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NSF PR 96-83 - December 15, 1996
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Researchers Sift Evidence Concerning Plate Boundary
Ancient Plate Boundary Exhumed
A series of papers presented at the annual conference
of the American Geophysical Union in San Francisco
could shed new light on the geological processes by
which continents grow, as well as on the nature of
the structures, known as faults, that lie along plate
boundaries.
For the past two years, a team of National Science
Foundation (NSF)-supported scientists has used seismic
techniques along a network of deep mountain fjords
near the border of British Columbia and southeast
Alaska to study what may be part of an ancient boundary
between two tectonic plates: the Kula plate, which
disappeared under Alaska about 50 million years ago,
and the North American plate.
"Scientists from various earth-science disciplines
have assembled evidence on the nature of the rocks
and rock formations found deep beneath the surface
of the earth," says Leonard Johnson, director of NSF's
continental dynamics program, which funded the research.
"As they present and discuss their findings, they
will be attempting to draw some conclusions about
whether they have, in fact, located the exhumed portion
of a major ancient strike-slip fault."
At the AGU conference, the researchers will assemble
the sometimes conflicting evidence gathered from their
various disciplines, and attempt to decide whether,
in fact, they have found the boundary. If it can be
demonstrated that the zone crossed by the seismic
section represents the deep portions of an ancient
strike-slip fault zone, now exhumed by erosion, this
region will provide unprecedented opportunities to
study the geological processes that occur in the lower
crust during strike-slip faulting. Understanding the
growth of continents and other large-scale structures,
such as mountain ranges, is of more than passing scientific
interest. The relative motions of continental plates
along fault lines create earthquakes. Knowing more
about the nature of these faults could help scientists
better understand earthquake activity.
Aboard the research vessel Maurice Ewing in 1994,
earth scientists used ultrasound techniques to create
a three-dimensional image of British Columbia and
southeast Alaska along the network of fjords that
crosses the mountains along the northwestern continental
margin. The ship navigated the fjords Portland Canal,
Dixon Entrance and Clarence Strait, towing a 2.5-mile
cable of sound-wave sensors called hydrophones and
an array of air guns. The project was coordinated
by Lincoln Hollister, a geoscientist at Princeton
University in New Jersey.
The air guns created bubbles by rapid release of compressed
air. These bubbles produced sound waves that traveled
to the ocean bottom and into the rock below. The sound
waves were reflected back from the rock formations
from up to 40 kilometers below the beds of the fjords
to the sensors towed behind the ship. Measuring the
refraction of the sound waves through the rock bed
gave the researchers information about the physical
properties of the layers through which the sound had
traveled.
Editors: The AGU conference takes place Sunday,
December 15, through Thursday, December 19, at the
Moscone Center in San Francisco. The papers from the
Accrete project will be delivered on Sunday, beginning
at 8:30 a.m. in Room 125. Many of the presenters will
be available also at the poster session that begins
at 1:30 p.m. in Hall D.
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