Download by Henry Simmons Before there was the Pangean supercontinent

by Henry Simmons
a r t h ' s continents are not forever fixed
in place. T h a t they are in slow and
restless movement over the surface
face of the earth is the grand unifying insight
of the plate tectonics revolution in geology
over the past 20 years. Simply by tracing the
outlines of the great lithospheric plates that
s u p p o r t the present oceans and continents,
and b y matching geological folds and other
features, geologists have been able to reconstruct a clear and persuasive picture of the
face of the earth as it was 200 million years
ago, early in the Mesozoic era. At that time,
almost all the earth's major land masses were
crammed together as a single supercontinent
which is n o w called Pangea.
T h e southern half of the vast structure—
G o n d w a n a l a n d — i n c l u d e d w h a t are n o w
South America, Africa, India, Australia, and
Antarctica. T h e northern half—Laurasia—u l t i m a t e l y i n c l u d e d p r e c u r s o r s of N o r t h
America, Greenland, northern Europe, Russia,
Siberia, C h i n a , and a s u b s e g m e n t called
Armorica that included southern Europe from
Portugal to Czechoslovakia. T h e rending of
this great block into continent-sized chunks,
b e g i n n i n g some 200 million years ago, and
their m o v e m e n t to the widely scattered
positions they occupy on the globe today, is
by n o w an oft-told tale.
But h o w did Pangea come to be assembled
in the first place? W h a t did the face of the
earth look like, say, at the beginning of the
Paleozoic era 370 million years or more
before Pangea? How did the primordial cores
of t h e p r e s e n t c o n t i n e n t s , the i n d i v i d u a l
b a s e m e n t slabs of Precambrian rock called
shields or cratons, move about the globe,
sweeping u p crust with their edges during
that third-of-a-billion years of the Paleozoic
era? W h a t are the finer details of the tectonic
process itself: mountain building (orogeny)
w h e n continental plates collide; the overt h r u s t i n g of older rock on y o u n g e r ; the
movement of microplates over huge distances
along faults on the earth's surface; and the
m e c h a n i s m of subduction itself, in which
the plate m a r g i n s descend t h r o u g h vast
trenches to rejoin the mantle? H o w far back
into the depths of time will paleogeographers
be able to carry their reconstruction of the
faces the earth has worn through time? A n d
w h a t is in store for the future?
Before there was the Pangean supercontinent
there were fragments that became it. And there was something
before that, and before that, and be...
MOSAIC March/April 1981
27
Magnetized pyzzle pieces
Geologists are bringing a great range of
investigative tools to bear on these related
questions. But their most valuable tool, the
one method that has consistently yielded
the most precise quantitative information
on the latitude and orientation of oceanic
and continental crust hundreds of millions
of years in the past, is paleomagnetism. It
involves the m e a s u r e m e n t of the n a t u r a l
remanent magnetism frozen into c o m p a r a tively undisturbed rock of k n o w n age.
This magnetic trace represents a permanent
record of the alignment of the rocks in relation
to the earth's magnetic field at the time they
formed. In the case of once-molten igneous
r o c k s of volcanic origin, the trace was
imprinted when the magma cooled; the rocks
bear the stamp of the orientation and intensity
of the local geomagnetic field at the time of
their formation. Similarly, clays and other
iron-bearing sediments can carry the imprint
of the prevailing field at the time and place
in which they were compacted and lithified.
"Imagine you have a globe with just two
continents—Europe and North A m e r i c a / '
proposes Rob Van der Voo, a geologist at
the University of Michigan who has made
these processes his specialty. "By measuring
the remanent magnetism in rock samples at
successive intervals backward in time, you
can generate an apparent polar wander path
for each continent. And you see that the two
paths are different for Europe and N o r t h
America.
" W e know, of course," he explains, " t h a t
it s the continents and not tue magnetic poies
that wander, but it's just handier to pretend
that it's the poles. W h e n we look back [250
million years] to the Permian period, for
example, we see that each continent tells us
the pole is in a different place. But we k n o w
the magnetic pole position must be the same
for each continent. So we move the continents
until the poles do overlap, and that tells us
the relative positions of the c o n t i n e n t s . "
Several assumptions are critical to paleomagnetic investigations. One is that the earth's
field is essentially a dipole, like the field of a
bar magnet, with one south magnetic pole
and one north. Another is that the magnetic
axis of the earth closely parallelled the
alignment of the rotational axis of the earth
in the past, as it-do.es now when averaged
over several thousand years.
" W e have good evidence," Van der Voo
observes, " t h a t the rotational and magnetic
axes have been roughly parallel for the last
500 million years; that for the last 100 million
years, at least, the rotational and magnetic
axes have on average not deviated from each
other more than 10 degrees; and that the
earth's obliquity has not been too different
28
MOSAIC March/April 1981
from what we see today." (Obliquity is the
a m o u n t of axial tilt of the earth relative to
the plane of the earth's orbit around the
sun.) Van der Voo is among those who hold
that the earth's rotational axis presumably
always has been tilted at about its present
66.5 degrees to the orbital plane—a statement
some of his colleagues will dispute.
