic environment, kinematic 

boundary conditions along fault-bound blocks are more important than the regional stress field in 

determining local tectonism. The shear zones that host the plutons probably were formed in late 

Paleoproterozic (~1.70 Ga; Sims and Stein, 2003) time. Extension concurrent with magma 

emplacement at ~1.4 Ga provided openings for magma emplacement (Sims and Stein, 2003). 

Continued or subsequent transpression along the preexisting shear zones caused local 

deformation of the intrusive bodies. In a continental setting, preexisting zones of weakness are 

preferentially reactivated during tectonism at the expense of development of new, pristine 

fractures (Holdsworth and others, 2001). 

Basement Structural Controls 

The continental-scale shears and faults that formed in late Paleoproterozoic time, after 

consolidation of the protocontinent have provided the primary structural controls for later 

Proterozoic tectonism and magmatism in this part of the North American continent. The close 

spatial relationship of the Transcontinental Proterozoic provinces (Van Schmus and others, 1993) 

with the belt of northeast-striking fundamental, intracontinental shear zones shown on plate 1 is 

interpreted to indicate that these structures provided first-order controls on localization of the 

igneous activity in the provinces as a whole (Sims, 2004, unpublished data), as well as at a local 

scale (for example, Ferguson and others, 2004). 

Petrogenetic studies of the igneous bodies (Anderson and Cullers, 1979; Stein, 1985; 

Frost and others, 2002) localized by the northeast-trending structures indicate remelting of lower 

crustal rocks as a major component of the magmas, stimulated by ponding of basaltic magma at 

the base of the crust. 

14 
The northwest-trending late Paleoproterozoic transcurrent fault system (pl. 1) was 

superposed on, and extends beyond the belt of northeast-striking shear zones along the southern 

margin of the Hudsonian craton (fig. 1b). It provided second-order controls on tectonomagmatic activity within the broad Transcontinental Proterozoic provinces. Specifically, it 

provided structural openings for igneous intrusions and structural basins for rhyolitic extrusions 

and quartz-arenite successions. Mid-Proterozoic tectonism involved kinematic interactions 

among the northeast-striking shear zones and individual strands of the northwest fault system. 

Continental Margins 

Both the western and eastern continental margins (pl. 1) are characterized by angular 

bends (promontories and embayments) that can be attributed to reactivation of intracontinental 

basement structures during the rift to drift transition in the interval Late Proterozoic-Cambrian 

period, which eventually led to continental separation on both margins. During rifting, 

intracontinental northwest-trending strike-slip shear zones, initiated chiefly in late 

Paleoproterozoic time (~1.70 Ga), were reactivated as transform faults, which accommodated 

differential lateral displacements. The rifted margins likewise follow preexisting structural 

fabrics, which acted as zones of weakness during extension accompanying the rifting (Sims and 

others, in press). 

The trace of subsequent passive-margin sedimentary successions and orogenic belts on 

both continental margins are inherited from the shape of the earlier late Neoproterozoic rifted 

margin. On the western margin, stratigraphic facies patterns of Neoproterozoic through 

Devonian successions (Poole and others, 1992) trend north-northeast from southern California to 

Idaho, where they abruptly turn to a northwest orientation, extending into northeast Washington, 

where they resume a northerly trend into Canada (pl. 1; fig. 1a). The northwest-trending 

segment in Idaho coincides with the Salmon River arch (Armstrong and others, 1977), which is 

now known (Sims and others, in press) to be a horst formed during reactivation of the mid15 
Proterozoic Clearwater fault (CF, fig. 1a) zone that shed clastic sediments into adjacent 

northwest-trending structural basins. The deformed belt has been called the “Central Idaho 

disturbance” (Sims and others, in press). 

On the eastern continental margin, angular bends along the margin (Thomas, 1977, 1983) 

are grossly similar in orientation and shape to those on the western margin. Rifted margin 

segments that generally trend north-northeast are offset by northwest-striking transform faults 

(pl. 1). Crustal separation of the eastern continent margin occurred penecontemporaneously with 

that on the western margin, as indicated by passive-margin volcanic-sedimentary successions of 

late Neoproterozoic to Early Cambrian age, followed by formation of a Cambrian-Ordovician 

carbonate bank. 

