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News and Trends in the Geosciences

 May 2000

News Notes
 Plate Tectonics
Missing tectonic link found

The relative motion and interaction between the Antarctic, Australia and Pacific tectonic plates have been poorly understood and have fueled disagreement over misfits in the global tectonic circuit. The region between Marie Byrd Land of Antarctica; Tasmania, Australia; and New Zealand was believed by scientists to have potentially held the final missing piece of the Antarctic tectonic puzzle.

Steven Cande of the Scripps Institution of Oceanography led a team of researchers on two extended research cruises into the Northern Ross Sea and South Tasman Sea. They collected copious bathymetric, magnetic anomaly and fracture zone data and published their findings in the March 9 issue of Nature. “The East-West Antarctica problem was the last missing link where we thought we could go out, collect data and actually solve the problem,” Cande says.

Bathymetric data have revealed part of the missing link — a trough running north-northwest out of the Ross Sea Embayment of Antarctica. Dubbed the Adare Trough, it is a graben structure, or a basin-like structure evident of crustal extension.

Paleomagnetic data provide evidence that it was also the site of an ancient spreading center that formed a triple-ridge junction between the Australia, Antarctic and Pacific plates during the Eocene and Oligocene.

“The biggest uncertainty remaining for the Cenozoic global circuit is the uncertainty of deformation within the Antarctic Plate,” says Tanya Atwater of the University of California, Santa Barbara. “Every development in pinning that down is a huge boon for all studies of the Circum-Pacific rim.”

At a mid-ocean spreading center, tectonic plates are pushed apart as new crust is extruded and added onto the plate. As newly formed continental crust cools, iron-rich minerals record the polarity of the geomagnetic field. The history of geomagnetic reversals is recorded at spreading centers, providing information about the duration and time of spreading.

Paleomagnetic data collected by Cande’s team revealed geomagnetic reversals in the sea floor that run parallel to the Adare Trough and recorded spreading along an ancient ridge that took place during the Eocene and Oligocene. The majority of spreading took place between 43 and 26 million years ago at an average rate of 12 millimeters per year, a rate comparable to the slowest rates of seafloor spreading observed today.

Cande’s team says spreading in the Adare Trough probably accounts for over 180-kilometers of extension and 40 to 90 degrees of relative rotation between East and West Antarctica. This corresponds to the opening of the Ross Sea Embayment, now overlain by the Ross Sea Ice Shelf. The paleomagnetic data are supported by the presence of Cenozoic sediments found in drill cores retrieved in 1999 near Cape Roberts, Antarctica.

There are many global geologic features associated with Pacific Plate motion that scientists have not been able to account for without reasonably confident motion constraints. The root of this problem has been the lack of evidence for sea-floor spreading centers along the Pacific Plate boundary.

“For many decades there has been a disconnect between absolute motion results using fixed hot spots under the Hawaiian-Emperor Chain and relative results using seafloor data,” Atwater says. “Any improvement in our understanding of how hot spots move will make a significant contribution to helping us understand global tectonics.”

By constraining motion in Antarctica, Cande’s team hoped, a correlation between volcanic arc movements around the globe could be made to determine fixed-mantle hot spots on Earth. They remodeled the formation of the 120-degree bend in the Emperor-Hawaiian Chain in light of the new Antarctic data. Cande and others did not observe any correlation with movement in other volcanic arcs in the Indian Ocean. They now believe that hot-spot migration, not plate movement over fixed mantle sources, is the more probable answer to the volcanic arc migration enigma.

Regionally, the reconstruction of New Zealand through time has long been a mystery for scientists who have sought to account for the 150-kilometer gap along the transform plate boundary that forms the Alpine Fault through the South Island of New Zealand. Accurate constraints on extension and rotation between East and West Antarctica have allowed scientists to reconstruct models that account for the Adare Trough spreading and to close the gap between the Lord Howe Rise and the Campbell Plateau, fixing the New Zealand misfit.

The Adare Trough discovery and associated motion between East and West Antarctica has also posed some new questions related to the uplift of the Transantarctic Mountains that flank the Ross Sea. Cande believes that spreading along the Adare Trough may have commenced at a very slow rate before 46 million years ago, coinciding with early uplift of the Transantarctic Mountains 55 to 50 million years ago. He and others are currently working on a proposal to continue research in the region that could potentially include an investigation of the relationship between ridge spreading and Transantarctic uplift.

Laura Wright

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