Subtle clues
Although paleomagnetism is a relatively
precise and quantitative tool, the amount of
information it can yield is restricted in several
ways. For one thing, since the earth's magnetic
field is approximately symmetrical around
its polar axis, a paleomagnetic trace cannot
yield any direct i n f o r m a t i o n about the
longitude at which a rock sample originally
formed. The magnetic inclination, or dip, of
the magnetic trace in the rock can give only
latitude precisely, along with the direction
to the paleopoles; the longitudinal, or eastwest, distribution of the blocks in ancient
times must be teased out by other, more
qualitative techniques.
A second limitation under which paleomagnetists labor has to do with the requirem e n t that the s a m p l e s remain relatively
undisturbed over large stretches of geological
time. This is necessary for the kind of clean
traces that give paleomagnetists a high level
of confidence in their samples. They m u s t
be sure they are l o o k i n g at an original
remanent imprint instead of some spurious
magnetic anomaly overprinted on the rock
long after it had originally formed.
T h e more ancient the rock, the less likely
that it will have escaped weathering or violent
metamorphic events that would hopelessly
alter its original magnetic signature. T h u s
trustworthy paleomagnetic evidence tends
to thin out as one looks further back in time.
This is a significant problem for paleomagnetic reconstruction of the wander paths of
several continents during a number of entire
periods in the 370 million years during which
P a n g e a formed. A n d the data are e v e n
sketchier for the era, the Proterozoic, that
extends back some 2.4 billion years b e y o n d
that.
O n e of the most a m b i t i o u s efforts to
reconstruct the face of the earth during the
Paleozoic (570 million to 228 million years
ago) has been mounted at the University of
C h i c a g o by A l f r e d Z i e g l e r , a i d e d b y
Christopher Scotese and Richard Bambach.
While paleomagnetic data provide the foundation for their work, they have combined
these with tectonic, lithologic, climatic, and
biogeographic evidence. They use these to
resolve ambiguities in the p a l e o m a g n e t i c
record and to establish an i n d e p e n d e n t ,
although more qualitative check on the overall
fit of their reconstruction.
Tectonic evidence includes a variety of
features: highly eroded fold belts indicative
of old sutures where now-widely separated
continental blocks were once welded together
by ancient collisions; linear formations of
andesite, an igneous rock indicative of the
kind of volcanic activity seen in the ring of
fire that today circles the Pacific basin; and
sharp breaks in the deep, early stratigraphic
record in places where y o u n g e r , overlying
fossil sequences are unbroken, a combination
suggestive of the merger of two previously
separated blocks. (See " O c e a n M a r g i n s :
the Scars of Creation," in this Mosaic.)
Major reconstructions
T h e fossil record also yields important
clues to the prevailing t e m p e r a t u r e and
moisture content of the e n v i r o n m e n t at the
time the sediments formed. The interpretation
of this record is based on the assumption,
supported by paleoclimatologists, that present global climate patterns were in place in
the past—that global atmospheric circulation, driven by differential heating of the
atmosphere, then as now was strongly affected by the w i n d - d e f l e c t i n g C o r i o l i s
influence of the earth's rotation. This would
dictate that surface winds blew from the
east in the low and high latitudes and from
the west in the middle latitudes then as they
do now.
A consequence of this is that equatorial
belts on the eastern margins of continents
tend to be far larger and wetter than on
western margins and that arid and desert
conditions are common at low latitudes on
the western margins. Evaporites—salt beds,
gypsum, and halites—indicate an arid paleoclimate; coal swamps indicate one marked
by high p r e c i p i t a t i o n a n d a b u n d a n t life.
Assuming these are indeed the conditions
that prevailed in the remote past, it is possible
Continental blocks in motion
W h e n it comes to describing the complex tectonic e v o l u t i o n of the e a r t h ,
involving many interlocking and simultaneous events, pictures can do the job
better than many words. Mindful of this,
Christopher Scotese, a doctoral candidate
in geophysics at the University of Chicago,
in 1979 prepared an eight-minute film
on the subject, with the assistance of Shell
Development C o m p a n y of Houston.
The film is a narrated computer animation of 540 million years of the earth's
c h a n g i n g g e o g r a p h y , f r o m the m i d Cambrian to the present time. It takes
the viewer back from the present to the
Mesozoic, when most of the earth's crust
was assembled into a single supercontinent, Pangea. It then goes beyond, to
the Cambrian period at the beginning of
the Paleozoic, when the continents were
widely scattered at low tropical latitudes.
The second half of the film is devoted
to the evolution of the modern ocean
basins, showing computer-animated
MOSAIC March/April 1981
29
color sequences of the spreading history
of the North and Central Atlantic.
In addition to the film, Scotese has prepared a pocket-sized flip-book of small
maps of the earth, d r a w n at 20-millionyear intervals from the Cambrian to the
present time. W h e n its pages are riffled
rapidly, the effect is to convey a sense of
the movement of the continental blocks
and the opening and closing of the ocean
basins just as these are presented in the
film. T h e booklet, Continental
Drift,
includes B a t m a n - s t y l e interjections—"Crash," "Rip!" and "Crunch"—denoting
the collision of N o r t h America w i t h
G o n d w a n a 320 million years ago, the
breakup of Pangea and opening of the
North Atlantic 160 million years ago,
and the collision of India with Asia 40
million years ago.
To see the world change, flip the lower,
right corner of this Mosaic off y o u r right
t h u m b . (It will work better with y o u r
t h u m b at the bottom of the corner, rather
than the side.) That should take you from
the dawn of the Cambrian to the assembly
of G o n d w a n a 250 million years later.
