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I
SUMMARY
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Plate Tectonics
ant] the Continents
A Review
1
INTRO D UCTION
B. CLARK BURCHFIEL
Massachusetts Institute of Technology
The plate-tectonics model has had marked success in ex-
plaining the first-order features of the earth; it has been
particularly successful in explaining the nature and age of
the crust of the ocean basins. The model accounts for the
distribution of earthquakes and volcanoes along plate
margins and the nature of deformations occurring along
the margins of the continents. The model, however, has
had less success in accounting for geological activity
within plates. especially the continental portions, and our
confidence in the usefulness of the model diminishes as
the age of continental crust increases.
The model has been applied in an attempt to explain
the tectonic features of the continents and to test the limi-
tations ofthe model in time and space. The accompanying
papers in this volume illustrate this application and the
limitations and methods used to study the continental
crust. The characteristics of plate boundaries, along
which the major deformation seems to occur, and the na-
ture of tectonic events that are not associated with plate
boundaries are summarized in this paper. Chapter 2 treats
the problems of understanding the pre-Mesozoic folder
15
r
than ~200 million years (m.y.)] evolution of the conti-
nents, which represents the majority of continental crust.
PLATE BOUNDARIES
The study of modern plate boundaries allows us the op-
portunity to examine the geological and geophysical ac-
tivity at the boundary and contemporaneous action ity adja-
cent to the boundary. Studies should be directed toward
establishing relationships between plate-bounda~y activ-
ity and the wide range of geological and geophysical
events, so that plate boundaries can be treated as dynamic
systems.
We know from modern and recent plate-bounda~y activ-
ity that conditions at the boundary may change rapidly. on
the order of a few million years, and in order to relate
geological events to plate-boundary activity contempo-
raneity of events must be established. Changes in plate-
boundary activity may lead to partial destruction, defor-
mation, or overprinting of the geological features of an
older system by a younger system. Interrelations between
the various parts of each system become lost or blurred.
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16
Such changes are the reason why the details of an ancient
plate-boundary system become hard to decipher. It is only
through the study of modern plate-boundary systems that
we can begin to comprehend fully the geological, geo-
physical, and geochemical processes and their interrela-
tions within these dynamic systems. The end product of such
studies would be to understand the processes and geo-
logical results of these systems from the earth's surface
into the underlying mantle. We are a long way from this
level of understanding, but continued investigations into
modem plate-boundary systems must continue as they
fonn Me fundamental basis for plate-tectonic analyses.
Even though there is great diversity in modern plate-
boundary systems, they do not represent the complete
range of potential plate-boundary systems. Therefore,
geological investigation of ancient plate-boundary sys-
tems may demand interpretations different from those of
modern systems.
Plate-bounda~y systems can be divided into divergent,
transfonn, and convergent types. Relative motion across
plate boundaries is oRen not orthogonal. particularly at
convergent boundaries; thus more complex combinations
of these types may also exist.
DIVERGENT BOUNDARY SYSTEMS
Divergent motion within continental lithosphere causes
extension of the lithosphere, producing elongate fault-
controlled depressions called riRs. If divergence contin-
ues' oceanic crust may develop and the continental l~tho-
sphere becomes part of two diverging plates. In many
cases ricing ceases before oceanic crust is developed, and
the nbc and its related features remain within continents.
Where divergent motion continues and oceanic crust is
fanned, the no strictures become buried and fonn the
basement for passive continental margins. Burke (see
chapter 4) has outlined many of the features associated
with continental rifts and has emphasized that to evaluate
these divergent systems properly we must establish con-
temporaneity of events within the system, the evolution of
riR systems, and the temporal stn~ctum1 relations of riRs to
adjacent oceanic crust.
A modern example of a rift system that has led to the
development of continental separation, connation of oce-
anic crust, and a divergent plate boundary is the East
African Red Sea, Gulf of Aden rift system. Studies in this
region have produced good, yet still incomplete, un-
derstanding of the evolution of a divergent plate-
boundary system. The East African rifts are in the riR
stage of development, whereas the Gulf of Aden has en-
tered the drifting stage. The Red Sea appears to be in a
transitional stage.
From studies ofthe East African~lfofAden area (see
Chapter 4) ant! from geophysical studies of continental
margins (see Chapter 5), an evolution of a divergent plate-
boundary system can be sketched. During the early riding
stage, the system consists of a broad linear belt of active
normal faults that produce elevated and depressed parts
B. CLARK BURCHFIEL
ofthe crust end characteristic patterns of sedimentary rock
types that commonly include great thickness of salt. This
early stage is also accompanied by volcanism, both al-
kaline and tholeiitic. and significant geothel~al activity.
The lithosphere is thinned over a broad linear zone, and
the asthenosphere rises beneath the riced zone. Dunng
continued ricing, this system enters the driR stage, in
which oceanic crust fonns between the separated conti-
nental lithosphere. Volcanism, structures, and associated
sedimentary basins may extend for hundreds of kilometers
away from the site that eventually develops into a diver-
gent plate boundary. Modification of continental litho-
sphere must take place beneath the entire affected region.
The divergent system thus affects a broad region of conti-
nental lithosphere, whereas divergent activity is more
restricted within oceanic lithosphere. The relationship
between upper crustal brittle faulting and deeper litho-
sphenc structural and chemical behavior is at present
poorly known and is certainly an important area for fixture
studies.
As the noted continental margins drift away from the
spreading ndge they become less active and finally form
a passive continental margin, where the transition from
continental to oceanic lithosphere takes place within a
single plate. Early subsidence along the passive margin
leads to the formation of continental shelf sedimentary
rocks, which overlap early-stage nk deposits. Further
subsidence can be explained by thermal decay away Tom
a spreading edge, and the formation of a constructional
passive margin consisting of shelf slope ant rise morphol-
ogy underlain by characteristic sedimentary-rocic assem-
blages.
The continued subsidence of some continental margins
indicates that they are not truly "passive," but important
dynamic processes are still active at the interface between
continental and oceanic lithosphere. Sediment loading
has been proposed as a mechanism for continued subsi-
dence, but it is insufficient to provide the proper magni-
tude of subsidence. Bott (1971) has argued that continued
subsidence along some passive margins may be the result
of lateral flow in the lithosphere across the ocean-
continent interface. Such ductile flow could produce its
own characteristic structures and modification of a pas-
sive margin that are superposed on those of the earlier rift
stage. By the time a passive margin has completed its
subsidence due to thermal decay, it is outside the influx
ence of the divergent boundary system. Its further devel-
opment is in an intraplate setting, and the processes that
affect it properly should be ~liscussed under the topic of
non-plate-boundary systems. But since passive-margin
development is clearly part of the evolution of divergent
systems it is considered here. Investigations of modem
passive margins at all stages of development are still in
their infancy, and a great deal is yet to be discovered
about their dynamics.
