New geosciences model explains ocean formation

http://www.physorg.com/news11864.html

Scientists at The University of Texas at Austin's Jackson School of
Geosciences and the Université Louis Pasteur in Strasbourg, France, have
developed a new model to explain how continents break apart to form new
oceans. Their discovery may improve targeting of deep-water oil and gas
reserves. A description of the model appears in the March 16 edition of
Nature.

Luc Lavier, a research associate at Jackson School's Institute for
Geophysics, and Gianreto Manatschal, a professor of tectonics at the
Université Louis Pasteur, set out to understand inconsistencies in the way
that tectonic models account for the geology of deep-water ocean basins.

In the process, they developed a new model that expands geologic theory.
The energy industry can also use the model to locate hydrocarbons in deep
water.

Deep-water oil and gas reserves are notoriously difficult to pinpoint. The
new model could make it more affordable to explore for potential reserves
in the Gulf of Mexico, coastal West Africa and other deep-water regions.

The model derived from experiments and geological reconstructions of the
evolution of the ancient Alpine and present-day North Atlantic margins.

"In recent years, academic and petroleum industry studies of continental
margins have shown that their evolution is more complex than that predicted
by earlier models," said Lavier. Energy companies have been especially
interested in learning more about the evolution of continental margins, he
said, because "the old models were not working."

The two prevailing models explaining continental break-up are known as pure
shear and simple shear. Both seek to explain how the Earth's outer
layers-the upper crust and underlying mantle, together known as the
lithosphere-stretch and deform in a process that breaks apart continents.

The pure and simple shear models explain features observed near the coasts
of continents but fail to explain the geology of the deep oceans. In
particular, pure and simple shear models do not do adequately predict a
process called mantle melting, which forms the volcanic seafloor in deep
oceans.

Lavier and Manatschal propose a new model that can explain both mantle
melting in deep waters and phenomena observed at the edge of continental
margins. In their model, a single fault in the Earth's crust and a single
fault in the mantle work in tandem to thin the Earth's top two layers and
uplift the mantle. These faults behave like a conveyor belt pulling up
lower crust and mantle.

Geologists have long observed the products of this process in nature.
Remnants of ancient oceans that have been discovered at high altitudes in
mountain ranges like the Alps and the Rockies narrate the story of the
break up of continents and the formation of new oceans.

"Our model represents a shift in the way we understand the evolution of
deformation and Earth's material properties during extension of the
continental lithosphere," said Lavier.

The new model also revises the concept of how heat is distributed in
offshore sedimentary basins that contain the world's major deep-water
reserves of oil and gas. Geologists use the temperature history of
sediments to predict where oil and gas is located. Refining the ability to
assess temperature history "is critical in areas such as the south Atlantic
where oil exploration is taking place in deeper waters in the search for
future resources," said Lavier.

The researchers' next steps are to go back to the field and look for
geological features that the model predicts. Lavier is applying the model
to other geological settings like the San Andreas Fault in California.

Supercomputers at the Jackson School's Institute for Geophysics helped run
the model, which benefited from advances over the last decade in computing
and the application of algorithms for material deformation to geology.

The research was supported in part by ExxonMobil Upstream Research Company
and Groupe de Recherche des Marges, a French academic consortium
co-sponsored by Total Oil Company.


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