Anatomy of North America: thematic geologic portrayals of the continent

Tectonophysics, 187 (1991) 117-134

Elsevier Science Publishers B.V., Amsterdam

117

Anatomy of North America: thematic geologic portrayals of the continent

Harold Williams a, Paul F. Hoffman b, John F. Lewry ‘, James W.H. Monger d and Toby Rivers e

a Department of Earth Sciences and Centre for Earth Resources Research, Memorial University of Newfoundland, St. John’s, Nfd., AIB 3X5, Canada

’ Geological Survey of Canada, 588 Booth St., Ottawa, Ont., KIA OE4, Canada

’ Department of Geology, University of Regina, Regina, Sask., S4S OA2, Canada d Geological Survey of Canada, 100 West Pender St., Vancouver, B.C., V6B lR8, Canada

e Department of Earth Sciences and Centre for Earth Resources Research, Memorial University of Newfoundland, St John’s, Nfld., AlB 3X5, Canada

(Submitted to editors September 17, 1987; received by publisher July 31, 1990)

ABSTRACT

Williams, H., Hoffman, P.F., Lewry, J.F., Monger, J.W.H. and Rivers, T., 1991. Anatomy of North America: thematic geologic portrayals of the continent. In: T.W.C. Hilde and R.L. Carlson (Editors), Silver Anniversary of Plate Tectonics.

Tectonophysics, 187: 117-134.

Six thematic tectonic maps are used to analyse the makeup of the North American continent. Themes are: (1) major

tectonic elements of the continent, (2) time of last major deformation, (3) time of first major deformation. (4) miogeoclines

and terranes by kindred, (5) suture zones and terrane boundaries by age, and (6) time of accretion. Features illustrated include

distribution of erogenic belts and their extensions beneath cover sequences to the continental edge, contrast between juvenile

and reworked crust in erogenic belts, geometry of ancient continental margins, distribution and classification of accreted

terranes, geometry of suture zones and courses of ancient oceans, and how the continent evolved from an assemblage of

Archean minicontinents to its present configuration. It is suggested that essentially similar plate tectonic processes controlled

continental breakup and assembly from the Archean onwards, albeit with gradual increase in size of continental lithospheric

plates and quantitative change in other parameters such as heatflow and character of the mantle.

Introduction

Ideas regarding the evolution of North America have changed radically since the advent of plate tectonics over 20 years ago. However, while new maps of some of the better known orogens have been produced (Williams, 1978; Howell et al., 1985; Wheeler and McFeely, 1987; Hoffman et al., in prep.), modem tectonic maps of the entire continent that reflect current thinking remain un- available. This paper briefly discusses six tectonic

portrayals of North America, presented in reduced format (pp. 123-128). The original compilations were made at 1 : 10 million scale, mainly as teach- ing aids; the reduced versions show the same

significant features and provide new insights into the makeup of the continent. They are intended as a focus for discussion and speculation on continental-scale tectonics and, it is hoped, will provide impetus for detailed tectonic map compilations at a more meaningful scale of 1 : 5 million. Themes for the six portrayals are as follows: (1) major tectonic elements, (2) time of last major deformation, (3) time of first major deformation, (4) miogeoclines and terranes by kindred, (5) suture zones and terrane boundaries by age, and (6) time of accretion. For completeness, Greenland is in- cluded in all figures but is unrestored with respect to eastern North America. The illustrations used

0040-1951/91/$03.50 0 1991 - Elsevier Science Publishers B.V.

118 H WILLIAMS ET AL..

here are available now in postcard format, in

colour, and as 35 mm slides, thus facilitating com-

parisons. Copies are available from the first author.

All of the maps are greatly simplified, already

outdated in some areas, and somewhat speculative

in regions where the data base is weak. However,

such failings, and other inevitable inaccuracies, do

not significantly detract from the primary purpose

of the exercise. More important is the practicality

and usefulness of maps produced according to the

selected themes. Above all, the portrayals provide

a vehicle to take ones imagination “out for a run”

while focusing on the broader aspects of the

evolution of the entire North American continent.

Maps are the way in which earth scientists com-

municate best and are the most practical way to

express quick, profound messages on the evolution

of North America, or indeed any other continent.

The first three maps (Figs. 1-3) are objective

compilations. The last three (Figs. 4-6) are more

interpretive, and developed, without inhibition or

apology, in accord with plate tectonic concepts,

even for Archean rocks (at least in principle). In

the spirit of this volume, the exercise represents an

especially practical application of plate tectonics.

Discussion of thematic maps (Figs. 1-6, pp. 123-

128)

Figure 1. Major tectonic elements of North America

This map portrays the first order components

of the continent; the Canadian Shield, Interior

Platform, Phanerozoic orogens and modern con-

tinental margins. It is the starting point for any

meaningful analysis on this scale. The arrange-

ment of tectonic elements, in particular the loca-

tion of Phanerozoic orogens peripheral to the Pre-

cambrian craton, greatly influenced early thinking

on the evolution of the continent and its geologic

mountain belts. North America has a gross sym-

metry of Phanerozoic elements not shared by other

continents. In the fixist view of permanent conti-

nents and oceans, the pattern suggested outward

growth of the continent by addition of younger

erogenic belts (e.g. Kay, 1951). In plate-tectonic

terms, it simply indicates that the modem con-

tinental margins of North America almost every-

where follow, and lie outboard of, Phanerozoic

orogens. The pattern breaks down in eastern

Canada, where the spreading axis of the Atlantic

crosses both the Appalachian Orogen and older

orogens of the Canadian Shield (Williams, 1984).