Ancient passive margins are well known from the late
Precambrian to the Recent. The marginal pants of most
deformed belts, now present as part of the internal struc-
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Plate Tectonics and the Continents: .A Rechew
ture of continents, contain rock assemblages and struc-
tures that indicate that they developed as part of a
divergent dynamic system. Similar passive-margin sedi-
mentary sequences are known as eroded remnants from
some middle Precambrian deformed belts, which sug-
gests that deep erosion could remove all sedimentary
evidence for former passive margins. Some structures, ig-
neous activity. and lithospheric modification by a di~er-
gent dynamic system could, however still be preserved.
Some middle Precambrian deformed belts show no sedi-
mentary record of a former passive margin, and the ques-
tion of whether one was ever present may have to rely on
studies of the geological and geophysical characteristics
of the crust beneath passive-margin sedimentary se-
quences. Archean passive margins are unknown, which,
along with much evidence, has suggested to many scien-
tists that during most of Archean time continental litho-
sphere was thin and ductile (see Chapter 15~. If plate-
tectonic processes were active in the Archean, they may
have operated at rates and scales on lithosphere and
mantle with characteristics unlike those of later Pre-
cambrian and Phanerozoic time.
Some ancient passive margins are eroded where deeper
parts ofthe lithosphere are exposed. In the southern Alps.
an entire section of the continental crust and several ki-
lometers of its upper mantle are exposed. Geological evi-
dence indicates that this area was the site of a divergent
plate boundary that evolved into a passive margin during
Mesozoic time. Study of ancient passive margins permits
the direct examination of geological features produced at
greater depths than those of modern passive margins.
During the early riding stage, several rids may develop
that do not progress to the fo~`ation of ocean crust. Dur-
mg the driPring phase they become inactive but retain the
geological features characteristic of rifting, such as exten-
si~e nonnal faulting, igneous activity, coarse elastic and
e`aporitic deposition, followed by local overlap by pas-
si~e-margin sediments. Studies of these failed rifts are
important because they yield evidence for the evolution
of adjacent passive margins and oceanic terranes which
for ancient e.Yamples may be completely destroyed.
Failed rids, commonly referred to as aulacogens. are
s<~metimes reaction ated by other dynamic systems fanning
either local mountain belts. .~bsiding basins (see Chapter
A. fir the locus `~f modern intraplate seismicitv and ~ult-
ing (see Chapter `). \tanv Ailed rifts contain important
gas and `,il reserves as well as being related locally to
lither types ot fire deposits.
T RA TV S FO R M B O UN D A R Y S Y S T E ~ S
Transtorrn h`~ndaries, ~ here taco lithosphere plates
slide past each <'ther horizontally, that affect c<,ntinental
lithosphere often Grin wide and complex belts `,t de~r-
mation. The San Andreas Fault ot Calitomia has linen
regarded as marking the transiorrn plate b<~ndarv be-
tu een the Pacitic and N<,rth .\merican plates. It is
I,<~` e~ en <'my fine of n meres north est trending tumults
17
of similar right-slip displacement. Some of these faults
have large displacements and may have served as the
plate boundary prior to the development of the present
San Andreas Fault. It is more likely that the relative mo-
tion between the two plates is a much broader zone,
stretching from the marine California borderlands inland
to at least western Nevada, a zone perhaps 500 km wide.
Thus, this transform boundary, lice most others within the
continental crust. is a broad zone of plate interaction and
may be called more properly a transform boundary
system.
Although tTansforrn boundary systems contain a wide
variety of associated geological features, the San Andreas
system can serve as an example. Associated with the
numerous strike-slip faults are areas affected by exten-
sion, such as pull-apart basins, or compression, such as the
folds and thrusts within the Transverse Ranges (Crowell,
1974~. ~rge-scale right-lateral structural bending and
faulting in southwestern Nevada and adjacent California
are probably part of the San Andreas transform system
(Figure 1.11. Blake et al. (1978) have reviewed the ;NTeo-
gene history of coastal California and have shown that the
development of the oil-rich Neogene basins is related to
evolution of the San Andreas system. In addition. recon-
structions of Mesozoic and early Cenozoic paleogeog-
naphy are complicated by the scrambling of paleogeo-
graphic belts and associated rotations of these temnes by
strike-slip faulting of late Cenozoic age (see Chapter 3)
related to this system.
Tra~el-time data from teleseismic studies of the upper
mantle in southern California by Hadley and Kanamori
( 1977) suggest that the San Andreas Fault does not offset
~ ridgelike body of high-velocity mantle extending from
40 t<, perhaps lOO km. The San Andreas Fault apparently
is confined to the upper 40 km of the continental litho-
sphere. The ridge of high-velocity upper mantle extends
150 km east of the San Anclreas before ending. Hadley and
Kanamori suggest that the upper lithosphere beneath
soothed California is decoupled along a surface of hori-
zontal shear. perhaps marked by low velocity, and that the
plate bo~ndarv at depth is further east than at the surface
(Figure 1.~). These interpretations are significant because
they raise the Cation of correlation between tipper
lithosphere and deeper plate and mantle dynamics. The
dec`~pling at fine or more layers in the lithosphere has
been suggested for other areas and other types of plate-
b`>~ndary systems (see Chapters 8 and 9; .\rmbruster et
aI., l9 ~ 8). This problem of lithospheric decoupling and its
etEects on correlating shallow and deep plate motions and
dynamics is clearly an important question that demands
farther attention.
The San Andreas Fault and other faults of this so stem
has e been the toc Ilk of studies concerned ~ ith earthquake
prediction and associated geological hazards. A ~ ide
Ronnie ot steadies hare been directed toward understand-
ing the dynamics of strain buildup. Cult initiation. and
movement. Beca~lse the San Andrea system has been
active from ;Il)OIlt 30 mu ago to the Recent. the opportu-
OCR for page 18
18
B. CLARK BURCHFIEL
it, -
~ ~ — - ~^ . ~ . . . _
_; +;
_~
... .
hi_
—~
˘~__~7~~
..~: `~ .~
,.~ ~ ~
_
Hi_
_~
I_
__
FIGURE 1.1 Looking northwest across southem Nevada ant across the Las Vegas Valley shear zone. The shear zone is marked by
right-lateral bending of all pre-\liocene rocks and marlcs the displacement near the eastern side of Me San Andreas transform system.
The bending and faulting along the shear zone account for a minimum of 70-km differential motion of late Miocene Age.
nity exists to understand the devils of this transfonn
system through time and its imprint on continental
geology.
Many large faults of proven strike-slip displacement are
known in the geological record, such as the Great Glen
Fault in Scotland. Other large linear faults or zones of
faults. such as the Kings Keweah Suture in California
(Saleeby, 1977) and broad Proterozoic shear zones of the
midcontinent region of the United States (Warner, 1978)
have been interpreted as ancient transforms. The fact is
that the importance of transfonn boundary systems in the
evolution of continents is essentially unknown. Where
ancient aseismic displacement may have occurred over a
broad zone, it is possible that we might not recognize it.
A deformed belt such as the Precambrian Limpopo Belt of
southern Africa is an intracratonic erogenic belt resulting
from horizontal shear between two undeformed crustal
blocks. The deformation described by Coward et al.
