Sub-sea rift spills secrets to seismic probe

Rice-led study yields first clues about the internal structure of Galicia margin.

The Galicia group — from left, Rice graduate student Nur Schuba, alumnus Ara Alexanian and graduate research assistant Mari Tesi Sanjurjo — discuss the northwest portion of the 3-D seismic volume at Rice’s Visualization Lab. Photo by Gary Linkevich
The Galicia group — from left, Rice graduate student Nur Schuba, alumnus Ara Alexanian and graduate research assistant Mari Tesi Sanjurjo — discuss the northwest portion of the 3-D seismic volume at Rice’s Visualization Lab. Photo by Gary Linkevich

A new study by the Rice University reveals insights about the development of the blame that isolates the continental and oceanic plates. This is the first study that describes the internal structure of a large three-dimensional section of the Galicia, a non-volcanic passive margin between Europe and the Atlantic basin that shows no signs of past volcanic activity and where the crust is remarkably thin.

That thinness made it easier to capture 3-D data for about 525 square miles of the Galicia, the first transition zone in the world so analyzed.

A Rice University-led seismic survey of the Galicia margin off the coast of Spain has produced new details about the passive rift that separates the oceanic and continental plates, and in particular the S-reflector, a prominent detachment fault within the transition zone. Illustration by Nur Schuba
A Rice University-led seismic survey of the Galicia margin off the coast of Spain has produced new details about the passive rift that separates the oceanic and continental plates, and in particular the S-reflector, a prominent detachment fault within the transition zone. Illustration by Nur Schuba

Advanced seismic reflection apparatuses towed behind a ship and on the sea depths empowered the scientists to display the Galicia. Despite the fact that the crack is covered under a few several meters of powdered shake and imperceptible to optical instruments, seismic apparatuses fire sound into the arrangement. The sounds that bob back tell scientists what sort of shake lies underneath and how it’s designed.

Among the information are the primary seismic pictures of what geologists call the S-reflector, an unmistakable separation blame inside the mainland sea progress zone. They trust this blame suited slipping along the zone in a way that helped keep the outside layer thin.

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

“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.”

The outcomes also suggest 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 led the Galicia to evolve differently, weakening faults and allowing for a longer duration of activity.

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
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

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 3-D dataset is like having an MRI,” she said. “We can dissect 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 paper is published in Earth and Planetary Science Letters.