Then start at the back, flipping the corner
off your left t h u m b , and see G o n d w a n a
take on the features of the earth as we
k n o w them today. (Additional information r e g a r d i n g the film a n d f l i p - b o o k
may be obtained by writing Christopher
R. Scotese, Department of Geological Sciences, University of Chicago, 5734 S.
Ellis Avenue, Chicago, Illinois 60637—or
call 312-753-8165.) •
to determine the latitude in which the sediments were deposited and the orientation of
the continents that they abut.
On the basis of all these kinds of data,
bolstering those from the p a l e o m a g n e t i c
record, Ziegler and his associates have prepared reconstructions of the Paleozoic globe
(one for each of the major periods of that
era) from the o p e n i n g of the C a m b r i a n
period some 570 million years ago to the
closing of the Permian, about 330 million
years later. Six major continental cratons
figure in the reconstruction: In addition to
Gondwana, they include Laurentia (most of
modern N o r t h America plus Greenland and
Scotland), Baltica ( S c a n d i n a v i a , P o l a n d ,
northern G e r m a n y , and Russia west of the
Urals), Siberia (essentially as it still is),
Kazakhstania (a triangular continent centered
on what is today Kazakh province in the
southern Soviet Union), and China. T h e
r e c o n s t r u c t i o n of the C a m b r i a n g l o b e is
remarkable in several respects. First, the six
continental blocks are all widely dispersed
and situated in low tropical latitudes; there
is no land at all north or south of 60 degrees
of latitude. And these protocontinents were
isolated from one another within a single,
huge, interconnected world ocean that covered
both poles.
By far the largest single Cambrian continent
was G o n d w a n a (Africa, S o u t h A m e r i c a ,
Antarctica, Australia, India, Tibet, Iran, Saudi
Arabia, Turkey—and present-day Florida).
Remarkably, the orientation then of G o n dwanaland and its major units was precisely
opposite to that of today. It was essentially
upside down, with Australia and Antarctica
in the tropical and m i d - l a t i t u d e s of the
Northern Hemisphere, North Africa in the
high southern latitudes, and S o u t h Africa
near the paleoequator. W h a t was its west
coast is east today. It was half the world
away from its ultimate location, and much
of the tectonic story of the Paleozoic is concerned with its migration. T o reach its final
location, the whole G o n d w a n a # s t r u c t u r e
had to move south, cross the South Pole,
and continue northward into the opposite
hemisphere. This shift required approximately 200 million years; it was completed
about 350 million years ago, in the early
Carboniferous.
Making mountains
As this maneuver proceeded, major tectonic
and geologic developments were occurring;
other, smaller c o n t i n e n t a l b l o c k s moved
a r o u n d . Each was on its s e p a r a t e p a t h .
Periodically they b a n g e d a n d jostled one
another, occasionally welding to or tearing
away from each other or one of the major
paleocontinental blocks.
A bare hundred million years into the
process, by the time of the Ordovician period
450 million to 500 million years ago, the
gradual closing of the ocean between Laurentia
and Baltica had led to mountain-building
episodes, called orogenies. Among these was
the Taconic orogeny that produced mountain
masses, or orogens, along the eastern margin
of Laurentia. Those now are the w o r n - d o w n
hills in New York south of the Adirondacks.
Continued closure during the next 50 million
years, in the Silurian period, u p t h r u s t the
Caledonian orogen along the western margin
of Baltica. And the physical collision and
welding together of Baltica and Laurentia in
the next period, the Devonian, did two things:
created the multiblock portion of Pangea
that is sometimes called Laurussia (Laurasia
without Siberia and China) and u p t h r u s t
the Acadian orogen in eastern Laurentia.
(See F o o t p r i n t s of a n c i e n t c o l l i s i o n s , "
accompanying this article.)
T h e Acadian u p l a n d s of the Nova ScotiaNewfoundland region today mark the suture
between the two previously separated blocks
(just as the Himalayas represent the suture
formed by the ongoing collision of India
and Tibet, which began 40 million years
ago, and the Alps evidence the collision of
Africa with Europe).
The grandest of all the Paleozoic collisions
began in the late Carboniferous, between
some 350 million and 400 million years ago,
when Gondwana and Laurussia came together
to create Pangea. This caused the Alleghenian
and s o u t h e r n A p p a l a c h i a n orogenies in
eastern North America and the Hercynian
orogeny across central Europe. The force
of this collision was so great, by some accounts, that the suture formed earlier between
Laurentia and Baltica gave way; the Baltica
unit was displaced northeastward along a
z o n e of w e a k n e s s — a t r a n s f o r m f a u l t extending from coastal N e w England across
Newfoundland and t h r o u g h Scotland. As
this movement went on, the ocean narrowed
b e t w e e n e a s t w a r d - d r i f t i n g Laurussia and
S i b e r i a , at that t i m e f i r m l y j o i n e d to
Kazakhstania. This caused further mountain
building along the eastern margin of Baltica,
where the Urals are today.
The vast mountain ranges thrown up by
the collision of G o n d w a n a with Laurussia
(the southern Appalachians and the Hercynian
belts across what is n o w Germany) eroded
to become the source of the vast amounts of
sediments transported by rivers and deposited
in the form of delta platforms. These became
marshy swamps. Because they were located
near the equator and were assured ample
rainfall and a constantly warm climate, the
growth of tropical plants was dense and
continuous. The vast coal deposits of eastern
N o r t h America, western Europe, and the
Donetz Basin of today's Soviet Union are
the legacy of these tropical swamps. The
name that designates this period, Carboniferous, commemorates its distinctive coalforming feature.