Figure 2. Time of last major deformation

This figure highlights the traditional structural

provinces of the Canadian Shield (Stockwell et al.,

1970) and their subsurface extensions, as well as

the Phanerozoic orogens and their extensions to

the continental edge. Undeformed cover rocks are

omitted. Comparison with Fig. 1 emphasizes the

varying reliability of the data base, e.g. where the

deformed rocks are exposed in the Canadian Shield

and where they occur beneath the Interior Plat-

form cover rocks, and where the Phanerozoic

orogens are exposed or where extended in sub-

surface to the continental edge. The Superior,

Slave, Wyoming and North Atlantic provinces

(cratons) were last significantly deformed in the

Archean (more than 2.5 Ga) and the Churchill,

Bear, Southern and Makkovik provinces in the

early Proterozoic (1.9-1.8 Ga). Successively

younger Proterozoic events define provinces which

occur progressively outward in the southeast part

of the continent; Central Plains (1.7 Ga), Mazatzal

(1.6 Ga), Grenville (1.0 Ga). The indicated pattern

of Precambrian elements is highly asymmetric,

with older deformed belts clustered at high lati-

tudes and younger Precambrian deformed zones

occurring to the southeast. Typically, the Archean

provinces or cratons are completely surrounded by

early Proterozoic deformed belts. “Patterned”

orogeneses is seen in the southeast, where succes-

sive sequential Precambrian provinces are fol-

lowed by the Paleozoic Appalachian Orogen and

finally the Atlantic continental margin, each sub-

parallel to its predecessor.

Divisions within the Phanerozoic orogens show

important differences along the length of these

belts. Some elements of the Canadian Appa-

lachians were last deformed in the late Pre-

cambrian (Avalonian), others during the Ordovi-

cian (Taconian), while the largest area was last

deformed during the middle Paleozoic (Acadian).

In the U.S. Appalachians, correlative rocks and

tectonic elements were overprinted by late Paleo-

ANATOMY OF NORTH AMERICA 119

zoic deformation (Alleghanian). These differences

reflect contrasting accretionary histories of differ-

ent segments of the orogen, which in turn are a

function of the complex closure of the Paleozoic

Iapetus Ocean during assembly of Pangea (Wil-

liams and Hatcher, 1983).

Central parts of the Cordilleran Orogen, last

deformed in the Jurassic and Cretaceous (Sono-

man and Columbian), were largely unaffected by

Cretaceous to Tertiary (Laramide) deformation. It

involved both the more outboard western elements

and extreme landward parts of the miogeocline;

despite the fact that Mesozoic accretion pro-

gressed from the miogeocline outwards (Monger et

al. 1982).

Figure 3. Time of first major deformation

Both the Precambrian craton and Phanerozoic

orogens remain clearly defined in this thematic

map, but their internal patterns contrast markedly

with those of Fig. 2. Large parts of the craton that

were significantly deformed in the Proterozoic

(1.9-1.8 Ga) represent overprinted Archean-de-

formed (more than 2.5 Ga) continental crust. This

is clear in the case of the Trans Hudson Orogen.

Similarly large parts of the mid to late Proterozoic

Grenville Orogen (1.0 Ga) involve reworking of

predeformed Archaean early (1.9-1.8 Ga) and

especially middle (1.6 Ga) Proterozoic crust. Simi-

larly the Phanerozoic orogens (e.g. Southern Ap-

palachians) incorporate rocks already affected by

earlier thermotectonism, e.g. Grenville inliers.

Comparison of Figs. 2 and 3 thus provides some

insight into the extent of reworking of older con-

tinental crust by younger deformational events,

most obviously in the Precambrian craton. The

higher proportion of reworked rocks in Protero-

zoic orogens compared to their Phanerozoic coun-

terparts may reflect deeper levels of erosion with

obliteration of miogeoclinal cover sequences and

overthrust upper crustal elements of accreted

juvenile terranes. On the other hand, it may be a

function of a predicted higher geothermal gradient

during the Proterozoic which could have resulted

in broader zones of ductile deformation in base-

ment rocks.

Structural provinces of the Canadian Shield,

defined by time of last significant deformation

and isotopic resetting, are less obvious in this

portrayal, save for the cratonic Superior, Slave,

Wyoming and North Atlantic provinces that com-

prise Archean rocks little affected by later events.

The factors that control extensive Proterozoic re-

working of Archean rocks in some areas and the

lack of reworking in other areas are of obvious

concern. This may reflect polarity of subduction,

toward the reworked areas and away from the

unreworked cratonic elements, and/or the pres-

ence of major Proterozoic “cryptic” collisional

sutures within abnormally wide reworked areas of

Archean crust, as suggested by recent work in the

western Churchill Province (e.g., Lewry et al.,

1985; Hoffman, 1987; 1988). Archean sutures are

also possible within some Archean structural prov-

inces (Hoffman, 1986a). The bulky, equidimen-

sional form of some Precambrian provinces re-

flects original shapes of discrete Archean minicon-

tinents. In other cases, shapes are the result of

disruption of larger Precambrian continents,

whereby original elongate erogenic belts were dis-

persed by subsequent ocean opening-closing

cycles or affected by major transcurrent faults.

