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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. • 36 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.