Tearing a Tectonic Plate in Two

Article excerpt

Tearing a tectonic plate in two

Fifty million years ago, a huge submerged plateau in the Indian Ocean split into two pieces that began to separate and now sit 2,000 kilometers apart. Previously, scientists seeking to explain the origin of this break have been unable to choose between two rival rifting theories, but geophysicists recently pulled up evidence from the ocean floor that they say settles the debate.

In May and June, the crew and scientists on Leg 121 of the Ocean Drilling Program (ODP) collected sediments from the top of a feature called Broken Ridge, which currently lies off the west coast of Australia and was half of the once-connected platform. Its estranged sibling, the Kerguelen plateau, is the world's largest submerged plateau and sits north of Antarctica (SN: 6/25/88, p.410). The original, intact plateau grew from the ocean floor more than 90 million years ago, when a series of volcanic eruptions poured out vast volumes of molten basalt onto the Antarctic plate.

During the Eocene epoch, a long rift cleaved the plate and cut off its northern section, which held the Broken Ridge part of the plateau. This renegade plate section latched onto its neighbor, the Indian plate, and started a journey northward, while the Kerguelen plateau moved southward.

As one explanation for the break, geophysicists had suggested this event may be an example of active rifting, a process now splitting apart East Africa. According to this theory, a rift can develop when a rising flow of hot magma buoys and warps the crust. Yet the newly collected sediments indicate Broken Ridge was sinking immediately prior to breaking from the rest of the plateau, which means active rifting did not cause the split, says Leg 121 co-chief investigator Jeffrey Weissel of Columbia University's Lamont-Doherty Geological Observatory in Palisades, N.Y. As conforming evidence, Weissel notes that instruments inserted into boreholes on Broken Ridge measured normal heatflow from the crust. …