The first study to spring from a Rice University-led 2013 international expedition to map the sea floor off the coast of Spain has revealed details about the evolution of the fault that separates the continental and oceanic plates.

A paper in Earth and Planetary Science Letters describes the internal structure of a large three-dimensional section of the Galicia, a non-volcanic passive margin between Europe and the Atlantic basin.

The Galicia shows no signs of past volcanic activity and has a remarkably thin crust. That thinness was the key for researchers to capture 3D data of around 525 square miles of the Galicia, the first transition zone in the world to be analyzed using this method.

Creating 3D Maps of Oceanic Faultline

Sophisticated seismic reflection tools were towed behind a ship and on the ocean floor, enabling the researchers to model the Galicia. The rift is buried under several hundreds of meters of powdered rock and invisible to optical instruments. Seismic tools, however, fire sound into the formation. The sounds that bounce back tell researchers what kind of rock lies underneath and how it’s configured.

Among the data are the first seismic images of what geologists call the 'S-reflector,' a prominent detachment fault within the continent-ocean transition zone. They believe this fault accommodated slipping along the zone in a way that helped keep the crust thin.

“The S-reflector, which has been studied since the ’70s, is a very low-angle, normal fault, which means the slip happens due to extension,” said Rice graduate student Nur Schuba, co-author of the study. “What’s interesting is that because it’s at a low angle, it shouldn’t be able to slip. But it did."

Rice University alumnus Brian Jordan, co-author of a new study on the Galicia margin based on an extensive seismic survey led by Rice, points out crustal faults that connect to the margin’s S-reflector. Photo by Gary Linkevich

“One mechanism people have postulated is called the rolling hinge,” she said. “The assumption is that an initially steep fault slipped over millions of years. Because the continental crust there is so thin, the material underneath it is hot and 'domed up' in the middle. The initially steep fault started rolling and became almost horizontal.

“So, with the help of the doming of the material coming from below and also the continuous slip, that’s how it is likely to have happened,” Schuba said.

Understanding Fault Evolution

The extensive dataset also provided clues about interactions between the detachment fault and the serpentinized mantle, the dome of softer rock that presses upward on the fault and lowers friction during slippage. The researchers believe that this interaction led the Galicia to evolve differently, weakening faults and allowing for a longer duration of activity.

The research is relevant to geologists who study land as well as sea because detachment faults are common above the water, Schuba said. “One of my advisers, (adjunct faculty member) Gary Gray, is jazzed about this because he says you can see these faults in Death Valley and Northern California, but you can’t ever see them fully because the faults keep going underground. You can’t see how deep they go or how the fault zones change or how they’re associated with other faults.

“But a 3D dataset is like having an MRI,” she said. “We can bisect it any way we want. It makes me happy that this was the first paper to come out of the Galicia data and the fact that we can see things no one else could see before.”

The National Science Foundation, the U.K. Natural Environment Research Council and the GEOMAR Helmholtz Center for Ocean Research supported the research.