Phanerozoic orogens display few Precambrian

deformed elements at present erosional levels but

some seismic reflection experiments indicate wide

stretches of older deformed basement in the deep-

er crust (Cook et al. 1979).

Figure 3 also provides an upper limit to age of

rocks, and in conjunction with Fig. 2 indicates

broad areas where the age of rocks can be gleaned

from the lack of older deformations (Fig. 3) and

preponderance of later deformations (Fig. 2). For

example, the lack of Precambrian deformations in

the Appalachian Orogen, coupled with the

frequency of Paleozoic events, implies that its

rocks are mainly of Paleozoic age. Similarly in the

Cordilleran Orogen, a lack of Paleozoic deforma-

tion (Fig. 3) and preponderance of Mesozoic de-

formations (Figs. 2 and 3) imply mainly Mesozoic

rocks. In contrast, the Grenville Orogen is com-

posed almost entirely of previously deformed

rocks; those first deformed during the Grenvillian

orogeny occur in small, widely separated areas

and make up less than 20% of the exposed orogen.

As a general rule, time of first deformation

approximates the age of rocks in erogenic belts.

120 H. WILLIAMS I!T Al.

The map illustrating last deformation is objec-

tive, being based on stratigraphic ages of de-

formed rocks and ages of unconformable cover

sequences, augmented by isotopic ages of

plutonism and metamo~~sm synchronous with

deformation. The map illustrating first deforma-

tion is more interpretive, especially in areas of

repeated structural, plutonic and metamorphic

overprinting. The necessary data for this map come

largely in the form of isotopic ages obtained from

minerals not easily reset during subsequent meta-

morphic events, e.g. U-Pb zircon. Nevertheless,

both maps are conceptually meaningful, regardless

of tectonic controls or presently available infor-

mation. A further breakdown of the prolonged

Archean interval, a difficult task not attempted

here, is essential to unravel the important earliest

crustal events.

Figure 4. Miogeoclines and terranes by kindred

The Archean structural provinces (Superior,

Slave, Wyoming, North Atlantic, North Kee-

watin *, South Keewatin *, Figs. 2, 3 and 5) are

all shown in the same pattern in this portrayal as

there is no attempt to separate their internal ele-

ments.

The Superior Province appears to have been

almost completely fringed by lower Proterozoic

miogeoclines, now extensively preserved in the

Circum-Ungava belt, Labrador Trough, Otish

Mountains and Penokean belt, but poorly defined

or obliterated along the southwestern margins of

the province. This implies that the Superior craton

was a discrete island minicontinent during at least

part of the early Proterozoic. The Circum-Superior

miogeoclines are essentially similar to those of the

Phanerozoic in most respects; moreover, a klippe

of predo~nantly oceanic rocks thrust upon the

Superior craton near Cape Smith (Hoffman, 1985),

resembles Ordovician Taconic allochthons in the

Appalachian Orogen.

* The North Keewatin and South Keewatin provinces are

renamed the Rae and Heame, respectively, and their

boundary, the Snowbird Line (Hoffman, 1988).

Lower Proterozoic miogeoclinal relics are also

preserved on the northwestern side of the Trans-

Hudson Orogen, in the Cree Lake and Foxe zones.

Continuity between these zones, and their farther

continuation through Baffin Island into the

Rinkian belt of Greenland, are matters of debate

(Lewry et al., 1985; Hoffman, 1987, 1988). The

best known and best exposed example of an Early

Proterozoic miogeocline is the fold and thrust belt

of the Wopmay Orogen that developed along the

western margin of the Archean Slave Province

(Hoffman, 1984; Hoffman and Bowring, 1984).

The stratigraphic analysis of this rniogeocline, from

its initiation and rift phase, through its passive

margin development, to final destruction rivals the

best analyses of Phanerozoic miogeoclines, such as

those existing for the App~ac~an and Cordil-

leran orogens. In contrast to the prolonged histo-

ries for the Phanerozoic miogeoclines (100 m.y. or

more), the Wopmay miogeoclinal cycle was short-

lived, about 10 m.y. This may not be typical of all

Early Proterozoic continental margins.

Rocks and structures of the Central Plains

Orogen truncate subsurface extensions of the

Penokean and Trans-Hudson orogens, consistent

with the Snowy Pass Supergroup of southeastern

Wyo~ng being a remnant of a Central Plains

miogeocline (Duebendorfer and Houston, 1987).

The younger Mazatzal, Southwest Province,

Trans-Labrador and Grenviile orogens, to the

southeast, are essentially devoid of preserved

miogeoclines (Condie, 1982; Green et al., 1985;

Bickford et al., 1986; Wardle et al., 1986; Sims

and Peterman, 1986).

