Earth before Pangaea [Preview]

The North American continent may be more nomadic than most of its inhabitants

It appears that after rifting from South America at the end of the Precambrian, North America moved quite far away. During the Cambrian period, when Gondwana was undergoing glaciation, North America was equatorial. Ocean floor was then subducted beneath the South American craton, and North and South America collided again during the Ordovician. We think that the older part of the Appalachian Mountains, which terminates abruptly in Georgia, was once continuous with Argentina's Famatinian belt. This construction places Washington, D.C., close to Lima, Peru, during mid-Ordovician times.

End of the Run
AFTER THE ORDOVICIAN collision, the continents separated again, apparently leaving North American limestone with its characteristic trilobites in northwestern Argentina. My Argentine colleagues and I have suggested that these rocks tore off the ancestral Gulf of Mexico, known as the Ouachita embayment. Blocks carried up by Andean volcanoes from below the limestones have recently been dated at around one billion years old, just like those of the Grenville province that probably occupied the embayment.

It is possible that the North and South American continents interacted again before North America finally collided with northwestern Africa to complete Pangaea. French geologists studying the Paleozoic sedimentary rocks of the Peruvian Andes have found that they are made of debris that must have eroded from a neighboring landmass. They assumed this continent, occupying the area now covered by the Pacific Ocean, to have been an extension of the Arequipa massif in Peru.

It may, however, have been North America. As Heinrich Bahlburg of the University of Heidelberg in Germany has pointed out, ancient warm-water North American fauna mingle with cold-water fauna of southern Africa and the Falkland (Malvinas) Islands in the 400-million-year-old (Devonian) strata of northwestern South America. Together with a deformation along the eastern seaboard of North America known as the Acadian orogeny, and the truncation of mountain structures along the South American margin, the evidence points to Laurentias sideswiping northwestern South America during the Devonian. There are even Ordovician limestones with South American trilobites--another calling card--at Oaxaca in Mexico. Only after North America finally moved away from the proto-Andean margin did the Andean Cordillera of the present day begin to develop.

Some 150 million years later North America returned to collide with northern Europe, Asia and Gondwana. Pangaea--with the Urals, the Armorican Mountains in Belgium and northern France, the Ouachitas and the youngest Appalachians as sutures--arose from the collisions of these continents. After a 500-million-year odyssey, North America had finally found a resting place. But not for long. In another 75 million years it separated from Africa as Pangaea broke up, to move toward its current position.

During the southern summer of 19931994--six years after my first glimpse of the Pensacola Mountains and glimmerings of North America''s odyssey--I returned to Antarctica. This time, with my colleague Mark A. Helper, two graduate students and two mountaineers, I explored the Shackleton Range and Coats Land near the Weddell Sea. According to my computer simulations, this is where North Americas Grenville rocks had projected 750 million years ago. Antarctic geologists have long regarded these areas as anomalous.

At the end of our visit to Coats Land, we roped together, picked up our ice axes and climbed back to another small aircraft. Weighing down our packs--and the aircraft, which groaned into the air--were the rock samples we had gleaned that day. In the laboratories of my colleagues Wulf A. Gose and James N. Connelly, we sat down to analyze those rocks.

Persuasive evidence
OUR IDEAS about how Earth looked before Pangaea, first described in this magazine in 1995, have stimulated a great deal of activity within the geologic community. They offered the first testable hypothesis regarding global geography in late Precambrian and early Paleozoic times--the critical era when single-celled organisms evolved into soft-bodied multicellular creatures, then invertebrates with hard shells and, ultimately, primitive vertebrates.

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