Why do the Caribbean Islands arc? Movement of Earth modeled to 3,000 km depth

The results, published August 19 in Nature Geoscience, give us a better understanding of how continents resist the constant movement of Earth's plates -- and what effect the continental plates have on reshaping the surface of Earth.

"Studying the deep earth interior provides insights into how the Earth has evolved into its present form," said Meghan S. Miller, assistant professor of earth sciences in the USC Dornsife College of Letters, Arts and Sciences, and lead author of the paper. "We're interested in plate tectonics, and the southeastern Caribbean is interesting because it's right near a complex plate boundary."

Miller and Thorsten W. Becker, associate professor of earth sciences at USC Dornsife College, studied the margin between the Caribbean plate and the South American plate, ringed by Haiti, the Dominican Republic, Puerto Rico and a crescent of smaller islands including Barbados and St. Lucia.

But just like the First Law of Ecology (and time travel), when it comes to Earth, everything really is connected. So to study the motion of the South American continent and Caribbean plate, the researchers had to first model the entire planet -- 176 models to be exact, so large that they took several weeks to compute even at the USC High Performance Computing Center.

For most of us, earthquakes are something to be dreaded. But for Miller and Becker they are a necessary source of data, providing seismic waves that can be tracked all over the world to provide an image of Earth's deep interior like a CAT scan. The earthquake waves move slower or more quickly depending on the temperature and composition of the rock, and also depending on how the crystals within the rocks align after millions of years of being pushed around in mantle convection.

"If you can, you want to solve the whole system and then zoom in," Becker said. "What's cool about this paper is that we didn't just run one or two models. We ran a lot, and it allowed us to explore different possibilities for how mantle flow might work."

Miller and Becker reconstructed the movement of Earth's mantle to a depth of almost 3,000 kilometers, upending previous hypotheses of the seismic activity beneath the Caribbean Sea and providing an important new look at the unique tectonic interactions that are causing the Caribbean plate to tear away from South America.

In particular, Miller and Becker point to a part of the South American plate -- known as a "cratonic keel" -- that is roughly three times thicker than normal lithosphere and much stronger than typical mantle. The keel deflects and channels mantle flow, and provides an important snapshot of the strength of the continents compared to the rest of Earth's outer layers.

"Oceanic plates are relatively simple, but if we want to understand how the Earth works as a system -- and how faults evolved and where the flow is going over millions of years -- we also have to understand continental plates," Becker said.

In the southeastern Caribbean, the interaction of the subducted plate beneath the Antilles island arc with the stronger continental keel has created the El Pilar-San Sebastian Fault, and the researchers believe a similar series of interactions may have formed the San Andreas Fault.

"We're studying the Caribbean, but our models are run for the entire globe," Miller said. "We can look at similar features in Japan, Southern California and the Mediterranean, anywhere we have instruments to record earthquakes."