Earth before Pangaea [Preview]

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

...and Margins
WITHIN PANGAEA there are some ancient continental margins that have no obvious counterparts. The Pacific margins of North and South America, Antarctica and Australia were all formed near the end of the Precambrian, between 750 million and 550 million years ago. The Appalachian margin of Laurentia--the ancestral shield of North America--also rifted away from another continent at that time. Since Wilson asked his famous question, the counterpart to this margin has usually been assumed to have been western Europe and northwestern Africa. But there is no firm evidence for such a juxtaposition.

In 1989 I led another field trip to Antarctica, as part of the International Geological Congress hosted by the U.S. The object of the expedition was to help bring Antarctic geology--long the private domain of a very small group of especially hardy souls (even among geologists)--into the mainstream of global earth science. Various experts on the Himalayas, the European Alps, the Appalachians, the Rockies and many other regions participated.

Soon after, one of these scientists, Eldridge M. Moores, was browsing in the library of the University of California at Davis when he came across a short article by Richard T. Bell and Charles W. Jefferson of the Geological Survey of Canada. They pointed out similarities between Precambrian strata in western Canada and eastern Australia and concluded that the Pacific margins of Canada and Australia might have been juxtaposed. Sensitized by his recent trip, Moores realized this would imply that the Pacific margins of the U.S. and Antarctica had been juxtaposed, a thought similar to my own. After some quick library research, he sent me a map highlighting the structural parallels in the interiors of the Laurentian and East Antarctic shields. "Is this crazy?" he asked.

Similarities in the internal structures of displaced continents can be powerful evidence of former juxtaposition. Moores drew particular attention to a report citing that along the Transantarctic Mountains--in a place called the Shackleton Range (after the famous British explorer Sir Ernest Shackleton)--lie rocks similar in age and character to those underneath much of New Mexico and Arizona. He also pointed out that roughly billion-year-old rocks like those characterizing the Grenville province--an aged band of rocks running along the eastern and southern margin of North America, from Labrador to Texas--had been found near one Antarctic shore. He called his hypothesis--the idea that the continents had been juxtaposed--SWEAT, for Southwest U.S.East Antarctica.

Fired up by the possibility that my question might finally have an answer, I reproduced Mooress reconstruction using the PLATES software at our institute at the University of Texas at Austin. The program allows us to group together pieces of continents and move them over the globe with geometric precision. A short time later my colleague Lisa M. Gahagan and I had removed any uncertainties about matching the boundaries: the scale and general shape of the two old rifted margins were indeed compatible. Moreover, the boundary between the Grenville rocks of Texas and the older rocks of Arizona and New Mexico projected into Antarctica--just where I knew there was a similar boundary under the ice, between the Shackleton Range and some tiny rock outcrops along the frozen shores of the Weddell Sea. It seemed as if the rocks right under my feet, those that form the Llano uplift in Texas and from which the Texas State Capitol was built, were reappearing electronically in Antarctica!

If the western edge of North America was joined to East Antarctica and Australia, then some other continent must have rifted off the Appalachian margin. Paul F. Hoffman, now at Harvard University, and I have suggested that the eastern side of North America's Laurentian shield was wedged against the Precambrian shields of South America, known as Amazonia and Rio de la Plata. In manipulating the three shields on the computer screen, it occurred to me that the Labrador-Greenland prominence of Laurentia might have originated within the recess in the South American margin between Chile and southern Peru, often referred to as the Arica embayment. Both the promontory and the embayment are believed to date from late Precambrian times. But while they are of the same size and general shape, they were extensively modified when the Appalachian and Andean mountain chains rose. So a precise geometric fit is not to be expected.

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