(1976) occurs across about 100 km of terrane and took
place at amphibolite and locally granulite metamorphic
grade. Structural evidence indicates that the deformation
was mainly by horizontal crystal shear. Can this defonna-
tional style be the deeper expression of transform motion?
An answer cannot be given wit}, our present state of
knowledge of transfonn systems within continental litho-
sphere.
CONVERGENT BOUNDARY SYSTEMS
Convergent plate boundaries, where one lithosphenc
plate passes beneath another, are Me sites of subduction
or recycling back into the mantle of the lithosphere
created at spreading ridges. The stable configuration at a
convergent boundary is where oceanic lithosphere is sub-
ducted because it is more dense than the underlying as-
thenosphere. Where oceanic lithosphere is subducted
beneath oceanic lithosphere a volcanic island arc chain is
present. Island arcs or continents may Boron past of a sub-
ducting plate and may eventually be drawn into a subduc-
tion zone. Because they are formed from material less
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Representative terms from entire chapter:
passive margins
Plate Tectonics and the C`'ntinent.s:.A Reeled
~ a_ SO rv AVOID—~
\
\
,~,~,,,,1J
FIGURE 1.2 Interpretive diagram of the proposed divergence
of the crust and mantle boundaries along the San Andreas Fault
in southern California (from Hadley and Kanamori, 1977. re-
printed Mom the Bulletin of the Geological Society of America,
with pez.~'ission).
dense than oceanic crust, they are more difficult to sub-
duct. The result is arc-arc, arc~ontinent' or continent-
continent collisions.
Study of modern subduction boundaries demonstrates
that geological, geophysical, and geochemical events and
features may extend for several thousand kilometers Tom
the boundary. From studies of events and features at mod-
ern convergent boundary systems, it is clear that ancient
analogs exist within continents, and it appears that most
material that forms continental lithosphere has been gen-
erated or reworked in convergent.boundary systems. It is
within such boundary systems that rocks formed within an
oceanic setting are added to continents.
NONCOLLISIONAL CONVERGENT SYSTEMS
In modern noncollisional convergent systems geological
features related to the system occur mainly within the
o~e~iding plate and only to a minor degree within the
lower plate. In fact, those features in the lower plate that
are part of the system—extension faults caused by flexing
of the lithosphere seaward of the subduction zone—are
mostly lost during subduction; this is not the case during
collisional events where the subducted plate is not oce-
anic lithosphere. It is the association of geological fea-
tures of the overriding plate that Is commonly presented
in the geological record and leaves a record of plate con-
vergence.
Hamilton (Chapter 3) has reviewed briefly some of the
elements of convergent systems. The outermost elements
of the system are the trench and outer nonvolcanic ridge
fanned from deformed packages of sedimentary rocks
containing occasional slivers of oceanic lithosphere, an
outer or forearc basin of undeformed sedimentary rocks,
and volcanic arc consisting of a characteristic sprite of vol-
canic rocks. Behind the volcanic arc, three dynamic set-
19
tines .~re pos.sit~le: (1) ``here the
JO
Cenozoic development of the southern part of the western
United States can serve as an example. Subduction of
oceanic lithosphere beneath the continental margin ofthe
western United States began about 240 m.y. ago and has
continued. at least locally, to the present. Dunug much of
Mesozoic time it appears that the convergent boundary
system generated compression within the overriding
plate and produced structural and magmatic effects more
than 1000 km eastward from the surface trace of the plate
boundary (see Chapters 3 and 6~. The sequence of ter-
ranes from west to east across the western United States
consists of a western terrane composed of accreted oce-
~nic sedimentary and volcanic rocks, `~ central terr`~ne
composed open Andean-type magmatic arc, an stem
terrace composed of belts of folds and east-directed
thrusts where older continental basement was reactivated,
find locally ~ foreland belt of faulted Precambrian cry.stal-
line rocks fonned by older plate-boundary systems. The
relationship between all these terranes is poorly under-
stood, but correlation with modern systems such as the
Andes will allow ~ betterunderstanding ofthe dynamics of
such systems. Study of the Cordil reran belt ~ ill permit us
to study relations between these terranes at crusta1 1~` els
not exposed in the modem Andes. These two types <,f
studies need to proceed together
During the later part of the early Cenozoic time, con-
~ergence continued and magmatic effects took place
across the western United States, but only minor deforrna-
tion occurred, with the exception of the Rio Grande Rift
that began about 29 m.y. ago (see Chapter 14~. From about
20 m.y. ago to the present, extensional tectonics have dis-
~upted the continental crust, folding the Basin and Range
province. Extension began in a back-arc sewing and
evolved complexly so that today part of the extensional
terrane is being overprinted by the San Andreas transfonn
system (see Chapters 8 and 9~.
This late Cenozoic development ofthe Basin and Range
extension has been recognized by several generations of
earth scientists, and many explanations for the extension
involve dynamics that would more properly be discussed
under the category of divergent or transform plate-
boundary systems. Recent studies have indicated that the
extension is an integral part of a convergent system. Re-
cent studies in the Basin ant! Range province exemplify
how new and exciting discoveries await us in unexplored
territory and how the melding of data from all earth-
science disciplines can yield completely new insights
into the development and evolution of the area. Field
studies about 10 years ago led to the discovery that many
subhorizontal faults, long considered Mesozoic thrust
faults, were Cenozoic in age. A few years later it was
determined that some areas affected by high-grade meta-
morphism and deformation, again considered Mesozoic in
age, were Cenozoic in age and locally could be related to
the subhonzontal low-angle faults. Tying together ail the
available geophysical. geological. and geochemical data
known for the Basin and Range province, Davis (see
Chapter 8) and Eaton (see Chapter 9) have developed
B. CLARK BURCHFIEL
models suggesting that the cr tstal rocks above to km e.,c-
tend by Iystic normal faulting and are detached along a
variably thick transition zone from the underlying crust
that is extended by ductile flow and dike intrusion Ero-
sional levels within the Basin and Range province have
exposed perhaps all three crystal levels. The ideas ex-
pressed in these models are new and unproved but
demonstrate that the potential still exists for major new
discoveries in regions that have been studied for several
generations.
Pre-.Mesozoic examples of noncollisional plate-bound-
ary systems are poorly known largely because the ;ll-
timate end of continental-margin evolution is collision by
either island arcs or other continental fragments. These
events commonly overprint the terranes generated prior
to collision. Because a Paleozoic ocean basin was present
in the site of the modern Pacific, evidence for Paleozoic
noncollisional convergent systems is present around the
Pacific margin, such as in Australia, South China (see
Chapter 16), Canada, and parts of South America and Ant-
arctica. Noncollisional systems are probably present
within some Precambrian deformed belts, but because
coIlisional events have overprinted these systems, they
have become difficult to recognize. Only through care fill
unraveling of the overpnuting events and recognition of
the characteristic features of the noncollisional systems
can these earlier systems be reconstructed.