The sinuous, disrupted and intersecting Pro-

terozoic miogeoclines of the North American cra-

ton form a pattern much like that of the African

continent where older Pr~amb~an provinces are

locally truncated and linked by late Precambrian

Pan-African belts. It is noteworthy that some

Pan-African belts are well-documented as result-

ing from plate tectonic processes, implying similar

controls for early to middle Proterozoic belts in

North America. The present complex tectonic pat-

tern of southeast Asia may be an analogue of early

Proterozoic North America (Hoffman, 1988).

Phanerozoic miogeoclines occur all around

North America. The Cordilleran miogeocline in


western North America is continuous from Alaska

to Arizona, and the Appalachian miogeocline in

eastern North America is continuous from New-

foundland to Mexico. Along the northern margin

of North America, the Cordilleran and Innuitian

miogeoclines can be linked by restoring the Alas-

kan North Slope terrane between the two at the

Beaufort Sea, and the Innuitian miogeocline and

Caledonian miogeocline of east Greenland are

continuous in northeast Greenland. Furthermore,

the gap between East Greenland and the Canadian

Appalachians is bridged by restoring the North

Atlantic (Williams, 1984). To the southwest, the

Cordilleran and Appalachian miogeoclines are

both truncated by the Sonora Megashear (Shurbet

and Cebull, 1987) and their rocks are translated

northward, possibly to reappear as suspect ter-

ranes-of the northern Pacific margin. The periph-

eral pattern and continuity of Phanerozoic

miogeoclines indicates that the late Precambrian

continent, Laurentia, was a discrete island much

like present day North America, though somewhat

smaller. The analysis also indicates that Laurentia

formed by late Precambrian breakup of a contem-

porary supercontinent, just as North America

formed by Mesozoic breakup of Pangea.

Suspect terranes, comprising either juvenile

crustal elements penecontemporaneous with ad-

jacent miogeoclines, or unrelated, mainly crystal-

line older terranes, occur within both Proterozoic

and Phanerozic orogens. The present challenge is

to document similar elements within the Archean

structural provinces, and some progress has been

made already in this regard (Card and Ciesielski,

1986; Hoffman, 1986a). Early Proterozoic juvenile

terranes are defined within the Trans Hudson

(Lewry et al., 1985; Van Schmus et al., 1987;

Chauvel et al., 1987), Penokean (Larue and Ueng,

1985) Central Plains (Bickford et al., 1986), Keti-

lidian (Allaart, 1976; Patchett and Bridgwater,

1984) and Grenville (Davidson, 1986) orogens.

They are generally of more limited area1 extent

than those of the Phanerozoic orogens, and

ophiolitic or chaotic terranes are sparse or un-

documented.

Much of the buried U.S. midcontinent may

comprise juvenile early to middle Proterozoic ad-

ditions. A broad area of granite and rhyolite

(1.50-1.44 Ga) is exposed in the St. Franqois

Mountains of Missouri and occurs widely in the

subsurface of Illinois, Indiana, Missouri and

Kentucky. Nd isotopic ratios (Nelson and De

Paolo, 1985) indicate derivation from crust having

a rather uniform model age of crust-mantle sep-

aration ca. 1.9 Ga, suggesting little or no involve-

ment of Archean crust. In contrast, granites of the

Penokean Orogen have model ages of ca. 2.3-2.1

Ga and were probably derived from crustal sources

containing mixed Archean and early Proterozoic

components.

Some crystalline tectonic elements, mainly

Archean, can be regarded as suspect relative to the

nearest well-defined miogeoclines; for example,

crystalline rocks in the vicinity of southern Baffin

Island are suspect with respect to the Circum-Su-

perior miogeocline to the south and the Foxe

miogeocline to the north.

Appalachian suspect terranes, most of them

composite or superterranes, are large and few

compared to Cordilleran examples (Williams,

1985). The orogen built up from the miogeocline

outward by successively younger Paleozoic accre-

tionary events. Boundaries between the miogeoc-

line and terranes, which amalgamated or accreted

during the Ordovician (Taconian Orogeny), are

marked by ophiolites and melanges, suggesting

head-on collisions. Boundaries between outboard

terranes, accreted later, are steep mylonite zones

or brittle faults, suggesting transcurrent oblique

motions (Williams and Hatcher, 1983). Some

ophiolitic and volcanic terranes occur as thin

Taconic allochthons, transported across the Ap-

palachian miogeocline, and a two-layered crust is

recognized in eastern parts of the ophiolitic Dun-

nage zone (Colman-Sadd and Swinden, 1984). To

the east, the Avalon terrane is a microplate, as its

rocks are embedded in the mantle (Keen et al.,

1986).

The Cordilleran orogen was assembled during

the Mesozoic, mainly by amalgamation and even-

tual accretion of two superterranes (Monger et al.,

1982). The two major accretionary episodes, in the

Jurassic and Cretaceous, are proposed as controls

for the contemporaneous metamorphic and

plutonic Omineca and Coast Plutonic belts, re-

spectively, in Canada. The greater number and

122 H. WILLIAMS ET AL

complexity of Cordilleran terranes compared to

those of the Appalachian Orogen reflect; (a) the

much longer history of the ancient Pacific Ocean

and persistence of the Cordilleran miogeocline,

from its late Precambrian initiation to Jurassic

destruction; (b) consequent greater width of

Pacific ocean crust destroyed by subduction; and

(c) greater dismemberment and dispersion of ter-

ranes by post-accretionary transcurrent faulting

(Coney et al., 1980).