By the late Permian, some 250 million
years ago, Pangea was almost completely
assembled. The major exception was China,
30
MOSAIC M a r c h / A p r i l 1981
which was still s e p a r a t e d from SiberiaKazakhstania by a narrowing sea. It did not
finally collide with the Mongolian region of
that block, to complete Laurasia, until early
in the Mesozoic. Meantime, however, rifting
developed in northeastern Gondwana, with
the result that Tibet, Iran, and Turkey pulled
away on a separate journey of their own,
ultimately to collide with Laurasia in the
mid-Mesozoic. That suture today is believed
to be represented by the mountains of the
Hindu Kush, the Pamirs, the Karakorum
Range, and the T ' a n g - k u - l a , or Tanglha, all
in central Asia.
MOSAIC yarch/April 1981
31
The paieoclimate
"The effects of continental collision reach
far beyond the simple suturing together of
two formerly separate continental b l o c k s / '
Ziegler and his associates observed in a recent
article in American Scientist. " T h e resulting
folding and thrust faulting cause the rocks
to 'pile u p ' on themselves, thus thickening
the continental crust in the zone of collision....
T h e area covered by continental crust is
[thus] decreased d u r i n g collision, just as the
area covered by a rug is less if it is crumpled
u p against a wall rather than spread flat on a
floor."
The clustering of the continents into Pangea
had an " e x t r a o r d i n a r y geographic consequence," the Chicago scientists note. " A n
enormous single, interconnected ocean developed. This 'world ocean/ sometimes called
Panthalassa, not only spanned the globe from
pole to pole b u t extended for 300 degrees of
longitude at the equator, twice the distance
from the Philippine Islands to South America
across the m o d e r n Pacific.
"Circulation in this giant ocean had to
have a major impact on Permian climates.
For example, the equatorial currents driven
by the trade winds flowed uninterrupted
a r o u n d five-sixths of the circumference of
the earth. T h e east-facing coast of Pangea,
against which these currents impinged, must
h a v e been extremely warm. Ancient Gulf
Streams would have circulated these warm
w a t e r s into h i g h e r l a t i t u d e s , causing an
especially strong climatic asymmetry between
eastern and western Pangea. The Permian
was indeed a time of geographic extreme.
ThePrecambrian frontier
" T h e Cambrian [300 million years earlier],
w i t h its isolated, e q u a t o r i a l l y d i s t r i b u t e d
continents and two polar-but-interconnected
oceans, was totally unlike the Permian....The
widespread shallow sea that flooded cont i n e n t a l p l a t f o r m s in the early Paleozoic
contrasts with the large proportion of exposed
land in the late Paleozoic. These two extremely
different periods also differed totally from
the m o d e r n world, which is the product of
the b r e a k u p of Pangea into nearly interconnected north-south continental belts and large,
semi-isolated ocean b a s i n s / '
W h a t are the prospects for determining
the earth's geography prior to the Paleozoic?
T h a t , says Ziegler, is the new frontier for
geologists and paleomagnetists. He believes
the next decade will see a tremendous expansion of knowledge about the Precambrian,
which began essentially with the young earth's
coagulation, some 3.8 billion years ago. (At
least, the oldest earth rock k n o w n is of that
age.) T h e oldest remains of living things—of
probably nonnucleated cells—appear to have
formed within the next 300 million years.
" I n the C a m b r i a n , " Ziegler observes, " w e
have for the first time a tremendous diversity
of fossil samples of multi-celled organisms.
T h e s e evolved r a p i d l y so that the paleontological record gives you nice, dense slices
to work w i t h . " T h e slow pace of evolution
in the 2.9 billion years that life spent in the
Precambrian, b y contrast, is divisible into
much larger and rougher-hewn segments.
That record, Ziegler observes, will be much
more difficult to decipher.
" O n e h o p e f u l sign a b o u t P r e c a m b r i a n
geology," Ziegler adds, "is that there is some
evidence of a supercontinent that broke u p
very late in the Precambrian or in earliest
Cambrian times. All over the earth you find
evidence of widespread, rift-type volcanism
occurring a b o u t 600 million years before the
present. T h e continental basalt formations
in southern India are an example....If we can
find what rifted from what, then we could
figure out more easily what happened, and
it's a lot better if you can work with one or
two large pieces rather than m a n y smaller
ones."
Many small ones
While Ziegler and his associates h a v e
concentrated on the big picture of continent
amalgamation and dismemberment in the
early Paleozoic, other geologists have focused
on the finer details of the process. O n e of
their most remarkable achievements has been
the confirmation of a bold proposal advanced
by J. T u z o Wilson of Canada a dozen years
ago: that m a n y provinces along the east and
west coasts of North America, as well as
parts of Europe, were once parts of other
continents. (See " T h e Earth's Great Domesday B o o k / ' Mosaic, Volume 1 1 , N u m b e r 4.)
Dennis Kent and Neil Opdyke of Columbia
U n i v e r s i t y ' s L a m o n t - D o h e r t y Geological
O b s e r v a t o r y have computed pole positions
from flat-lying Middle and Upper Devonian
sequences of clay and sandstone in the Catskill
M o u n t a i n s of N e w York State. These were
compared to samples of similar age from
e a s t e r n M a s s a c h u s e t t s , M a i n e , and N e w
Brunswick. Comparing the data indicates
that coastal N e w England and easternmost
C a n a d a formed at more southerly latitudes
than they would ultimately come to occupy.
