Rice researchers: 3-D seismic survey will give best-ever view of ancient North Atlantic rift
An international team of scientists and technicians led by Rice University will spend 45 days in the North Atlantic this summer to gather the most detailed information ever about the geology of the ocean basin that formed at what was once the center of Pangaea.
Geologists Dale Sawyer and Julia Morgan of Rice and Donna Shillington of Columbia University are leading the $6 million international project to study the Galicia rift northwest of the Spanish coast where, unusually, sediment has not deeply buried formations that have existed at the bottom of the ocean for millions of years.
A National Science Foundation (NSF) grant to Rice of more than $1.2 million will put five faculty members and graduate students on the 50-plus crew aboard the Seismic Vessel Marcus G. Langseth, owned by the NSF and operated by the Lamont-Doherty Earth Observatory.
The Langseth stopped in Galveston last month, where Sawyer took stock of its tools. He and Morgan have been waiting for their ship to come in since proposing the project eight years ago. “We had to wait for other seismic studies, creating a critical mass of work in the Atlantic in order to bring the ship from the Pacific Ocean to the Atlantic,” Sawyer said.
For 45 days the ship will trace a spiraling path — Sawyer compared it to mowing a lawn — over the rectangular 64-by-22.4 kilometer target. Throughout the journey, the 15 scientists and their technicians and students will analyze data and make critical decisions on optimizing data quality.
Sawyer will represent Rice on the ship, joined by graduate students Sarah Dean, Brian Jordan and Mari Tesi Sanjurjo and adjunct faculty lecturer Steve Danbom. All will begin analyzing the massive amount of data the ship will collect while on board, but the results will take years for geologists to fully process and understand, Sawyer said.
“Between 225 and 110 million years ago, the Atlantic Ocean opened up between Africa and North America, and the breakup propagated northward, opening a new ocean and pulling Spain and Portugal apart from what is now Newfoundland,” Sawyer said. “What makes this unusual is that it is a volcanic-starved rift.”
Elsewhere on the planet, lava flowed upward into rifts and created oceanic crust. “We found in the last 20 years that most margins are volcanic-dominated,” he said. “They actually pull apart, lots of magma comes up and the sea-floor spreading process begins immediately. Galicia is at the other end of the spectrum. Volcanic-dominated margins are thought to be caused by unusually high heat in the Earth’s mantle, while magma-starved margins are caused by cooler mantle rocks.
But the volcanic crust hasn’t reached the Galicia, where sections of the Earth’s mantle in the form of peridotite lie just under a thin layer of sediment. At this rift, the crust is neither oceanic nor continental, but of a different type. These formations may tell geologists a great deal about the rifts that appeared when the great continent split and began evolving into the map we know today.
“The sediments are thin,” Sawyer said, “so we can do seismic characterization and potentially drill into these rocks without having to go through 10 or 15 kilometers of sediment.
“One of our objectives with the 3-D survey is to find the best places to drill,” he said. “Now we can see the fans of sediment deposited on the broken and tilted continental crust blocks, but we don’t know when they broke and how quickly.” The images should allow them to gather core samples at the right places. “Then we have paleontological evidence we can date, and then we can start to know.”
To learn all this, the team will make 3-D images of the upper 20 kilometers of the rocks under the ocean. The RV Langseth will tow four cables, each 6 kilometers long, carrying nearly 2,000 hydrophones. The towed cables cover a width of 600 meters as they rake through the water. Constantly on the move, the ship fires an array of compressed air guns towed behind the ship. Airgun shots will be fired every 37 meters (once about every 16 seconds), and the seismometers will sense the reflections that come back from the seafloor and from rock layers. Using sound to see what eyes can’t, the signals are translated into an accurate three-dimensional image of the geological terrain below.
Before the Langseth sails from Vigo, Spain, on June 1, the German Research Vessel Poseidon and scientists from Germany and the United Kingdom will drop about 80 ocean-bottom seismometers in a grid on the same patch of ocean floor that the Rice-led team will survey.
“The ocean-bottom seismometers give us much better information about the speed of sound through the rocks, and that tells us a lot about what kind of rock the seismic wave is traveling through,” Sawyer said.
Several oil companies are interested in this work, Sawyer said. Although the sediments to be studied are thin and unlikely to yield oil or gas, other places in the world with similar magma-starved rifting and thick overlying sediments are virtually certain to contain hydrocarbons. Images taken through the thin Galicia sediments will provide information about what to expect in hydrocarbon-bearing areas elsewhere, he said.