COLLISIONAL CONVERGENT
BOI;NDARY SYSTEMS
Island-arc terranes are fanned by noncollisional conver-
gent boundary systems, but, because oceanic cmst ult -
mately is subducted. the end product of all island-err
development is collisional with a continent. The dy-
namics of island-arc systems can be studied Tom modern
examples, but the features of these systems become dis-
rupted during their inevitable collision. Dunng arc-
continent collision, new crust is added to the continent,
but in many cases collided arcs are reworked by later
superposed plate-boundary systems before they become ~
part of typical continental lithosphere. Continent-
continent collisions simply rearrange continental ma-
terial, but the processes leading to collision may add some
new continental material.
Several arc - Continent collision systems are at present
active and in different stages of evolution. In most
arc~ontinent collisions, the features of noncollision
systems are present in the island arc, but where conti-
nental lithosphere is present in the subducting plate and
enters the subduction zone, considerable defonnation
may take place within the subducting plate, extending the
effects ofthe convergent boundary system. The following
modem convergent systems show an increase in the
extent of disruption of the subducted! continental litho-
sphere: (1) northwest Australia, (2) Taiwan, and (3) north-
ern New Guinea. In each case, the continental passi~e-
margin sedimentary rocks, and in the latter two cases en en
Plate Tectonics and the Continents: A Re';zeu~
the underlying continental crust, have been cut by thrust
faults locally as much as 20~ km from the site of the
collision boundary. Because island arcs are backed up by
oceanic lithosphere, following collision with a continent,
the new ocean~ontinent interface is potentially subject
to the development of a new plate-boundary system. If the
geometry of the new convergent system subducts oceanic
lithosphere beneath the collided arc, rocks of the collided
system are overprinted by a noncollisional continental-
margin arc system and become deformed and metamor-
phosed to form the polyphase terranes common within
continental crust. The transitions from arc collisions
are not easy to interpret, and some of the volcanic rock
associations developed following collision in New
Guinea are difficult to relate to plate-tectonic processes
Johnson et al., 1978~.
Ancient mountain belts abound with evidence of arc-
continent collisions and overprinting by younger plate-
boundary systems (see Chapters 3, 6, and 16~. Because of
the common overprinting of collided arcs, the reconstruc-
tion of the plate-bounda~y systems related to the arc, its
collisional history, and overprinting episodes are complex
indeed. Study of older and more deeply eroded collided
arc tenanes gives us the opportunity to examine the de-
tails of the structure, geochemistry, metamorphism, distri-
bution of rock types, and evolution of arcs and collisional
margins that lie at depth below modern systems where
the details remain obscure. Studies of these arc terranes
will clearly extend back into Archean rocks of continents
where arc systems have been identified. There are,
however, significant differences between modem colli-
siona] systems and Archean ones (see Chapters 2 and 15~.
These differences must be understood, because they
relate to the evolution of the earth, its lithosphere, and its
continental regions.
Continent~ontinent collisions occur where oceanic
FIGURE 1.3 Distribution of tectonic
styles in Asia along the Indian-Asian
collisional system (Mom Tapponnier
and .Molaar, 1976). Dark area, region of
thrusting and crustal thickening;
domed area, regions of strike-slip fault-
ing; lined area, region of nonnal
faulting and crustal thinning. Cor-
respond~ng stress states are also indi-
cated.
"
~1
lithosphere between continents is completely subducted
in one or more subduction zones. Remnants of the oceanic
lithosphere with or without associated island-arc terranes
may be preserved within the collisional boundary.
Active systems of continent~ontinent collision are
present in the Alpine-Himalayan mountain belt. Studies
of this modem collisional system have shown that once
collision has taken place, but convergence between the
to o plates continues. geological activity w ithin the
system broadens throughout both plates and becomes
very diffuse. In fact. the geological activity may become
developed in so many local structures that it is difficult, if
not impossible. to define a single plate boundary.
Nowhere is this type of activity better exemplified than
in Asia. Molnar and Tapponnier (1975) have traced the
collisional history between India and Asia from collision
about 50 m.y. ago to the present. Since collision, India and
Asia have converged by more than 2000 km. This postcol-
lisional convergence has caused deformation within the
Asian plate for a distance of 3000 km from the collisional
boundary (Figure 1.3~- In addition, the Indian plate was
disrupted by south-directed thrust faults. Slolnar and
Tapponnier have suggested that the faults within the
fanner Asian plate form a pattern that is explainable by
indentation mechanics (Figure 1.~. India is displacing
large parts of Asia laterally as it continues northward.
Many ofthe active faults follow structural lines fanned by
older collisional events within Asia (see Chapter 16~.
Thus, it appears that such features as the extensional
opening of the Baikal Rift are part of a convergent system
whose initial collisional boundary was 3000 km away.
Within this plate-boundary system complex patterns of
volcanism' crystal defor~nation, and lithospheric modifi-
cation are taking place. Once such a system becomes inac-
tive, it is obvious that detailed geological, geophysical,
and geochemical studies will be necessary to establish
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A,
~~ in/ iN;
FIGURE 1.4 Plane indentation of semi-infinite rigid-plastic
media by different rigid dies (from Tapponnier and !vlolnar,
1976). Allows indicate sense of shear along slip lines. Principal
stresses as, and a3 bisect small quadniaterals delineated by slip
lines. The names of corresponding major tectonic features are
indicated. (a) Flat rigid die; (b) rigid wedge, similar to the situa-
tion that occurs at the Pamirs (western end of the Himalayas); (c)
flat triangular indenter and hallowed~ut medium, similar to
situation that arises at the Himalaya-Bunnan syntaxis (eastern
end of the Himalayas).
B. CLARK BURCHFIEL
contemporaneous relations between widely diverse geo-
logical features and events.
Ancient examples of continent~ontinent collisions are
common in the geological record. Huang (Chapter 16) has
described the tectonic evolution of China in such a way
that it is obvious that China has been built by a complex
series of arc and continent collision events that began in
the late Precambrian, continued through the Paleozoic and
Mesozoic, to the most recent collision of India during the
Cenozoic. The ancient sutures can be identified in many
places, but the far-reaching effects of convergent systems
can produce a complex paKem of extensional, transla-
tional, and compressional features, which Is only now
being recognized. Burke (Chapter 4) has suggested that
the Keweenawan rift system, which fonns a major crustal
structure of the central United States (see Chapter 11),
may be related to a coIlisional system within the Gren-
ville (lOOO-m.y.mld) deformed belt (see Chapter 15~.
Compound convergent systems may contain charac-
teristics of two types. Such plate boundanes are char-
actenzed by relative motions that are oblique or change
rapidly because of continual shifts of rotational poles.
Along the western part of the Java subduction system, the
relative motion is oblique to the boundary and under-
thrusting takes place in the Bench, whereas strike-slip
faulting talces place in the overdying plate and is super-
posed on the volcanic arc terrane. Further north, along the
same plate boundary, the motion becomes strike-slip, ant]
extension occurs behind the boundary in the Andaman
Sea.
