Researchers solve mystery that has plagued researchers of tectonics

Researchers
solve mystery that has plagued researchers of tectonics

…………………………………………………………………

BY JADE BOYD
Rice News Staff

Geologists at
Rice University have located the oceanic portion, off the
southern African coast, of a boundary between two immense
continental plates, solving a mystery that has plagued tectonic
researchers for more than 35 years.

The northern boundary between the west African (Nubian)
plate and the east African (Somalian) plate has long been
identified as the East African Rift Valley. From the time
plate tectonics was proposed in the mid-1960s, geologists
have speculated about whether, and in what direction, the
boundary continues from the south end of the rift valley,
beyond which seismic and volcanic activity disappear.

Rice geologists Richard Gordon and James Lemaux II, along
with geologist Jean-Yves Royer of the Institut Universitaire
Européen de la Mer, report their findings in the
April issue of the journal Geology. Comparing records of
magnetic variations in the seafloor of the southern Indian
Ocean, they located the intersection of the Nubian, Somalian
and Antarctic plates within a 100-kilometer-wide region
known as the Andrew Bain Fracture Zone Complex. The submarine
complex, located south of Africa, is more than 1,000 miles
long and, at its southern end, intersects the northern boundary
of the Antarctic plate.

“This boundary has been elusive because there is very
slow movement between the Somalian and Nubian plates,”
said Gordon, the W.M. Keck Professor of Geophysics. “Both
plates are moving away from Antarctica, but the Somalian
plate is moving slightly slower, so the relative movement
between the African plates is only about two millimeters
per year.”

Gordon, Royer and Lemaux, now with BP Exploration (Alaska)
Inc., determined movement rates for the African plates by
studying the magnetic profile of the sea-floor on both sides
of the slowly slipping fault zone. New seafloor is continuously
created as the African plates pull away from Antarctica.
Because Earth’s magnetic field changes polarity about
every 500,000 years, the seafloor appears as a series of
bands, each with reverse polarity from the next. Like rings
of a tree, these bands can be used to date the creation
of seafloor, and they can be matched up from opposite sides
of the fault zone to gauge how far plates have moved relative
to one another.

The Rice researchers compared the magnetic signature of
a single band in 237 locations — six times as many
as in prior studies of the region — to determine the
relative movement of the plates and to pinpoint the location
of the boundary zone between the two African plates.

The research, funded by the National Science Foundation
and the French National Scientific Research Center, allows
geologists to better understand the relationship between
the African plates, including the tectonic processes that
created the East African Rift. The findings should also
allow geologists to improve software models that predict
the tectonic motion, giving a clearer picture of what’s
likely to happen not only in the rift valley, but also in
other areas such as the Himalayas and Tibetan Plateau, where
the Indian and Eurasian plates collide.

About Jade Boyd

Jade Boyd is science editor and associate director of news and media relations in Rice University's Office of Public Affairs.