As the sea closed between Gondwana and
Laurentia during the Carboniferous, and the
two continents welded themselves together,
a northeast-trending megashear formed. The
Acadian (New England-eastern Canada)
complex was evidently squeezed to its present
location—like a marble from between a boy's
k n u c k l e s — s o m e 1,500 kilometers to the
northeast, over the equivalent of 15 degrees
of latitude. " T h e northward movement of
the s o u t h e r n continents produced this mega s h e a r , " Kent explains. "It's as if a huge
section of continental plate got squeezed out
and pushed aside."
I n t e r e s t i n g l y , there is close a g r e e m e n t
between the pole positions computed for
the Acadia province and for the British Isles
in those late Devonian and early Carboniferous times. This suggests that the two
regions were possibly both attached to the
N o r t h A m e r i c a n craton and t h a t Britain
s h a r e d with the N e w E n g l a n d - M a r i t i m e
Provinces region its motion relative to North
America during the Carboniferous.
A n o t h e r displaced s e g m e n t , or terrane
( " W e ' r e r u n n i n g out of words to preempt;
n o w we have to respell them," explains one
g e o l o g i s t ) , is Florida, w h o s e rocks bear
similarities to those of Africa. O p d y k e and
Kent believe Florida was once associated
with Africa and was rafted into Laurentia
during its collision with the Gondwana plate.
32
MOSAIC March/Aoril 1981
In their r e c o n s t r u c t i o n , only a n a r r o w ,
triangular marginal basin remained, once
the junction between Laurentia and Gondwana was formed by closure of the Atlantic;
Florida fit neatly into the center of this
shallow marginal basin.
Open the Atlantic
Michigan's Rob Van der Voo has looked
still further back in time to determine the
tectonic history of coastal N e w England and
eastern Canada. He proposes that, some 600
million years ago, when the late Precambrian
was ending and the Cambrian beginning,
t h e y were part of a separate c o n t i n e n t a l
plate called Armorica (after the Armorican
m a s s i f in F r a n c e ; it i n c l u d e d s o u t h e r n
England and Wales, the Iberian Peninsula,
France, and south-central Europe).
"Until the late Cambrian," says Van der
Voo, "we see a paleomagnetic match between
Armorica and Gondwanaland. They obviously
rifted a p a r t sometime later; by the late
D e v o n i a n [100 million years ago] we see
that Laurentia, Baltica, and Armorica have
come together. Where did Armorica collide
with Laurentia? I believe the Taconic orogeny
represents the suture, and that occurred about
440 million years ago, in the late Ordovician.
By the Silurian [only ten million years later]
the two blocks have actually joined. T h e n
Baltica comes back into the picture, producing
the Caledonian orogeny in northeast Laurentia
and other orogenies in several places in Europe
in the late D e v o n i a n . " (See "Footprints of
ancient collisions," accompanying this article.)
"In the Carboniferous," says Van der Voo,
"Gondwana collides with Laurentia and forms
orogenies extending from the Ouachita [the
O z a r k and Wichita Mountains] through the
S o u t h e r n Appalachians and Alleghenies to
the Hercynian, deforming all of Armorica.
This is a far more extensive and grinding
type of collision than the earlier ones; the
Hercynian orogeny was huge, covering an
area at least equal to the Himalayan orogeny
of today."
It is during this period that Armorica,
with its Acadia c o m p o n e n t , was p u s h e d
n o r t h e a s t w a r d along the t r a n s f o r m fault
proposed by Kent and O p d y k e , with Acadia
docking on the North American continent.
T h e n , as G o n d w a n a began to break away
from North America in a rifting and seafloorspreading episode 160 million years later, in
the Jurassic period, a new ocean ridge—now
the Atlantic's mid-ocean ridge—developed
between Acadia and the rest of Armorica.
For the past 160 million years the two
c o m p o n e n t s have been pushed to opposite
sides of the steadily opening Atlantic, as
have South America and Africa.
T h e picture that emerges from this work
is one of great continental plates rifting and
MOSAIC March/April 1981
33
peeling apart, of ocean closure and mountain
building, of the suturing of accreted material
to old continental crust followed by new
episodes of rifting of great continental plates.
T h e triple junction formed by the Red Sea
and Gulf of Aqaba, both n o w opening, and
the volcanic activity extending southward
from their intersection through East Africa's
Great Rift Valley, are an example of such
rifting t o d a y . (See " A C o n t i n e n t C o m e s
U n g l u e d , " Mosaic, Volume 8, N u m b e r 6.)
N o t only are bits of continental crust transferred from one plate to another, but sometimes, as in the case of Armorica, a block
m a y f i n d itself a p a r t of t h r e e d i f f e r e n t
plates in succession in a mere 500 million
years.
Wrangellia, etc.
T h e picture is considerably more complicated for western N o r t h America, Alaska,
and the continental blocks rimming the Arctic
basin. Large areas of the western United
States and Canada and the south of Alaska
h a v e b e e n f o u n d to consist of s e g m e n t s
formed elsewhere and transported by tectonic
processes, sometimes over great distances,
to their present sites.