Similar types of compound boundaries can be rec-
ognizec} in ancient systems within continents. Within the
Cordilleran belt of Canada and Alaska, both collisional
and noncollisional convergence during late Mesozoic
time was associated with major strike-slip filults that were
fanned within the continent several hundred kilometers
from the plate boundary (Davis et al., 1978~. The evolu-
tion of the Pyrenees has been interpreted as a combina-
tion of transfo.~ and both extensional and convergent
systems at different times (Mattauer and Seguret, 1971~.
Complex combinations of boundary systems can also
result Mom collisions that take place along initially i'Tegu-
lar boundaries. Dewey and Burke (1974) hypothesized
that continent~ontinent collisions along irregular bound-
aries could result in fragmentation and complex relative
motions of small fragments. McKenzie (1972), Mom stud-
ies of active seismicity in the eastern Mediterranean re-
gion, showed that dunng such fragmentation complex
relative motions take place. It can be argued that the mod-
em seismicity gives us only an instantaneous picture of
the boundaries and relative motions of small fragments,
and, given longer periods of geological time, the entire
belt of defonnation could be regarded as a continuous or
at best a semicontinuolls system of deformation. Analysis
of the motions of small fragments suggests that their mo-
tions may not be a reflection of motion at depth. Within
the alpine belt, analysis of fragment motion suggests that
the continental fragments are probably decoupled fat
Plate Tectonics and the Continents: A Reeve?*
levels within the crust andlor within the lithosphere and
their motion is not directly reflected by deep lithospheric
or asthenospheric flow (Burchfiel, in press). Such studies
as those of Hadley and Kanamon (1977) along the San
Andreas Fault and Davis (Chapter 8) and Eaton (Chapter
9) for the Basin and Range province add evidence that
such detachments are possible. Some geophysical evi-
dence suggests that for these zones decoupling may be
related to low-veloci~ crystal layers. A1I these data and
interpretations suggest that lower lithosphere now found
beneath such continental fragments is exotic and may
have an evolution different from the crust that overlies it.
These types of complexities are only now being rec-
ognized, and the extent of such activity must be under-
stood because it could modify greatly the models we de-
velop for the evolution of continental crust.
ON-PLATE-BOUNDARY SYSTEMS
There are many structural and magmatic features within
continents that have no apparent relation to plate-
boundary activity. Some of the features regarded above as
related to plate-boundary systems are so far removed from
the boundary that they may be independent of such
systems and should be included in the category of non-
plate-boundary systems. Included in non-plate-boundary
systems, but not discussed in this chapter, are features of
the interiors of oceanic plates.
INTRAPLATE MAGMATIC ACTIVITY
Intracontinental magmatic activity that is not related to
plate-boundary systems takes the forTn of small plutons,
plugs, laccoliths, dikes, diatremes, kimberlites, and some
volcanic rocks. They have a complete range of compos~-
tions from ul~amafic to alkalic. Sources of material for
these roclcs are highly varied. Some basaltic igneous rocks
and radiogenic isotopic data suggest that they are devel-
oped from a deep mantle source, possibly below the as-
thenosphere, that was homogenized about 2.0 b.y. ago
(see Chapter 13~. Other basaltic rocks such as those ofthe
Absaroka volcanic rocks, Snake River plain, and at Yel-
lowstone. are derived from subcontinental mantle that has
been attached to the overlying continental crust since the
last major thermal event to affect these rocks (see Chapter
14~. The data indicate the continent and upper mantle in
this area have been together since 2.8 b.y. ago.
AII these types of igneous activity require perturbations
in the mantle to cause Ideal melting. \\ hat these perturba-
tions may be is uncertain and clearly represents problems
for considerable future study. Whatever the causes, these
rocks record events that contribute to the evolution of
already formed continental lithosphere. Many of these
igneous rocks contain fragments, .xenol~ths (see Chapter
12), that give its a sampling of the rock types present
~ ithin the lithosphere and a~thenosphere. If their source
can be properly placed, and their age determined, they
23
yield additional valuable evidence concerning the en ol~-
tion of continental lithosphere. These xenoliths. however
naust be interpreted cautiously as they come from per-
turbed areas within the earth and may not be representa-
tive of the mantle or lower crust (Irving, 19 ~ 6).
INTRAPLATE VERTICAL MOVEMENTS
lithe development of basins and arches within continental
areas appears to be unrelated to plate-boundary activity.
Basins contain sedimentary accumulations that record the
history of subsidence. The arches, on the other hand, are
uplifted areas, and much of the record has been removed
by erosion. and the history of their movement is more
difficult to decipher. Vertical movements of the crust are
the dominant processes in the formation of basins and
arches, but the nature and cause of these movements is
virtually unknown. Intracratonic basins are superposed
on already formed continental crust of varied age and
structure. The Michigan Basin overlies an older late Pre-
cambrian riR terrane, whereas the Illinois Basin overlies
an older Proterozoic folded belt that was affected by later
Precambrian rhyolitic magnnatism. Some basins are ac-
companied by early igneous activity and nonnal faulting,
whereas others are not. The variety of basin settings has
made it difficult to arrive at an explanation for their forma-
tion. A number of models have been proposed that rely on
thermal, eustatic, tensional, and compressional mecha-
nisms (see Chapter 7~. None of these, however. has of-
fered a completely satisfactory explanation. Basins are an
important feature of continental tectonics because many
contain important accumulations of hydrocarbons.
Basin and arch development within continents indi-
cates late-stage modification of already formed conti-
nental lithosphere. What types of modifications talce place
at depth are unknown but must be understood before a
complete evolution of the lithosphere can be deciphered.
Recent studies of unconfonn~ties and the fluctuation of
sea level within basins has suggested a similarity between
basins within a single continent and on several continents
(Sloes, 1972~. Studies on continental margins show similar
histories, indicating that these features may be a world-
wide phenomenon. If this proves to be the case, at least
part of the sedimentological controls could be related to
plate-tectonic activity.
Broad epirogenic movements, as with their more re-
stricted counterparts, basins and arches, may also lead to
modification ofthe continental lithosphere. Broad vertical
movements, such as the elevation of the Colorado Plateau
and adjacent Rocky Mountain area in the Cenozoic. must
involve changes within the mantle. What these changes
are and how they affect the continental crust and litho-
sphere are essentially unknown.
INTRAPLATE DE FORMATION
Even though continental lithosphere away from active
plate-bounda~y systems is considered rigid. it is not en-
24
tirely inactive. Recent studies have shown diffuse seismic
activity to be present throughout the eastern part of the
United States (see Chapter 7~. Some of these earthquakes
have been large, with magnitudes greater than 7. A1-
though intraplate continental seismicity is generally at a
very loci level, recent studies have shown that the seis-
micity occurs in several linear belts. It can be demon-
strated that some of these linear belts are controlled by
older trends (see Chapter 11) related to Precambrian,
Paleozoic, and Mesozoic structures. Thus. mapping of
older structures, both at the surface and their continuation
beneath younger sedimentary cover within the conti-
nents, may be useful in understanding the trends of mod-
em and potential seismicity.