A small area of mixed volcanic and sedimen-

tary rocks at the northern end of Ellesmere Island

is interpreted as an Innuitian suspect terrane

(Trettin, 1987). It includes an Ordovician ophiolite

complex at M’Clintock Inlet, records Ordovician

deformation, and has Grenville basement rocks.

These features are all typical of the western flank

of the Appalachian Orogen.

The Caledonides of east Greenland represent a

deformed, intruded and metamorphosed miogeo-

cline only. Its suspect elements were presumably

removed by spreading of the North Atlantic along

an axis coincident with the Ipetus suture.

Figure 5. Sutures and terrane boundaries

This figure portrays both well-documented and

probable collisional sutures representing closure

of former oceans, and other major terrane

boundaries whose significance and evolutionary

history are less well established. Some terrane

boundaries mark collisional zones across former

oceanic tracts; others are intracontinental trans-

current faults. As with miogeoclines and terranes

(Fig. 4), Archean sutures and terrane boundaries

are not depicted, but their existence is likely,

because of documented orogenies in Archean pro-

vinces.

Early to middle Proterozoic sutures and ter-

rane-boundaries surround and separate the

Archean Superior, Slave, Wyoming and North

Atlantic cratons, following the courses of the

Trans-Hudson, Wopmay, Penokean and Central

Plains orogens. Some of these orogen, e.g. Trans-

Hudson, are two-sided or symmetrical with oppos-

ing contemporary miogeoclines preserved. Others

are one-sided, asymmetric with only one miogeo-

cline. The asymmetric Precambrian orogens are

viewed as either accretionary, without collision of

opposing miogeoclines (like the Cordilleran

Orogen), or more likely collisional orogens with

opposing miogeoclines removed by subsequent

breakup along former collisional zones (like the

Appalachian Orogen).

The Thelon Front, between the Slave Province

and North Keewatin Province is interpreted now

as a collisional suture and part of a much longer

intracontinental shear zone, the Great Slave Lake

shear zone (Hoffman, 1986b; 1989). The geometry

of the Slave province against the North Keewatin

resembles the indentation of southeast Asia by

India at the Indus Suture. The indentation of the

North Keewatin was accommodated by 600-700

km of dextral offset on the Great Slave Lake shear

zone. The shear zone is marked by a wide zone of

mylonite and it is traceable in subsurface from the

foothills of the Rocky Mountains 1300 km north-

ward to the Thelon Basin. Another feature of the

shear zone is an 80 km wide belt of granitic and

dioritic intrusions along an exposed length of 1000

km, interpreted as a pre-collisional composite

magmatic arc and post-collisional zone of anatectic

batholiths (Hoffman, 1989).

Another important crustal break, the Intra-

Keewatin or Snowbird Line (Hoffman, 1988) sep-

arates the variably reworked North Keewatin and

South Keewatin divisions of the former western

Churchill Province. It may represent either a colli-

sional suture between former minicontinents or a

major intracontinental transcurrent fault (Hoff-

man, 1987; 1988). A suture between the Makko-

vikian/Ketilidian and North Atlantic province of

Labrador and southern Greenland is implied be-

cause the Ketilidian contains early Proterozoic

juvenile rocks (Allaart, 1976; Gower and Ryan,

1986; Patchett and Bridgwater, 1984). Evidence

for other sutures crossing north Greenland and

Baffin Island is weak.

A middle Proterozoic Central Plains suture is

indicated by apparent truncation of the slightly

older Trans-Hudson Orogen (Sims and Peterman,

1986) and by documented relations across the

Cheyenne Lineament (e.g., Duebendorfer and

Houston, 1987). In eastern Labrador, in what is

now the Grenville Province, there must be a cryptic

suture separating the early Proterozoic Mak-

ANATOMY OF NORTH AMERICA

Fig. 1. Tectonic elements of the North American continent.

TERTIARY TO RECENT

0 (Eurekan, Cascadian)

CRETACEOUS TO TERTIARY

ilaramide)

JURASSIC TO CRETACEOUS

(Columbian)

JURASSIC

m (Sonoman)

PALEOZOIC

[ Late (Alleghanian, Antler)

0 Middle (Acadian, Ellesmerianl

Early (Wonian)

PROTEROZOIC

0 Late to Cambrian (Avalonian)

Middle to Late (Grenvillian)

Early to Middle (Central Plains)

m Early (Hudsonian)

ARCHEAN CRATONS

m (Kenoran)

(Major erogenic events)

Fig. 7. Time of last major deformation. North -\merican continent. undeformed cober rocks omitted.

pp. 123-128

CRETACEOUS TO TERTIARY

f--J Mekan, LaramW

TRIASSIC TO jURASSlC AND OLDER

(Coiumbian~

PALEOZOIC

Late W@anhn, Ant&r)

Middle (&cadiam, Ellesmerian~

m Early flaconian‘ M%rintcxk)

PROTEROZOIC

Late to Cambrian fivalonian)

Middle Gzntral P&ins, Labradoran)

Early IHttdsonian)

ARCHEAN CRATONS

m (Kenoran and o&r)

Mjof orqgenic ewnts)

Fig. 3. Time of first major deformation. North American continent. undeformed cover rocks omitted.

