The East Coast of the United States, a passive margin formed during the continental breakup of Pangea, holds vital clues to understanding the dynamic processes that shaped our planet. While passive margins are traditionally considered devoid of active faulting and magmatism, a groundbreaking study is revealing the hidden complexities beneath the surfaces.
Researchers from The University of New Mexico recently published a paper in the Journal of Geophysical Research: Solid Earth titled "Discontinuous Igneous Addition Along the Eastern North American Margin beneath the East Coast Magnetic Anomaly."
Pangea was the most recent supercontinent. Tectonic plates move on the Earth's surface and collide over very long cycles, forming what scientists call a "supercontinent." Over time, these supercontinents start to break apart, forming a continental rift. Pangea is now separated, but over 200 million years ago, the U.S. East Coast began forming as part of the rift.
Scientists Lindsay Worthington, professor of Earth & Planetary Sciences, and Collin Brandl, Ph.D. student, participated in a large community experiment on the east coast of the United States to answer the question, "How do supercontinents break apart?" The experiment was called the ENAM Community Seismic Experiment (ENAM stands for Eastern North American Margin, the geologic term for the east coast of the U.S.). The experiment was centered on Cape Hatteras, N.C., and contained both onshore and offshore seismometers, allowing the science to cross the shoreline.
Being a community experiment, the data was open and accessible to any scientist interested. Here at UNM, Worthington and Brandl worked on a subset of the data that follows the margin from south to north, "along-strike." The data from this orientation is a bit different.
"It is a little bit different when people tend to study these passive margins. They've analyzed data perpendicular to the coastline," Brandl said. "Having this data that goes along-strike instead gives you a different view of how things are happening. From the start, we wanted to see what the margin looks like in this direction, which hasn't been done. I would say a significant majority of this study has been done in the other direction."
The researchers utilized ocean-bottom seismometers, sensors that are deployed from a ship and sink through the water to the ocean floor. These instruments measure ground movement, specifically energy that travels through the earth and shakes the instrument, which then records that motion. The data from these ocean-bottom seismometers was used to uncover a variable distribution of magma-derived rock buried beneath the seafloor.
"We have instruments called ocean bottom seismometers. They're small-refrigerator-sized instruments that get deployed onto the seafloor with a lot of other equipment so that they can safely make it down to the bottom of the ocean and back to the surface," said Brandl.
"Our first goal is to figure out the sound velocity of the rocks beneath the seafloor, which depends on the specific type of rock present. Sound energy travels slower through some and faster through others," Brandl said. "With some context from other experiments, we can use the sound velocity to figure out which rocks are present beneath the seafloor. On a basic level, we are doing something similar to the classic question, "If I drove 20 miles and it took an hour, how fast did I go?" We know where all sound sources were set off and where the instrument was, so we know the distance. The seismometer has a clock on it, so we know the timing of how long it took for that energy to go from wherever it was sent to the instrument."
Brandl states that with those two elements and some specialized codes, they were able to determine the velocity, which gave them insight into the kind of rocks they were dealing with.
"Once we know the sound velocities, we can look at variations along the margin. We were interested in understanding the volcanism and magmatism occurring while Pangea was breaking apart," Brandl said.
This discovery sheds light on the extensive volcanism and magmatism during Pangea's breakup and poses intriguing questions about the continental rift's influence on the Mid-Atlantic Ridge's structure.
"This region started as a supercontinent, and then it started to rift apart. And that rift was similar to what's happening in East Africa. The East African rift is a big system with all these unique segments. Some places have a lot of volcanoes, and some places have none," said Brandl.
"What we found the most significant was that there's a lot more variability in the magmatism and volcanism than people used to think. We found that there are these pockets where there's a lot of magmatism and pockets where there was very little magmatism," Brandl.
They ultimately were able to link some other recent studies of this area to what's being observed today at the Mid-Atlantic Ridge.
"Mid-ocean ridges are these really long chains of volcanoes in the middle of the ocean, and that's where the new ocean crust is made. It's almost like a factory where you have magmatism and volcanism that make new oceanic crust. If you look closely at a map of the Mid-Atlantic Ridge, you can see that it's broken into segments, and each segment is where new crust is formed," said Brandl.
Their findings, highlighting gaps up to 30 km wide in magma-derived rock thickness and extent, open new avenues for comprehending continental rifting. The team at UNM decided to follow specific segments of the Mid-Atlantic ridge due west, and the boundaries of these segments aligned with the gaps in magmatism along the east coast.
"We think that we're seeing how structures in the Pangean rift influenced what's happening today at the mid-Atlantic Ridge," said Brandl.
Brandl states that while passive margins aren't known for active faults or volcanism, they are helpful places to understand these things. These concepts are even relevant right here in New Mexico.
"For New Mexico, understanding something like this could be helpful. We sit within the Rio Grande Rift which is technically still active. The inactive volcanoes on the West Mesa and the Sandias were uplifted by a rift border fault. In Socorro, there's a magma body beneath the surface. The rift here, while not successful in the sense that it didn't break apart the continent, is similar to what you might observe in East Africa or Pangea," explained Brandl. "It's the same type of boundary where things are pulling apart, and there is faulting, volcanoes, and magmatism."
This publication was chosen as an Editor's highlight for the journal and was featured on EOS.org.