It is not entirely clear that intraplate continental seismic
activity is unrelated to plate tectonics. Studies of earth-
quake focal mechanism, in site stress measurements, and
hydraulic fracturing studies hare produced data showing
the present stress field within the eastern United States to
have a general pattern with the maximum compressive
stress trending northeast, but with local complexities.
Sbar and Sykes (1973) suggested that the regional stress
field is related to the motion of the North American plate
and that the earthquake zones are controlled by the pres-
ence of unhealed fault zones. If intraplate fault activity is
related to plate motion and to the reactivation of older
structures, perhaps some intraplate igneous activity is
also related to these reactivated fault and structural lines
of weakness in the crust. Lipman (Chapter 14) has sug-
gested that the temporal progression of volcanism asso-
ciated with the Yellowstone volcanic field as well as some
older Mesozoic and Cenozoic volcanic trends follow Pre-
cambrian structural trends. It is clear that the relations
between plate tectonics and intraplate tectonic and ig-
neous activity require continued research to determine
how much intraplate activity is related to plate motions
and how much is related to independent causes.
CO?
Plate Tectonias and the Cvntinents: .A Rev~e2c
`'phic~lite T'~lt i`'uth`` estern Sierra Ne~ <~da. C.~lit`'mi.~. in
.~'rth.Arneric<`n Opl~i`,l~tes R. C. Cr~leman `~nd \~. P. lruin
eds.. Orexon Dep Ge<'l and .Mineral Industries Bul l. 9~. pp.
141-1~.
S bar. !~. L.. .ind L Svkes (19~3). Contemporarv c`'mpres!siv e
stress and seismicity in eastern .North America: an example of
intraplate tectonics. Ceot. Soc. Am. Bull. R54, 1861-1882.
25
Sloss L. L. ( 19 ~ _). S`nchron~ of Phaner(>zolc sedimentarv-
tectonic e`ents <~t the .~,rth .American craton and the Russian
platform. Int. Ge`31. C()ngr.. 24th .se.ssi`,n. sect. 4. E'P -~32.
Tapponnier P.. and P. Slolnar ~ 19 ~ 6). Sl m-line field theow and
large-scale continental tectonics .\iat'`re 264. .319-~.
Warner L. .~. ( 19 ~). The Colorado lineament: ~ middle
Precambrian wrench fault system Geol. Soc. .Am. B``ll. s~g
161-1~1.
Problems of Pre-Mesozoic
Continental Evolution
LEON T. SlLvER
California lustitute of Technology
INTRODUCTION
By virtue of its remarkable ability to explain currently
observed dynamic processes of ocean basins and conti-
nental margins, and to account for much of the geological
record of the last 200 million years (m.y.), the plate-
tectonics model Is a rational starting point in interpreting
the earlier evolutionary history of the earth. The model's
numerous predictions facilitate continued testing and ups
grading of its own validity. Since early Mesozoic time,
modem continental margins have developed with distinc-
tive physical and chemical characteristics that can serve
as bases for comparative studies of older margins, which
have been incorporated within the continents in the past.
The recent progress of geological studies in North Amer-
ica ant! the diversity of its geological endowments make
the United States a particularly appropriate region for
many of these essential investigations.
There is, however, some risk in directing these research
programs solely to the paradigms of plate tectonics. Our
This paper is Contribution Number 3271. Division of Geological
and Planetary Sciences, Califomia Institute of Technology, Pasa-
dena, Califomia 91125.
26
current knowledge of the earth's evolutionary history in-
cltldes evidence for many unidirectional changes in earth
properties and suggests the need for a cautious approach
to the extrapolation of modem geodynamics to earlier geo-
Iogical eras.
Earth energies, forces, processes, material distribu-
tions, temperatures, gradients, rates, and other factors all
combine to constrain earth dynamics. and all have
changed with time. If we are to understand the past fully.
Other versions of the plate-tectonics model and other
models also must be considered. It is vital not to be
seduced into allowing modern plate tectonics to assume
premature dominance of interpretations of earth history,
and therefore the analysis of the early geological and geo-
chemical evolution of the continents deserves a signifi-
cant fraction of our continental research efforts.
The immense bibliography required to document all
the assertions, suggestions, questions, and other pro~oca-
tions contained in this brief chapter would outstrip it in
length I omit them here (with one deferential exception)
but suggest that the reader will find most of them con-
tained in the references in the other papers contained in
this volume, especially the papers by Hanson (Chapter
Problems of Pre--Ues~zv~c Continental Evolution
13), .\f uehlberger (Chapter
(Chapter 12~.
TIME CALIBRATION OF THE
TECTONIC RECORD
15), and Kay and Kay
It is the essential dimension of time, recorded in the
paleontological time scale. in the various radiometric
systems, and in the compelling paleomagnetic record,
that revealed for plate tectonics its true complex dynamic
character. Extension and testing of plate-tectonic con-
cepts to the earlier geological record will continue to
require time calibration. For the earlier Phanerozoic, fos-
sil zonation is still the most precise time-resolving tool
available. Its intercalibration with radiometric techniques
is required for establishment of dimensions of physical
time and for extension to unfossiliferous segments of the
geological column. Continuing efforts in this direction
deserve high priority. The paleomagnetic pole-reversal
sequences and the apparent pole-migration paths have
not yet been integrated into a continuous record that can
be used for precise age assignments, but significant prog-
ress is being made in this direction. The establishment of
a comprehensive magnetostratigraphic record in the sedi-
mentary and igneous columns of the Phanerozoic de-
serves high priority.
For the first 4 billion years (b.y.) of earth history, various
radiogenic isotope systems in rocks and minerals, increas-
ingly supplemented by biological, chemical, and rema-
nent magnetic information, will be the principal tools for
both general time assignments and precise time resolu-
tion. The characterization of long-term evolutionary addi-
tions of radiogenic 87Sr, 206Pb, 207Pb, Mob, and '43Nd to the
major crust and mantle reservoirs provides important ge-
netic and temporal information on the nature and history
of the source regions of continental rocks.
Current research in the precise measurement of ism
topic systems promises time resolution on the order of
~ 0.1 percent in the oldest crustal rocks, under favorable
conditions and with suitable lithologies. It is essential,
however, that integrated petrological and geochemical re-
search into the methods for interpreting the geochronol-
ogy of high-temperature, high-pressure metamorphisms
be continued. Among the most challenging problems in
tectonics are the time constraints for erogenic evolution at
depth in the lower crest and upper mantle leading to
granulite and eclogite facies metamorphism.
CONTIN E NTAL TE CTON IC S FROM I H E
PR E-ARCH EA ~ TH RO U G H TH E
PALE OZO IC
THE INTERPLANETARY CONNECTIO N
The stunning revelations of 20 years of space exploration
in our 4.~-b.y.-old solar system, capped by direct lunar
2`
sampling and field studies and by sophisticated orbital
probing of Nlars, Mercury. Venus, and ttie Jovian systems,
have focused scientific attention on the first 5~00 m.y.