Fig. 4. ~iogcocli~e~ and terranes of the North Amcrica~ continent cl~~~~ed by minored, ~~defo~ed anwr rocks omitted.

MESOZOIC TO CENOZOiC

PALEOZOIC

PROTEROZOic ‘_ I . ’ Middle to Late

I Middle

- Early to Middle

Sutures and &wane boundaries iden- tify the courses of ancient oceans destroyed by colffsfonal and accre- tiOMF/ events. The intricate array, with some boundaries dating back to the early Proterozoic, indicates the corn.. piexity of successive assembly and breakup episodes recorded in North American rocks. Where the stratigraphic record of miogeoclines is obfilerated, sutures may be the only evidence of ancient oceans.

Fig. 5. Sutures and terrane houndarics of the North American continent. undeformed cover rocks omitted

TERTIARY &s&j

CRETACEOUS

PALEOZOIC

0 Late

Middle to Late

Middle

Early

PROTEROZOIC

Middle to Late

Middle

Early

ARCHEAN CRATONS

Fig. 5. Time ofaccretion for the North American continent. undeformed cover rocks omitted


kovikian and middle Proterozoic Labradorian

orogens (Ward et al., 1986). Similarly, middle to late Proterozoic sutures are implied in the Gren- ville Province and may occur as terrane boundaries around an island arc sequence of middle Protero- zoic age in Ontario (Brown et al., 1975; Davidson, 1986), and around middle Proterozoic ophiolite and melange in the Llano Uplift in Texas (Garri- son, 1981). The New York-Alabama Lineament (Zietz, 1981), defined by strong magnetic and Bouguer gravity anomalies, is a possible unex- posed suture or terrane boundary within subsurface extensions of the Grenville Province.

Paleozoic sutures mark the course of the Iape- tus Ocean along the Appalachian Orogen. In Canada, wide vestiges of Iapetus crust and mantle occur as discrete, thin allochthons thrust upon the Paleozoic eastern margin of North America. A thicker oceanic vestige, the Dunnage terrane (Fig. 4), is 150 km wide in northeast Newfoundland and sits above a collisional zone between the miogeocline and the outboard Gander terrane (Keen et al., 1986).

The Avalon and Meguma terranes in the Canadian Appalachians and the Carolina, Bruns- wick and Tallahassee terranes in the U.S. Appal- achians (Fig. 4) have European and African affinities, and evidently travelled great distances to their present positions; thus they must lie outboard of a suture. In the Gulf Coast area, late Paleozoic sutures truncate early and middle Paleo- zoic sutures and terrane boundaries, indicating final accretion during Alleghanian Orogeny in the southernmost Appalachians and Ouachitas. One such suture, marked onshore by the Brunswick magnetic anomaly and traceable along the off- shore shelf-edge as the East Coast Magnetic Anomaly, is a major collisional zone (Nelson et al., 1985a). Along the Atlantic shelf-edge between Georgia and Nova Scotia, it was the locus for opening of the Atlantic Ocean (Nelson et al., 1985b).

Middle Paleozoic accretion in the Innuitian Orogen involved a number of small terranes (Tret- tin, 1987) that are unseparated in Figs. 4 and 5.

Mesozoic and Cenozoic sutures and terrane boundaries of the Cordilleran Orogen, only a few of which are shown in Fig. 5, reflect a complicated

pattern of accretion (Coney et al., 1980). They separate mainly volcanic juvenile terranes and chaotic terranes of ophiolites and melanges. Blueschists are preserved along some boundaries in the Canadian Omineca belt and occur as blocks in the Franciscan Melange of California. The high pressure-low temperature conditions required for blueschist metamorphism are exclusively related to a subduction environment. The geometry of modem subduction beneath Vancouver Island in Canada has been defined recently by onland deep seismic experiments (Yorath et al., 1985).

Phanerozoic sutures and terrane boundaries are subparallel to the modem continental margins along much of the Appalachian, Cordilleran and Innuitian orogens, but they are sharply truncated by the present margin northeast of Newfound- land. Early to late Proterozoic sutures of the Grenville, Makkovik and Ketilidian provinces are also truncated by the modem margin in this area.

The ages of sutures and terrane boundaries (Fig. 5) date rocks and structures of adjacent miogeoclines and terranes (Fig. 4). The patterns shown in the two figures are complimentary and provide insight into the controls and geometry of last and first deformations depicted in Figs. 2 and 3, respectively.

Figure 6. Time of accretion

This illustrates the way in which the North American continent grew by accretionary episodes to attain its present form and internal pattern of geologic mountain belts. The stable deformed rocks of ancient orogens are the frozen record of a mobile crust that changed continually to shape the present continent. Progressively younger accretionary events affected adjacent contemporary rniogeoclines through several sequential cycles of geologic mountain building.

