Earth before Pangaea [Preview]

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

Crystalline enigma
MY SUGGESTION provides a possible explanation for a long-standing enigma of Andean geology. Along the otherwise youthful and active Peruvian margin are found 1.9-billion-year-old crystalline rocks. Hardolph A. Wasteneys, then at the Royal Ontario Museum, dated zircon crystals from the Arequipa massif, along the coast of southern Peru. He demonstrated that these rocks were highly metamorphosed when North Americas Grenville Mountains were formed, 1.3 billion to 0.9 billion years ago. They may therefore represent a continuation of the Grenville province of eastern and southern North America into South America.

The hypothesis of a South American connection for the eastern margin of Laurentia unexpectedly brought my career full circle. I grew up in Scotland and cut my geologic teeth on its rocks. Northwestern Scotland and the submerged Rockall Plateau--off the western margin of the British Isles--remained part of North America until the North Atlantic Ocean basin had almost finished opening. Scotland was at the apex of the Labrador-Greenland promontory. When nestled (electronically) in the Arica embayment, the rocks of the Scottish Highlands that I studied for my doctoral degree in the 1960s appear to continue into equally old rocks of Peru and Bolivia. Given how well studied the Scottish Highlands are, they may provide critical tests for a former North America-South America connection.

Assuming the SWEAT hypothesis and the Pan-American connection, we can try to reconstruct the global distribution of continents and oceans in the late Precambrian. Most geologists believe that the relative areas occupied by continents and ocean basins have not changed since the late Precambrian. If, therefore, Antarctica, Australia, North America and fragments of South America were fused into a pre-Pangaean supercontinent, now named Rodinia, then there had to have been vast oceans elsewhere. Ophiolitic relics caught up within the continents indicate that these oceans lay between India and today's East Africa (the Mozambique Ocean) and within Africa and South America (the Pan-African and Braziliano oceans, respectively).

Between roughly 750 million and 550 million years ago these ocean basins were destroyed, and all the Precambrian nuclei of Africa, Australia, Antarctica, South America and India amalgamated into the supercontinent of Gondwana. It was during this time interval that the Pacific Ocean basin opened between Laurentia and the East Antarctic-Australian landmass. Isotopic dating of volcanic rocks in Newfoundland shows that the ocean basin between Laurentia and South America did not open until the beginning of the Cambrian. North America may therefore have separated out in a two-stage process.

Reconstructing the travels of North America requires an essential piece of information: the magnetization of ancient rocks. Such data allow geologists to figure out the latitude and orientation of the rocks when they formed. But because Earth's magnetic field is axially symmetrical, paleomagnetic measurements cannot tell us about the original longitude of the rocks. Present-day lava from Iceland and Hawaii, for example, could reveal to a geologist 100 million years from now the latitudes and the orientation of these islands but not their vast difference in longitude. It would not be apparent that the islands are in different oceans.

Traditional reconstructions of Laurentia always place its Appalachian margin opposite northwestern Africa during the Paleozoic era. I decided to plot the relation of North America to Gondwana differently, taking advantage of the fact that the longitude of the continent is not constrained by paleomagnetic data. It turned out that North America could have made what one of my graduate students referred to as an "end run" around South America during the Paleozoic, starting from next to Antarctica.

When Luis H. Dalla Salda, Carlos A. Cingolani and Ricardo Varela of the University of La Plata in Argentina saw the sketch of the end run, they became excited. They had recently proposed that a Paleozoic mountain belt, whose roots are exposed in the Andes of northern Argentina, could have formed when another continent collided with Gondwana. Moreover, the western margin of this Famatinian belt includes Cambrian and Lower Ordovician limestones (between 545 million and 490 million years old) containing trilobites characteristic of North America. Perhaps, they reasoned, this is a "geologic calling card" left behind when North America collided with South America during the Ordovician period, 450 million years ago.

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