Chemical clues reveal ancient geography

by Erin Wayman
Thursday, January 5, 2012

Reconstructing the history of supercontinents requires careful detective work. A variety of geological processes wipes the evidence clean, like a burglar who smears away his fingerprints. Yet even the most cautious criminals leave clues behind — and so do supercontinents.

For geological sleuths mapping Earth’s ancient geography, locating the trailing edge where a continent broke free from a supercontinent is relatively easy; it’s identifying the drifting continent’s leading edge that is more difficult. But now researchers say they have found a way to help geologists retrace the steps of these elusive parts of former supercontinents.

Earth has witnessed several supercontinent cycles over its 4.6-billion-year history. Most recently, the planet’s continents came together to form Pangaea about 300 million years ago. Then, about 100 million years later, the continents began to break up and drift away from each other. As the continents pulled apart, large basins opened in their wake and became new oceans. Today, where the trailing edges of these continents descend into the ocean, they have formed broad, shallow continental margins, such as along the eastern coast of North America.

Finding remnants of these trailing edges in the geologic record helps researchers learn more about the geographic positions of former supercontinents, says Brendan Murphy, a geologist at St. Francis Xavier University in Nova Scotia, Canada, and lead author of the new study published in Geology. Identifying the leading edges of continents, however, is difficult because these edges are more tectonically active than trailing edges, Murphy says. Leading edges are constantly reshaped by subduction, which fuels volcanism and mountain building.

But Murphy and his colleagues have found a way to see past all of that geological reworking. The key is to follow the paths of slabs of lithosphere, called terranes, that pile up on the leading edges of moving continents.

Drifting continents are like bulldozers. They push up and collect bits of built-up ocean lithosphere, such as islands, which then merge with the continent and become a new lithospheric patch on the continent’s leading edge. Later, some of those patches, along with the continent’s original lithosphere, become a source of magma for volcanoes in the area’s subduction zone.

Ocean lithosphere is younger than continental lithosphere, Murphy says, and therefore has a distinct chemical signature, which is preserved in the magma. Therefore, geologists can tell if magma erupting from a subduction zone’s volcanoes was derived from recycled terranes made of ocean lithosphere or from recycled continental lithosphere, he says.

Geologists identified this distinct signature on a known part of the leading edge of western North America after it separated from Pangaea. To ensure that this technique could be used to locate the remnants of any supercontinent, Murphy and his colleagues tested it on an even older supercontinent: Rodinia.

Rodinia formed about 1.1 billion years ago and started to break up sometime between 800 million and 750 million years ago. Geologists have identified several complexes of igneous rock associated with the northern margin of Gondwana, one of the continents that broke free from Rodinia. Murphy and his colleagues studied three of these complexes found in Mexico, eastern North America and Spain.

After studying the isotopic composition of these rocks, the researchers found the same type of chemical signature that is found at the leading edge of western North America. First, they determined the rock’s lithospheric material was too young to be part of Rodinia. Therefore, the lithosphere must have come from terranes accreted onto Gondwana after Rodinia broke up. In fact, the team suggests the lithosphere formed in the ocean surrounding Rodinia sometime between 1.1 billion and 750 million years ago. And by about 600 million years ago, they say, the mantle beneath those accreted terranes produced the magma that formed the igneous complexes.

That the leading edges of North America and Gondwana share the same kind of chemical signature suggests the technique could be applied to any supercontinent, Murphy says.

By differentiating magmas formed from terranes and magma formed from continental lithosphere, geologists can identify the leading edge of a former supercontinent. The only question now is how far back in time this technique will hold up, he says.

“It’s inherently important to try to understand the history of a tectonic plate,” says John Goodge, a geologist at the University of Minnesota at Duluth. And Murphy’s method helps reveal the tectonic history of these terranes and “adds another element to what a margin looks like.” Yet, he says, having a better understanding of a leading edge’s history won’t necessarily help geologists fully constrain supercontinent geography — it may trace the outlines of the continents, but it won’t help geologists figure out where on the planet these ancient continents used to be.

Murphy acknowledges that supercontinent reconstruction requires a combination of approaches, and says that this tool is not a “silver bullet.” But, he says, adding a tool to the geologist toolbox is never a bad thing.

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