Tectonic Mystery Solved

Geologists at Rice University have located the oceanic portion 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 Earth Science. “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 seafloor 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 polarity reversed 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 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.

—Jade Boyd