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Atlantis Massif: Expedition studies structure in Atlantic Ocean

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Alcibiades
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« on: June 30, 2012, 11:33:41 pm »

Atlantis Massif: Expedition studies structure in Atlantic Ocean
March 21, 2012 By Bernd F. Laeschke Leave a Comment




La Jolla (global-adventures.us): Rising about 14,000 feet (4,267 meter) from the sea floor in the North Atlantic Ocean, the Atlantis Massif is a dome-shaped undersea structure approximately 10 miles (16.1 kilometers) across. It formed approximately 1.5-2.0 million years ago in a very different way than the majority of the seafloor in the oceans. Unlike volcanic seamounts, which are made of the basalt that's typical of most of the seafloor, Atlantis Massif includes rock types that are usually only found much deeper in the ocean crust, such as gabbro and peridotite.

An expedition aboard the research vessel Joides Resolution, known as Integrated Ocean Drilling Program (IODP) Expedition 340T, marks the first time the geophysical properties of gabbroic rocks have successfully been measured directly in place, rather than via remote techniques such as seismic surveying. With these measurements in hand, Atlantis Massifscientists can now infer how these hard-to-reach rocks will "look" on future seismic surveys, making it easier to map out geophysical structures beneath the seafloor.

"This is exciting because it means that we may be able to use seismic survey data to infer the pattern of seawater circulation within the deeper crust," says Donna Blackman of the Scripps Institution of Oceanography in La Jolla, co-chief scientist for Expedition 340T. "This would be a key step for quantifying rates and volumes of chemical, possibly biological, exchange between the oceans and the crust."

The Atlantis Massif sits on the flank of an oceanic spreading center that runs down the middle of the Atlantic Ocean. As the tectonic plates separate, new crust is formed at the spreading center and a combination of stretching, faulting and the intrusion of magma from below shape the new seafloor. Periods of reduced magma supplied from the underlying mantle result in the development of long-lived, large faults. Deep portions of the crust shift upward along these faults and may be exposed at the seafloor. This process results in the formation of an oceanic core complex, or OCC, and is similar to the processes that formed the Basin and Range province of the Southwest United States.

"Recent discoveries from scientific ocean drilling have underlined that the process of creating new oceanic crust at seafloor spreading centers is complex," says Jamie Allan, IODP program director at the U.S. National Science Foundation (NSF). "This work significantly adds to our ability to infer ocean crust structure and composition, including predicting how ocean crust has 'aged' in an area," says Allan, "thereby giving us new tools for understanding ocean crust creation from Earth's mantle."

The Atlantis Massif is a classic example of an oceanic core complex. Because it's relatively young, it's an ideal place to study how the interplay between faulting, magmatism and seawater circulation influences the evolution of an OCC within the crust.
"Vast ocean basins cover most of the Earth, yet their crust is formed in a narrow zone," says Blackman. "We're studying that source zone to understand how rifting and magmatism work together to form a new plate."

The research vessel Joides Resolution first visited Atlantis Massif about seven years ago; the science team on that expedition measured properties in gabbro. But they focused on a shallower section, where pervasive seawater circulation had weathered the rock and changed its physical properties.

For the current expedition, the team did not drill new holes. Rather, they lowered instruments into a deep existing hole drilled on a previous expedition, and made measurements from inside the hole. The new measurements at depths between 2,600 to 4,600 feet (800 to 1,400 meters) below the seafloor include only a few narrow zones that had been altered by seawater circulation and/or by fault slip deformation. The rest of the measurements focused on gabbroic rocks that have remained unaltered thus far. The properties measured in the narrow zones of altered rock differ from the background properties measured in the unaltered gabbroic rocks.

The team found small differences in temperature next to two sub-seafloor faults, which suggests a slow percolation of seawater within those zones. There were also significant differences in the speed at which seismic waves travel through the altered vs. unaltered zones.

"The expedition was a great opportunity to ground-truth our recent seismic analysis," says Alistair Harding, also from the Scripps Institution of Oceanography and a co-chief scientist for Expedition 340T. "It also provides vital baseline data for further seismic work aimed at understanding the formation and alteration of the massif."

The image shows the fault lines bordering the Atlantis Massif. Map: NOAA/Public Domain

http://www.global-adventures.us/2012/03/21/atlantis-massif-expedition/
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