O n e of the most intensively studied of
these is Wrangellia. Its fragments have b e e n
found in at least six different places in western
N o r t h America. These extend over some 17
degrees of latitude from Hells C a n y o n on
the Idaho-Oregon border through western
Canada to southeastern Alaska. (See " T h e
Earth Beneath the Poles," Mosaic, V o l u m e
9, N u m b e r 5.) The structure is underlain b y
thick limestones and island-arc basalts a n d
capped by limestone and other sediments,
suggesting deposition in equatorial or low
tropical latitudes.
John Hillhouse of the United States G e o logical S u r v e y , w h o has done e x t e n s i v e
paleomagnetic investigations of the Wrangellian lavas, confirmed three years ago that
the igneous Wrangellian rocks formed within
15 degrees of the equator. He has yet to
d e t e r m i n e w h e t h e r this occurred in t h e
Northern or Southern Hemisphere, however.
"If we can learn w h e t h e r the e a r t h ' s
magnetic field was normal or reversed during
the part of the Triassic [225 million to 200
million years ago] when these rocks cooled,
we will clean u p the picture a good b i t / ' he
declares. " W e should be able to tell at least
in which hemisphere the Wrangellian rocks
formed." Because the remanent magnetic
inclinations that Canadian geologists have
f o u n d in s o m e s o u t h e r n p o r t i o n s of
Wrangellia actually point north and slightly
u p w a r d , he suspects that a Southern Hemisphere origin might well be the case.
A number of other displaced, suspect, or
enigmatic terranes have been found in western
N o r t h America, including one in Nevada,
another in British Columbia, and a third
t e r r a n e , w h i c h is w e d g e d b e t w e e n t h e
Wrangellian fragment there and the continental craton. These fragments, sometimes
called microplates, consist of island-arc
volcanic rock, ocean crust, and marine sediments, although at least one looks as if it
might have been a former continental margin.
Like Wrangellia, m o s t of these c r u s t a l
blocks formed far to the south of their present
p o s i t i o n . W i t h V a n der V o o , S h e r m a n
Gromme of the Geological Survey has shown
that what is being called the Alexander terrane,
which incorporates the coastal islands off
northern British Columbia at 55 degrees north
latitude, originally formed in the Paleozoic
at a b o u t 40 degrees of t o d a y ' s latitude.
G r o m m e and Hillhouse have found more
recently that all of the Alexander terrane's
Simmons, a science writer for many years,
says if he had to do it over he'd be a
paleontologist.
northward movement occurred during the
Paleozoic and that it was firmly b o n d e d to
N o r t h America shortly after that era ended
and the Triassic period began, 225 million
years ago.
The northward movement of Wrangellia,
however, occurred after the Triassic. So,
despite the fact that the two terranes are
adjacent today, they obviously were once
independent of one another.
Seeking longitudes
While the paleomagnetic technique can
d e t e r m i n e paleolatitudes of origins with
considerable precision, it cannot ordinarily
make direct statements about paleolongitude.
Sometimes, however, longitude can be inferred. T h e special circumstances in which
this is possible require (1) a continental mass
that has long been in place—in this case the
N o r t h A m e r i c a n shield—and that h a s an
in-place paleomagnetic record long enough
to include an a p p r e c i a b l e s e g m e n t of an
a p p a r e n t polar w a n d e r path; and (2) an
accreted terrane with a record of its o w n
long enough to overlap that wander path at
b o t h e n d s . C o m p u t a t i o n s based on the
differences in those traces of the s a m e
apparent wandering of the magnetic pole
can produce an approximate longitude.
Gromme believes he has such a combination
in the Chugach terrane, the second most
recent segment to have accreted to N o r t h
America. The Chugach arches around the
bight of the Gulf of Alaska south of the
border-ranges fault, from Prince William
Sound to the islands off British Columbia.
T h e Chugach record, says Gromme, "goes
from the late Cretaceous to the Oligocene, a
span of 50 to 60 million years." This was a
time when N o r t h America was pretty m u c h
where it is now; differences in the two records
result from movement of the C h u g a c h and
make the computations feasible.
Gromme and his associates checked the
orientation of magnetic particles in Chugach
rock. The particles pointed, of course, in the
direction of the earth's magnetic pole at the
time the rock formed. " W e saw a 90-degree
counterclockwise rotation between the expected and observed poles," Gromme reports.
" T h e game you play is to make a cutout and
then move it around until the two poles line
up. Then you can get both the original latitude
and some idea of longitude. It turns out to
be far south in the Pacific in a direction
somewhat northwest of H a w a i i , " at a surprising distance from the earth's poles.
T h e validity of this m e t h o d d e p e n d s
critically u p o n the a s s u m p t i o n that the
Chugach has not been deformed or bent
into its present shape since its origin. Gromme
thinks he can show that the Chugach, despite
its bowed shape, has not been so shaped by
tectonic processes subsequent to its formation.
Building Alaska
Until only recently, earth scientists almost
despaired of finding a coherent and internally
consistent tectonic explanation for what has
been going on in Alaska, in part because of
the melange of different segments forming
its southern margin. N o w the clouds seem
to be lifting. Michael C h u r k i n and James
Trexler, also of the Geological S u r v e y , have
proposed a four-step process for the evolution
of Alaska since the early Jurassic 175 million
years ago.