In this analysis, there is a need for a systematic nomenclature that identifies the North American element (offspring) that resulted through the breakup of a preceding landmass, or parent supercontinent. Just as present North America is the offspring of Pangea, and Paleozoic Laurentia the offspring of a late Precambrian landmass (Pan- Africaland), names are necessary to denote off-

130

spring and parent supercontinents for Pre-

cambrian breakup cycles. One such scheme, sum-

marized in Table 1, uses names of contemporary

Precambrian orogenies, modified by suffixes for

the various offspring-parent relationship. Thus,

Hudsoniu (a North American element of un-

known original shape and extent) is the offspring

of Hudsonland (a fictitious supercontinent.

The existence of Archean deformed belts and

their truncation by fringing Proterozoic orogens

implies that the Archean structural provinces, or

minicontinents, were previously part of a larger

landmass (Kenorland). Late Archean (Kenoran)

orogeny, recorded isotopically in most Archean

provinces, was probably temporally related to as-

sembly of such an Archean landmass.

There is no documentation that rocks and

structures in any particular Archean province were

ever continuous with those of another. The

Archean provinces are all mutually suspect with

respect to one another. Prior cycles of Archean

breakup and assembly are also possible.

The breakup of Kenorland led to the formation

of the North American minicontinents, the Super-

ior, Slave, Wyoming, North Atlantic, and possibly

the North Keewatin and South Keewatin. Some,

like the Superior, were discrete island continents,

with their early Proterozoic peripheral miogeo-

clines still preserved. Others, like the Wyoming

H WILLIAMS ET AL

and South Keewatin are truncated and of un-

known earlier geometry. Other parts of Kenor-

land, dispersed by rifting, now lie beyond the

confines of North America.

The Archean minicontinents were assembled

(or reassembled), together with intervening juvenile

terranes accreted to continental margins, in the

early Proterozoic ca. 1.95-1.85 Ga (Hoffman,

1988) during development of the Trans-Hudson,

Wopmay, Penokean and Makkovikian/ Ketilidian

orogens. Other orogens, now largely obliterated,

developed adjacent to the Thelon Front and

Snowbird Line (Fig. 5). This early Proterozoic

phase of assembly, although probably di-

achronous, was rather brief, about 100 m.y. It

produced what has recently been termed the Pan-

American system (Lewry, 1987). It resulted in

stabilization of the northern two thirds of the

North American craton, that has remained unaf-

fected by younger orogeneses, except for diminu-

tion through several episodes of breakup. The

name Hudsonia is proposed for the present North

American faction of the supercontinent (Hudson-

land) formed by the early Proterozoic Pan-

American event.

Evidence for at least three middle to late Pro-

terozoic accretionary episodes is found in the

southeastern one third of the North American

craton. The Central Plains Orogen, comprising

TABLE 1

Scheme to denote North American offspring elements that resulted through breakup of preceding parent supercontinents

Parent Time of North American Time of

supercontinent amalgamation offspring separation

(Ga) (Ga)

Kenorland 2.5 and older

Hudsonland 1.9-1.8

Central Plainsland 1.7

Labradorland (Mazatzaland) 1.6

Grenvilleland 1.0

Pan-Africaland 0.8-0.6

Pangea (Pangealand) OS-O.3

Superior, Slave, Wyoming, North Atlantic,

North and South Keewatin

Hudsonia

Central Plainsia

Labradoria (Mazatzia)

Grenvillia

Laurentia

North America

2.5-1.9

1.x-1.7

1.7-1.6

1.6-1.0

1 .O-0.6

0.6-0.5

0.3-0.0


mainly juvenile rocks, formed at about 1.7 Ga. This modified the southern part of Hudsonia. The Mazatzal (Condie, 1982) and newly defined Trans-Labrador (Thomas et al., 1986; Wardle et al., 1986) orogens further modified the same general area at about 1.6 Ga, and were followed by the Grenville Orogen formed at about 1.0 Ga. The southern margin of Hudsonia was affected there- fore by at least three mountain building episodes, each with extended periods of juvenile accretion and/or collisions at contemporary continental margins. Orogenesis was patterned with successive deformed zones subparallel and to some extent overlapping preceding ones.

Accretion onto the southern margin of the North Atlantic, Superior and Wyoming provinces, which began before or during their confluence into Hudsonland, continued for at least 200 m.y. after their unification. It is not yet certain if some additions are unique to individual Archean provinces or if they are separately exposed parts of single belts accreted after the unification of Hud- sonland. In general, igneous crystallization ages decrease southward: about 1.9-1.8 Ga for Penokean, Makkovikian and Ketilidian; 1.8-1.7 Ga for Central Plains; and 1.7-1.6 Ga for Mazatzal and Labradorian (Hoffman, 1988).

The full extent and geometry of these middle to late Proterozoic mountain belts are unknown; however Labradorian and/or Grenvillian events are recorded in deformed rocks of Scandinavia (Gower and Owen, 1984), East Greenland (Max, 1979), the United Kingdom and Ireland (Piasecki et al., 1981) and Northwest Africa.