Orogenesis along both continental margins was prolonged and episodic but culminated at 

widely different geologic times. Tectonic transport on each margin was directed craton-ward. 

On the western margin, orogenesis during the middle Paleozoic (Antler orogeny) and late 

Paleozoic (Sonoma orogeny) terminated with the Cretaceous Sevier orogeny (Burchfiel and 

others, 1992). These orogenic episodes did not materially change the zig-zag shape of the 

continental margin. On the eastern margin, west-vergent orogenesis began in the Ordovician, as 

indicated by earliest clastic-wedge sediments (Thomas, 1977) and terminated in the late 

Paleozoic with development of the Appalachian-Ouachita system. 

Dynamics 

The assembly of the Hudsonian craton (fig. 1a) in Paleoproterozoic time (Van Schmus 

and others, 1993) and the addition of the Central Plains orogen (Sims and Peterman, 1986) and 

its general equivalent, the Colorado province (Bickford and others, 1986) to the craton 

previously have been generally attributed to plate tectonic processes, mainly the suturing of 

accretionary arcs to Archean nuclei—the Superior (Card, 1990) and Wyoming (Sims and others, 

2001b) provinces. The nature, origin, and significance of the post-assembly Proterozoic shear 

16 
zones and faults have not been well understood previously. Yet it is these structures that 

provided the prevailing architecture for subsequent geologic evolution within the continent. 

Model for Subcontinental Mantle Deformation 

The systematics of major regional post-assembly Precambrian basement structures 

throughout the United States continent point to a common causal mechanism for their 

development. The model presented here accords with new geodynamic models for North and 

South American plate motions, based on seismic anisotropy beneath the continents (Silver, 

1996), that invoke mechanical coupling and subsequent shear between the lithosphere and 

asthenosphere such that a major driving force for plate movement is deep-mantle flow. The 

model of subcontinental mantle deformation has been proposed to explain the Andes Cordillera 

in South America and by analogy the Rocky Mountains (Russo and Silver, 1996), and also 

deformed mantle peridotite xenoliths in the Slave craton in northwest Canada (Kennedy and 

others, 2002). The inadequacies of plate tectonic theory to explain continental deformation have 

been pointed out previously by Molnar (1988). 

Two orthogonal sets of shear zones and faults are predominant in the continent: (1) 

northeast-striking, partitioned ductile shear zones, and (2) northwest-trending strike-slip ductilebrittle faults. The northeast-striking shear zones are interpreted as resulting from northwestsoutheast shortening (Nyman and others, 1994; Karlstrom and others, 1997; Sims and Stein, 

2003), apparently formed during the interval 1.76 to 1.70 Ga (Shaw and others, 2001). The 

northwest-trending (1.7ʊ1.5 Ga) transcurrent fault system consists of west-northwest to 

northwest synthetic faults and northerly-trending antithetic transfer faults; it is attributed to 

transpressional-transtensional deformation (Dewey and others, 1998), that is, strike-slip 

deformation that deviates from simple shear because of a component of shortening or extension 

orthogonal to the deformation zone. 

17 
The northeast- and northwest-oriented shears and faults mimic orthogonal teleseismic 

images of the upper mantle (van der Lee and Nolet, 1997; Dueker and others, 2001). The 

northwest-trending fault system mirrors northwest-oriented high-velocity seismic anomalies, 

indicative of cool mantle; the northeast-striking shears chiefly mirror low-velocity anomalies of 

the same trend, indicative of warm mantle (or melts). These data support the postulate of 

Karlstrom and Humphreys (1998) that the mantle structure in the Rocky Mountain region was 

formed in Proterozoic time and has been modified little subsequently. Possibly, this postulate is 

applicable to the stable interior of the United States as well. The northwest-oriented mantle 

fabric guided later mechanical crustal dislocations, and the northeast-oriented fabric guided 

episodic magmatic activity as well as crustal shortening. 