In their model, the Brooks Range is an
extension of the original N o r t h American
craton. During the early Jurassic, the protoPacific separated it from Eurasia. Later in
34
MOSAIC March/April 1981
the Jurassic, both the opening of the Atlantic
and circumpolar drift narrowed the oceanic
strait between Alaska a n d the Siberian platform. It was 125 million years ago, according to this model, that a poleward-drifting
Pacific p r o t o p l a t e called Kolyma collided
and wedged itself b e t w e e n Alaska and the
Eurasian plates. T h e Brooks R a n g e a n d
Seward Peninsula began to bend and buckle
as Siberia, Kolyma, a n d Alaska collided.
By the early Tertiary (the Eocene epoch),
some 50 million years ago, the combination
of Kolyma's poleward motion and the pincerlike closing of N o r t h America to Eurasia
sealed the Alaska-Siberia junction. Part of
the Pacific Ocean basin was virtually landlocked.
Within the 50 million years remaining
between the Eocene and the present, however,
circumpolar drift further deformed the land
link between Alaska and Siberia at what
became the Bering Strait. T h r o u g h o u t the
entire process, microplates originating somewhere in the Pacific were grafting themselves
to Alaska so that it g r e w significantly in area
south of the Brooks Range.
Pacifica
Just where did all these fragments of crust
accreting to Alaska and other Pacific margins
originate? In 1977, A m o s Nur and Zvi BenAvraham, an Israeli scientist working with
N u r at S t a n f o r d U n i v e r s i t y , a d v a n c e d a
startling suggestion: T h e y proposed that some
blocks of alien crust plastered to land masses
around the Pacific m a r g i n actually could be
fragments of a hypothetical continent called
Pacifica. It would have been located to the
northwest of Australia in the early Jurassic,
they propose, when Australia was still an
integral unit of G o n d w a n a l a n d . T h e rifting
and seaf loor spreading between segments of
Pacifica would have forced the individual
elements to migrate in different directions;
they would have collided with other continental margins by the late Jurassic or early
Cretaceous.
Nur and B e n - A v r a h a m base their Idea on
the bold notion that all seafloor-spreading
episodes must originate under continents
and not in ocean crust. T h e y argue that the
insulating effects of thick continental blankets
could cause the buildup of heat in the earth's
mantle and the creation of a magma sufficiently hot to break through the thick crust,
forming a triple junction along which volcanic
rifting would occur. Such a spreading episode
would be similar to that operating today in
the Red Sea, Gulf of A q a b a , and the African
rift valleys.
The paleogeographic community has been
cool to the Pacifica hypothesis for a variety
of reasons. O n e of the greatest difficulties is
that most of the itinerant blocks can be seen
MOSAIC March/April 1981
35
as segments of volcanic island arcs or of
other oceanic features, i n c l u d i n g oceanic
plateaus, rather than as continental residue.
Stoppers in the trench
More recently, Nur and Ben-Avraham have
proposed that the large number of submerged,
thick-crusted plateaus, rises, and ridges in
the Pacific (31 major ones covering about 10
p e r c e n t of the Pacific floor) are m o d e r n
migrant terranes. " T h e y are moving with
the ocean plates in which they are embedded,
fated eventually to join continents adjacent
to the subduction zones that ring the Pacific,"
they propose. F u r t h e r m o r e , they offer, the
oceanic plateaus and the itinerant blocks on
land " m a y provide one of the major missing
links in geodynamics—the link between plate
tectonics in oceans a n d accretion tectonics
on the continents."
Footprints of ancient collisions
A c c o r d i n g to the tectonic model of
crustal processes, when the sea closes
between a pair of plates carrying lighter
continental crust,, the coastal shelf and
slope sediments commence to pile up,
overthrust, fold, and uplift into metamorphosed highlands. W h e r e these episodes of uplift occur simultaneously over a
large area, they can be identified as a
tectonic orogeny m a r k i n g the narrowing
of an ocean basin between two continental
land masses, and ultimately the actual
collision.
T h e identification of these extensive
lineaments contributes significantly to an
u n d e r s t a n d i n g of plate movements and
collision in the remote past. Several major
orogenies have been identified in eastern
North America and Europe. They mark
key tectonic events over the past 500
million years.
Tacotiic: A mountain-building event
in the late Ordovician (440 million years
ago) that piled u p the area south of the
Catskills of N e w York and the Green
Mountains of Vermont. T h r u s t faulting
along this province extends from Pennsylvania to the St. Lawrence Valley and the
west coast of N e w f o u n d l a n d . It is conjectured that this faulting occurred when
a s e p a r a t e c o n t i n e n t a l b l o c k called
Armorica became s u t u r e d to the North
American plate sometime before the late
Devonian epoch, some 350 million years
ago. A major part of this block, carrying
what is now Ireland, southern England,
France, and the Iberian Peninsula, broke
away, when the Atlantic Ocean opened,
to become a part of the European plate.
Acadian and Caledonian: A mountainbuilding episode in eastern North America
and northern Europe, which commenced
with the narrowing of the ocean basin
between those two blocks and culminated
400 million years ago, in the Silurian,
with their welding together. The part called
Acadian, in eastern N o r t h America, is
outboard of the Taconic, extending from
the M e r r i m a c area of M a s s a c h u s e t t s
northward through Maine into the central
volcanic belt of N e w f o u n d l a n d . The part
called Caledonian traces the same southw e s t - n o r t h e a s t t r e n d , e x t e n d i n g from
Wales through Scotland and Scandinavia
to the Arctic island of Spitsbergen.
