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Scientists lead international team in Tibet to probe role of erosion in mountain-making

When Peter Zeitler and Anne Meltzer look at a map of the world, they see the subcontinent of India pushing like a relentless snowplow into the soft underbelly of East Central Asia.

Across the front of the plow, like blocks of freshly stacked snow, the Himalayan Mountains patrol Nepal and Tibet.

At either side of the plow, the world’s tallest mountain range ends abruptly in spectacular gorges and overlooks powerful rivers that contribute to the turbulent forces that are continuing to shape the Himalayas.

Zeitler and Meltzer, professors of earth and environmental science at Lehigh, are leaders of an international team of 16 researchers from seven institutions that has received $2.2 million from the National Science Foundation to study the interaction of erosion and tectonic processes in the vicinity of Namche Barwa, the highest massif (mountain mass) in the eastern Himalayas.

The project, titled "Geodynamics of Indentor Corners," is being funded through NSF’s highly competitive Continental Dynamics Program, which supports only a handful of integrative, collaborative projects each year.

Namche Barwa, which rises to 23,000 feet, is one of two "corner regions" on the edges of the Himalayas where subsurface rocks are twisting and rotating in response to the Indian subcontinental encroachment.

Like Nanga Parbat, the 26,000-foot massif dominating the western end of the Himalayas, Namche Barwa stands in an area characterized by highly active tectonic processes below the earth and extreme erosion on the surface.

Thanks in part to their location in the watersheds of two of Asia’s largest rivers – the Indus winds around Nanga Parbat and the Tsangpo around Namche Barwa – the two massifs are witness to some of the most dramatic orogeny, or mountain-building processes, in the world, Zeitler and Meltzer wrote recently in GSA Today, the magazine of the Geological Society of America.

"[In these] unusual, highly active antiformal basement massifs," the geologists wrote, "the deep gorges of the Indus and Tsangpo rivers expose, uniquely to our knowledge, [about] 7,000 meters of relief, actively deforming metamorphic rocks, and granites as young as Pleistocene."

In both cases, Zeitler and Meltzer believe, the rivers erode gorges deep enough to weaken the earth’s crust, encouraging an upward surge of hot metamorphic and mountain-forming rock that they have termed a "tectonic aneurysm."

The geologists hope to answer several questions relating to the geodynamics of continental collisions. How much erosion is occurring? Is the edge of Tibet sliding away, geologically speaking, from the main body of Tibet? How do surface processes like erosion interact with tectonic (solid-earth) processes to shape the earth’s crust during mountain-forming? The last question is "enigmatic in older [mountain regions] but resolvable in young and active regions such as the India-Asia collision," the researchers wrote in their proposal to NSF.

"We believe that the manner and rate at which surface rocks are being chiseled away by erosion affects what is happening 10 to 30 kilometers below the earth’s surface," says Zeitler.

In their investigations, the researchers will combine short-timescale seismological, geomorphic and GPS (global positioning system) measurements with observations made over a longer temporal scale, including geochronologic, petrologic and structural measurements.

These results will be integrated with three-dimensional modeling and analyzed against the backdrop of a fourth dimension – time.

Zeitler and Meltzer recently led the Nanga Parbat Continental Dynamics Project, a five-year study by a team of 26 researchers from France, New Zealand, Pakistan and the U.S. They described the results of that project in articles published in several journal, including Tectonics and GSA Today.

Their colleagues in the Namche Barwa project represent Stanford University, the Massachusetts Institute of Technology, Otago University (New Zealand), the University of Maine, the Chengdu Institute (China), the University of Washington, and the State University of New York at Albany.

In addition to analyzing and dating samples of surface rocks, the researchers will deploy an array of up to 70 seismometers that will listen to earthquakes, including local and regional events as well as those on different continents. By comparing the arrival times and form of seismic waves generated by the earthquakes, the seismometers will help researchers use a form of tomography to infer the composition and temperature of rocks in the subsurface, as well as detect the presence of any melts.

The seismometers will be made available to the researchers through Incorporated Research Institutions for Seismology (IRIS), a university research consortium of which Meltzer is former director.

Zeitler and Meltzer spent one month in Tibet last fall and nearly two months this summer, and will be returning with students for several more trips over the next four years.


by Kurt Pfitzer
kap4@lehigh.edu

Posted on Friday, August 23, 2002

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