Of history in the terrestrial planets. The phenomenology
of initial planetary aggregation and differentiation has
been developed with extraordinary results for the earth's
moon.
Intemal lunar differentiation must hare been com-
pleted in the first 100 or 200 m.y. Catastrophic surficial
modifications from external bombardments are inferred in
the interval Mom the moon's formation to about 3.9 b.y.
ago. Comparative studies of the surfaces of the other ter-
restrial planets also reveal profouncl effects of early im-
pact histories.
On the earth, the oldest rocks, peculiarly enough, are
identified at close to 3.8 b.y. old and represent early dif-
ferentiated granitic crust. Is this timing simply fortuitous?
Are there still older rocks to be found in the continents?
Or is this a most significant time convergence implying a
critical shared episode in the genesis of the earth-moon
system? Is this a time of common cessation of plane-
tesimal infalls? Planetary fission? Planetary near~olli-
sion? Some other phenomenon?
In these, the earth's oldest rocks, the record commonly
appears to be a montage of multiple episodes of igneous
and metamorphic activities. No direct evidence for impact
features has been reported; but impact involvement can-
not be precluded. Can various constructional elements of
the montage be stepped away to unveil cataclysmic im-
pact products? What is certain is that crustal dynamics was
well established more than 3.5 b.y. ago but in fonns and
with rates that are still obscure. Was this ngi&-plate tec-
tonics as it is modeled today?
THE ARCHEAN NEED NOT BE ARCANE
The earliest geological era for which there exists a signifi-
cant record is the Archean, extending from 3.8 to 2.5 b.y.
ago. It is no longer quite so cryptic or mysterious as it was
when first tentatively identified. In North America,
Greenland, Eurasia, Africa, and Australia, the earliest
continental records have been extended back to 3.3 3.8
b.y. Although the rocks approaching 3.8 b.y. of age invari-
abIy are strongly metamorphosed and complex, they es-
tablish a minimum age for planetary differentiation and
for the production of continent-forming crustal materials.
Such rocks are found in west Greenland and in ~linne-
sota.
In the Barberton Mountains of South Africa and in the
Pilbara region of western Australia, there exist essentially
unmetamorphosed sedimentary and volcanic strata that
are clearly 3.2 to more than 3.4 b.y. old. In their existence
is the earliest record ofthe initiation ofthe very-long-term
crustal stability, which is the unique distinction of conti-
nental structures. In the rocks of the Barberton ~loun-
tains, there is evidence for primitive life forms. Clearly.
these regions contain the best surviving records of the
early surface environments of the earth. At these sites the
28
crust. the atmosphere, and the hydrosphere interacted to
produce at their interfaces the premonitory biological,
chemical, and physical environments in which biochem-
ical assemblages have evolved into the life forms and the
life environments that we know.
Widely recognized features of Archean shields are the
numerous great linear arrangements of mafic volcanic
rocks called greenstone belts, commonly more than 1000
km in length and hundreds of kilometers in width. Be-
cause immensely valuable massive sulfides and other
types of ore deposits are associated with them. the details
of their local structure and stratigraphy, petrology, and
mineralogy hare received considerable attention. How-
ever, their broad spatial distributions, their isolation in
"seas" of ancient granitic gneisses, their spatial and tem-
poral relations to granite crust development are still in-
completely understood. Among a number of features that
distinguish Archean greenstone belts are abundant.Ug-
rich, Si-poor basaltic lavas, which are unlike most recent
basalts. These ancient rocks, called komatiites, require
much higher melting temperatures, probably reflecting
the much steeper temperature gradients that prevailed in
the crust and upper mantle of the Archean earth. The
relationship of these rocks to ancient crust is not under-
stood. Were they part of ancient global oceanic floors sub-
sequently deformed into linearity? Are they part of
ancient island arcs produced at convergent plate bound-
aries? Do their geometry, age ranges, and chemistry com-
pare directly with Phanerozaic systems?
Although Archean continental crustal rocks are present
in large areas of the northern United States, their com-
plete distribution and range in ages are still not estate
fished. Their lateral boundaries with younger rocks are
invariably concealed or complicated by igneous intrusion
or intense shearing. Their vertical distribution and litI2o-
spheric roots (extension to or below the base of the crust)
are unknown. Their relationship to ancient ocean basins
is not clear. Were there once supercontinents, subse-
~luentlv tr`~gmented and disaggregated prior to the Pro-
ter<'z<'ic? Whether rigid" c~us~1 plates, as we know ~em,
existed in the Archean has not been established. This is a
central question for understanding early continental tec-
tonics.
PROTEROZOIC PERIMETERS IN
SPACE AND TIME
It is in the Proterozoic era, e.~ctending from 2500 to about
600 m.y. ago, that the opportunities for comparison of
pre-Phanerozoic tectonics to plate tectonics are most
favorable. The boundary in time between the Archean
and Proterozoic eras is commonly set arbitrarily at about
2~00 m.y. ago. The boundaries in space between Archean
and Proterozoic rocks in North America are far more
diverse and potentially offer critical into,~.ation on tec-
tonic processes.
The last major magmatic culmination in the Archean
appears to be recorded on all continents in the interval
oO0-96~0 m.y. ago. In the Allowing hall aeon, 900~2500
LEON A. SILVER
m.y. ago, well-documented evidence for significant oro-
genic and magmatic arc den elopment is surprisingly
sparse. Cratonic sedimentary sequences w ith plateau ba-
salts (diabases) and felsic volcanic rocks are the dominant
litholog~es of this period and are well preserv ed on all the
continents. There is, therefore, a significant possibility
that at approximately 2500 m.y. ago a global tTansforma-
tion in the earth's dynamic systems occurred and was
followed by 500 m.y. of comparative continental stability .
This possibility is still incompletely seen, much less
documented. The search for a more complete erogenic
record of this seemingly quiet period needs new mo-
mentum on all continents.
A distinctive aspect of the sedimentary record found in
the cnatonic and circumcratonic basins <'f the early Pro-
terozoic is the abundance and diversity of sedimentary ore
deposits. The banded iron formations, which are the prin-
cipal world source of iron ores, reached maximum devel-
opment between 2500 and 18()0 m.y. ago. The great
uranium-rich conglomerates of the Canadian Huronian
formations and the South African gold-uraniu~r.
glomerates (e.g.' Witwatersrand) were formed in ;-
interval. In each case, the sources of the metals -
localization mechanisms are in need of better a.
In the United States, iron, uranium, and gold ores are
major targets for exploration; research into the early Pro-
terozoic history of the continent conceivably may pros ide a
substantial contribution in these important areas of re-
source development.