Hercynian: A vast uplift in the late
C a r b o n i f e r o u s , 300 million years ago,
which marks the collision of Gondwana
and North America and northern Europe
(then combined into a single unit, Laurussia). In N o r t h America, the lineament
extends from the Wichita-Ozark range
through the s o u t h e r n Appalachians and
the Alleghenies. In Europe, the Hercynian
extends from Ireland and southern England through central France and Germany
to southern Poland, the most prominent
remnants being the Massif Central of
France, the H a r z , Vosges, and Bohemian
Mountains, and the Black Forest region.
In Africa the ancient suture is represented
in the uplifted fold belts of Mauretania. •
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MOSAIC March/April 1981
They suggest that when such ridges arrive
at s u b d u c t i o n zones, the buoyant, lighter
rocks of the plateaus, ridges, and rises refuse to
s u b d u c t with the heavier underlying crust.
T h e trench becomes gorged with the lighter
material and disappears. This disrupts the
downgoing slab, and seismicity and volcanic
activity cease. T h i s actually seems to be
happening today, they note, in the high-stress
tectonic e n v i r o n m e n t of the eastern Pacific
which accounts for more than half of the
energy released by all the world's earthquakes.
There the direct collision of the Juan Fernandez and Nazca ridges with continental
crust is creating prominent gaps in the chain
of volcanism and deep seismicity. The cessation of seismicity and volcanism is only
temporary, however; after the lighter ridge
crust of the old trench becomes part of the
continent, a new trench will form just seaward
of the accreted material and normal oceanic
s u b d u c t i o n will resume.
In low-stress tectonic environments, like
those of the northern and western Pacific,
N u r and Ben-Avraham suggest, the ridge
material is not forced into the continental
crust and embedded there. In such regions,
after an interval in w h i c h the trench is
plugged, the subduction zone jumps, sometimes great distances, to the seaward side of
the plateau, and normal oceanic subduction
can resume. Unlike the high-stress style of
MOSAIC March/April 1981
37
collision, where only slight adjustments occur
in plate margins, this process results in major
changes in plate boundaries and may have
been responsible for the construction of the
Aleutians and the formation of the marginal
sea behind them..
The two scientists at Stanford, together
with D a v i d J o n e s of the U n i t e d States
Geological S u r v e y , suggest f u r t h e r that
drifting terranes may play a very large role
in the deformation of mountain-producing
belts. " W e suggest the possibility that all
orogenic belts, even those classified as
subduction tpe, may in fact be the result of
collisions not with major continents but with
oceanic plateaus whose origins include extinct
arcs, s u b m e r g e d c o n t i n e n t a l f r a g m e n t s ,
clusters of sea mounts, and hot-spot traces,"
they say. A d d s B e n - A v r a h a m : " W e are
challenging the idea that oceanic subduction
can be responsible for mountain buildjng
and are arguing that only collisions can do
that, including collisions with these s u b merged oceanic plateaus and rises."
Things to come
It is a p p a r e n t that the tectonic processes
that have s h a p e d the face of the earth
t h r o u g h o u t its time are continuing today
and that the earth's geography will change
radically in coming tens of millions of years.
Southern California and Baja California will
tear loose from the mainland and be carried
n o r t h w a r d to form a new mountain range in
the Aleutians, similar to the ranges in British
Columbia and southern Alaska. Additionally,
" t h e Mediterranean is closing, the Alps are
n o w being overthrust, and you see back-arc
spreading in Corsica and Sardinia," notes
Neil O p d y k e . " T h e whole region has small
s u b d u c t i o n zones, quakes, volcanoes, and
y o u n g mountains. You could see one hell of
a collision there in the next few tens of
millions of years."
Fred Ziegler believes that because of its
great age and its inordinate thickness for
oceanic crust, the North Atlantic will develop a
s u b d u c t i o n zone and throw u p an Andesstyle m o u n t a i n chain along the eastern coast
of N o r t h America in the next 50 million
years. Moreover, "If you project the present
tectonic patterns far enough into the future,
say 200 million years, you find that you
eventually coalesce the different continental
blocks into a single super-continent," o b serves O p d y k e . " T h a t ' s what happens
when you disperse objects on a sphere; sooner
or later, they have got to come together
again, even if they all move in different
directions."
T h u s the awes.ome, r a n d o m dance of the
continents and the ocean basins will continue
far into the future. Ultimately, however, it
will come to an end and the planet's crust
will finally be frozen in place. This will
h a p p e n when the cooling earth deprives the
tectonic process of the heat energy required
for m a n t l e c o n v e c t i o n a n d plate motion.
(See " W h a t D r i v e s the E a r t h ' s P l a t e s ? "
Mosaic, Volume 10, N u m b e r 5.)
" A s the continental shields continue to
thicken and to develop substantial viscous
anchors, one can expect the motion of the
plates eventually to cease, bringing to an
end the plate-tectonic phase of the earth's
e v o l u t i o n , " H e n r y Pollack and David
C h a p m a n of the University of Michigan
wrote in a recent article in Scientific American.
" T h u s , for the diminishing band of earth
scientists who still adhere to a nonmobile
view of the earth, there may be some small
solace in the fact that the earth will eventually
conform to their concept of it. They must be
patient, however, since that time is probably
some two billion years hence." •
The National Science Foundation contributes
to the support of work discussed in this
article principally through its Geology and
Geophysics
Programs.