The general coherence of regional lithospheric structures and mantle anisotropy within 

the continent strongly suggests that the overall continental lithosphere is coupled to the upper 

mantle, and that deformation within the continent is chiefly the result of subcontinental 

deformation, as discussed by Silver (1996). The absolute motion of the continent is 

demonstrably southwestward (for example, Schutt and Humphreys, 2001). Russo and Silver 

(1996) have shown that drag resulting from subcontinental mantle convection is a more powerful 

driving force for deformation than plate-generated forces (that is, ridge-push and slab-pull). As 

stated previously, they proposed that the Andes Cordillera and, by analogy, the Rocky Mountains 

are products of subcontinental mantle deformation. 

The kinematics of regional basement structures within the United States continent 

suggest that deformation since at least early Proterozoic time has been predominantly 

transpression. Transcurrent lithospheric structures formed during Proterozoic mantle 

deformation are oriented obliquely to the southwestward (absolute) motion of the North 

American plate. Stress caused by traction between the asthenosphere and lithosphere during the 

southwestward drift focused on preexisting block boundaries repeatedly have reactivated 

18 
basement zones of weakness, thus localizing sedimentation, magmatism, and generation of ore 

deposits (Sims and others, 2002). 

Suggestions for Future Research 

Geologic investigations in the Archean Superior province in the Canadian Shield 

(Peterman and Day, 1989; Kamineni and others, 1990; Bauer and Hudleston, 1995; Card and 

Poulsen, 1998; Park, 1981) have disclosed a system of right-lateral transcurrent shears and faults 

in Archean rocks that is similar kinematically to the ductile-brittle faults in the United States 

sector of the continent. Shear zones in the southern part of the province have been interpreted as 

being late Archean in age (Bauer and Hudleston, 1995), whereas those in the western part of the 

Superior province are interpreted as being Paleoproterozoic (~2.3 Ga) (Kamineni and others, 

1990). These structures, and others in the Superior province, deserve further study. 

As a working hypothesis, we suggest that intracontinental shearing, driven by 

subcontinental mantle deformation, was initiated in Archean time in the cratonic nucleus within 

the Superior province and that it progressively affected younger accreted rocks to the south as 

they became coupled to the mantle below. An implication of this postulate is that continent-scale 

transpression had a dominant role during the early evolution of the North American continent. 

19 
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28 
Figure Captions 

Figure 1. Generalized Precambrian geologic map of continental United States. A, Archean and 

Paleoproterozoic accretionary crustal provinces assembled prior to 1.70 Ga. B, Late 

Paleoproterozoic and Mesoproterozoic provinces formed during intracontinental 

deformation. CB, Cheyenne belt; CF, Clearwater fault; DS, Dillon suture, GF, Grenville 

front; GL, Great Lakes tectonic zone, LF, Llano front; MCR, Midcontinent rift; Nd, Nd 

line of Van Schmus and others, 1996; Nf, Niagara fault; SAF, San Andreas fault; VF, 

Vermilion fault; WL, Walker Lane. 

Plate 1. Preliminary Precambrian basement structure map of the continental United States—An 

interpretation of geologic and aeromagnetic data. 

29 
AF 

CB 

DS 

GL 

Nf 

WL 

CF 

TMO

MCR 

THO SP 

Western 

continental 

margin MRV PO

W Nd P

? 

CPO 

MV 

Eastern 

continental 

margin 

EX PLANAT ION 

Acretionary arc terrane 

Central Plains orogen 

H udsonian craton 

Mojave province 

T rans-H udson orogen 

T rans-Montana orogen 

Penokean orogen 

S uperior province 

Stratigraphic contact 

High-angle fault 

Wyoming province Strike-slip fault (1.7 Ga or younger); arrows indicate predominant relative displacement 

Minnesota R iver province Thrust fault of Paleoproterozoic age (suture) 

e 1a. 
LF 

SAF 

MCR 

Western 

margin of Nd continental 

crust 

Southern margin of Hudsonian craton

Approximate

northern

limit of

Transcontinental

Proterozoic

provinces

G

F

Eastern 

margin of

continental 

crust 

EXPLANATION 

R Midcontinent rift system (~1.1 Ga) 

Transcontinental Proterozoic provinces 

R Southern granite-rhyolite province (1.4–1.35 Ga) 

R Eastern granite-rhyolite province (1.5–1.4 Ga) 

 Rhyolite-quartz arenite belt (1.76–1.65 Ga) 

Approximate contact

Grenville thrust fault

Strike-slip fault

re 1b.