Broadly viewed, widespread Proterozoic orogenies in
North America appear only in two significant time in-
tervals, 200~1600 m.y. before the present (B.P. ) and
1300 900 m.y. B.P. In Canada, these are recognized as the
Hudsonian and Grenvillian progenies, respectively. Most
of the United States and Mexico appears to be underlain
by crustal materials that seem to have been initiated in
these two great episodes (see Chapter 15 for geographic
distributions). Although their total distribution is incom-
pletely known, especially under the Phanerozoic cover of
modern continental margins, it commonly has been in-
Erred that the major Proterozoic erogenic belts are
sutured against the Archean crustal remnants. Suggested
examples are along the Grenville front In Ontario and
Quebec, the Nash Fork-.\lullen Creek shear zone in
Wyoming, and in portions of Minnesota and Michigan.
The geological style of each ofthese "sutured" perimeters
appears to be sufficiently distinctive. however, to make all
such boundaries worthy of specific study. As the proposed
loci of continent-continent collisions. or transforms. they
constitute a major class of targets for the study of Precam-
brian crusta1 dynamics.
Are there unique elements of the Proterozoic tectonic
record that differ from more ancient or more recent tec-
tonic products? Perhaps! At feast two possibilities may be
considered.
The midcontinent gravity high ofthe United States (see
Chapter 11) is an anomalous geophysical feature with no
precise analog recognized anywhere else on earth. It is
the consequence ot a late Proterozoic continental rifting
Problems of Pre-`Uesozo~c Continental Evolution
event in which older Proterozoic and Archean granitic
crust was invaded in an arc uate belt by great masses of
basaltic magma over a distance of about 2000 Ian, ~ idths of
50 150 km, and probably to depths of 20 30 km (base of
granitic Christ). The timing of this catastrophic noting
event is remarkable in that most of it occurred about
1100 1120 m.y. ago, essentially synchronous with a major
plutonic magmatic are culmination recorded throughout
the not too distant Grenville province. If the Grenville
province was developing at a convergent plate boundary.
Or was introduced by continent~ontinent collision at this
time, the Keweenawan basaltic rip has no clear equiva-
lent in comparable plate-tectonic settings developed in
the last 200 m.y. Does this reflect a fundamental change
in crustal or lithospheric strength and rigidity from the
Proterozoic to the Mesozoic?
Extensive geochronological studies have suggested a
second remarkable Proterozoic phenomenon in North
Amenca. In the interval from 140~1500 m.y. ago, in
newly developed mid-Proterozoic crust, a magmatic
event of enormous magnitude and with distinctive lithol-
ogies occurred without identifiable relation to synchro-
nous orogeny or sedimentation. From Labrador to Cali-
fomia (and beyond?) hundreds, perhaps thousands, of
ano~osite-syenite and rapakivi granite plutons with sur-
face diameters up to 100 km perforated the continental
crust in a belt at least 6000 km long and 1000 km wide.
Within that belt these plutons became the most important
voIumetnc constituent in the upper crust. The volume of
magma is staggering for an anorogenic setting; the ma-
tenal and energy sources are a first~rder problem in
crust-mantle evolution; the state of crystal stress that ac-
companied the emplacement is still speculative. FIas this
type and magnitude of event been recorded in the Pha-
nerozoic or in the Archean? If so, the record has not yet
been read.
In summing up some of the problems of Proterozoic
dynamics in such cursory fashion, it is perhaps best to
return to a first-order question. Can the nearly 2 b.y. of
Proterozoic time essentially be represented by two great
erogenic intervals, each approximately 300 400 m.y. long
and separated from each other and We preceding Archean
and succeeding Phanerozoic erogenic culminations by a
quiet interval of comparative durations. In the detailed
record, we know of some regionally significant orogenic
events in the so-callecI quiet intervals, but the promi-
nence of these two great erogenic periods, corresponding
to the Hudsonian and Grenvillian, is clearly seen not only
in North Amenca but on several other continents. What
does this indicate for ep~sodicity, even penodicity, in
global dynamics during this time interval? Is there possi-
bly an extension of periodicity as well as episodicity
beyond the Proterozoic? Were these orogenies shared
events on the margins of supercontinents that subse-
quently were fragmented and redistributed? Such propo-
sitions represent some of the outstanding questions for
those who would extend post-Paleozoic plate tectonics
into the Precarnbnan.
29
ARE PALEOZOIC PLATES \fORE
PALATABLE THA^N' PRECA\1 BRIA`NT ?
In a summary discourse of this nature. c.~.sual pr<~ocati<'n
is easier than thoughtful analysis. Current views `'f plate
tectonics may not be adequate to explain even the P.`leo-
zoic orogenies. \Vith the extraordinary [~,dv `~f Pale`'z<'ic
faunal data on provinciality. ecology, and climatology.
with increasingly detailed stratis~raphic and pale`'ma~z-
netic records with over 100 years <'f intense ~e<'l`~gic`~l
study of Paleozoic erogenic provinces on troth sides <'f the
Atlantic Ocean. plus an essentially straightforward rec<'n-
struction of Pangaea at the end of the Paleozoic. ~ critical
analysis of the effectiveness of the plate-tectonics mode]
in the Paleozoic seems possible. Indeed. an impressive
number of rational interpretations of venous early Paleo-
zoic plate-tectonic features have been made. They range
from old marginal geosynclines and island arcs (northeast
North America Great Bntain) to ocean floor (Bay of
Islands) to closing oceans (Iapetus) to continental colli-
sions (Appalachian and Caledonide orogenies) to a co-
herent supercontinent (Pangaea). Given the immense
complexity yet coherency of most of We diverse data in-
puts, it would be difficult not to accept what I. Tuzo ``il-
son ( 1966) suggested were the consequences of the riding,
spreading, and closing of an ancient ocean in the early to
mid-Paleozoic (the Wilson cycle).
The Devonian to Permian history of the same region,
however, involved a widespread, perplexing erogenic
episode (the Hercvnian), which is found superimposed on
the earlier orogenies, without the obvious plate-tectonic
features Mat characterized He earlier events. Intense fold-
ing, high-grade metamorphism, and granitic mobilization
of a different chemistry are some of the consequences of
the Hercynian (Allegheny) erogenic interval.
.\ large number of studies attempt to relate the Her-
cynian episode to hypothetical subduction and conver-
gence, but almost always without broad acceptance. Many
thoughtful students of plate tectonics recognize in this
case that there are additional dimensions to the phenom-
ena of continental dynamics that have not yet been elabo-
rated in the existing models.
One of the most exciting rewards of the study of pre-
Mesozoic continental evolution will be. obviously. the ex-
tension ;md improvement occurrent tectonics models. It its
those workers ``ho exercise admiration for. and careful
restraint in. the use of these models from whom me can
expect the next generation of state-of-~e-art tectonics con-
cepts. The greatest rewards, perhaps, may be found in our
increased ability to approach the continental dynamics
record as the key to the global dynamics record and, in
turn, utilize terrestrial tectonic evolution as ~ key to the
comparative tectonic history of the inner planets of our
solar system.
RE FE RE NC E
Wilson, 1 T. (1966). Did the Atlantic close and then reopen?
.`Jature 211, 676.