Late Precambrian (Pan-African) deformed zones, so widespread in the African continent, are absent in North America, except for local late Precambrian orogeny recorded in the Appalachian Avalon terrane (Figs, 2, 3 and 4). The Pan-African event is equated with the assembly of a late Pre- cambrian supercontinent (Pan-Africaland). Con- tinuity of miogeoclines around the periphery of the North American (Laurentian) craton, all ini- tiated in late Precambrian to Cambrian time, indicates that the North American (Laurentian) craton was an internal part of the Pan-Africaland supercontinent. Its breakup isolated Laurentia, the early Paleozoic predecessor of North America.

Laurentia remained an island continent until initial early Paleozoic accretionary events recorded in the Appalachian Orogen. Middle Paleozoic accretion further modified its eastern margin, and the adjacent Iapetus Ocean was eventually destroyed during late Paleozoic assembly of Pangea. At that time the Appalachian Orogen was in the heart of a supercontinent (and Newfoundland was the hub of the Pangean world).

Breakup of Pangea, mainly along former Paleozoic collisional zones, led to formation of Mesozoic North America. Opening of the Atlantic Ocean was approximately coeval with the onset of Cordilleran accretion on the opposite side of the continent. Greenland, which was part of North America (Hudsonia) since early Proterozoic Pan- American assembly, was isolated from the rest of the North American continent late in the opening history of the Atlantic.

The Paleozoic record of Laurentia and its evolution to North America serves as a useful model for considering the earlier Precambrian accretionary and breakup events implied in Table 1.

Discussion and summary

The thematic maps described above together provide a vehicle for discussion and broad analysis of most major aspects of North American architecture and evolution. Other important related features not represented in these maps might legitimately be presented in a similar fashion. For example, the portrayal of dyke swarms and rift zones, by age, would facilitate analysis of important extensional episodes and their spatial- temporal relation to other major tectonic events (Fahrig et al., 1986). Are dyke swarms and rifts everywhere a prelude to significant spreading episodes? Age and lithology of other anorogenic intrusions, age and lithology of major magmatic belts, and age of rocks in intracratonic basins are other worthwhile themes.

Our portrayal and analysis emphasizes the essential similarities in character, spatial distribution of lithotectonic elements, and geodynamic evolution of Precambrian and Phanerozoic erogenic belts. We are impressed with the similarities, rather

132 H. WILLIAMS t-1 AI.

than differences, among rocks and structures of

orogens, regardless of age; implying persistence of

plate tectonic processes back to at least the latter

part of the Archean Era. We do not, however,

ignore or underestimate possibly significant

quantitative differences between the Phanerozoic

and earlier Precambrian periods. These seem to

reflect gradual changes in crust-mantle evolution

and inevitable progressive accretionary expansion

of continental lithosphere, rather than major

qualitative change in lithosphere-mantle geody-

namics.

It is quite evident, from analysis of the North

American and other continents, that both the total

area1 extent of continental lithosphere and average

size of individual contemporary continental masses

has gradually increased from the Archean on-

wards. It is this progressive change, rather than

fundamental differences in lithospheric geody-

namics and mantle processes, that is reflected in

most of the observed differences between Early

Proterozoic and Phanerozoic lithotectonic ele-

ments. By extrapolation, it may account for many

of the even greater differences in the nature of the

Archean-deformed cratons. Other significant fac-

tors may include progressively lower spreading

rates and total spreading ridge length (the latter to

some extent implicit in our discussion) over time,

consequences of gradual decrease in total heat

flow, and effects of changing lithospheric levels of

geologically significant geotherms. Such factors

may control extensive volcanism and emplacement

of mafic sills in some Proterozoic foredeeps, such

as those connected with the Wopmay Orogen and

the Circum-Superior miogeocline, and their ab-

sence in Phanerozoic continental margin evolu-

tion. Absence of documented Proterozoic blue-

schists may reflect more general subduction of

hotter, younger oceanic lithosphere at contem-

porary destructive plate boundaries or other dif-

ferences in thermal regime. Certainly, the greater

abundance of ultramafic lavas during the Archean

and, to a lesser extent, in the early Proterozoic

must reflect significant progressive changes in

thermal regimes within the mantle.

Other differences may be related to changes in

the atmosphere-hydrosphere-biosphere, as for

example the widespread spectacular development

of sedimentary iron formations in the Early Pro-

terozoic Circum-Superior miogeocline.

Compilation of good thematic maps of North

America, although time consuming, is a relatively

easy exercise. The geologic data base is strong and

rapidly expanding, and it is accessible to all in a

single language. Maps are beautiful, fun, informa-

tive, and even profitable in some cases. We offer

our full support, loud applause and deep respect

to those brave souls who may surface to attempt

this task.

Acknowledgements

Thanks are extended to H.P. Trettin and W.R.

Muehlberger, who supplied material relevant to

the compilatons and unselfishly shared their latest

thoughts and ideas. We also thank G.A.G. Nunn

and A.P. Nutman for discussion, C. Conway and

G. McManus for preparation of figures, the

Canadian Geological Foundation and D.F. Strong

through Memorial University for financial support

in producing the coloured figures, and the Natural

Sciences and Engineering Research Council of

Canada and the Department of Energy, Mines

and Resources for supporting our work in the

field.

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Documents

Anatomy of North America: thematic geologic portrayals of the continent