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Geotimes
 Published by the American Geological Institute
Newsmagazine of the Earth Sciences

 July 2000


Highlights
Structure and Tectonics

Christian Teyssier
 
       After 2,000 stations are installed in the United States, including Alaska, as well as 
         Canada, Mexico and the continental margin, USArray will cover the above area.
         GSA Today.

The year 1999 was fertile for structural geology and tectonics. A good summary of the major questions in structural geology appeared in the 20th anniversary volume of the Journal of Structural Geology. A number of authors reflected on the way one should approach structural studies, from an observational standpoint to a full use of solid and fluid mechanics. The special volume also showed that the processes and theory of faulting are very strong areas of research. The series of earthquakes that punctuated the year, especially those in Turkey and Mexico, reemphasized our poor knowledge of faulting and the need for field studies to complement seismological monitoring.
A large-scale geoscience initiative is being developed around EarthScope, a project supported by the National Science Foundation that will significantly impact research in North America. Part of EarthScope, the USArray program is an unprecedented effort to seismically image Earth’s deep interior under the North American continent. USArray will also scan the continent with Big Foot, a network of portable seismometers, over 10 years. Results will display coarse lithospheric structures as well as mantle heterogeneities down to the core-mantle boundary.

Of interest to tectonicists, USArray will also deploy about 2,000 additional seismometers over specific target regions to investigate fundamental geologic problems at the crust/lithosphere scale. These regions have yet to be determined, but the potential for new insights and discoveries is great. Target regions will be imaged at an unparalleled resolution and in three dimensions, allowing a better understanding of their geologic histories.

Such probing of North America will address key questions on the assembly of the continent and the relationship between lithospheric and crustal dynamics. The success of this initiative depends on the synergy that will develop among the various disciplines. It is clear that tectonicists have a major role to play.

A good example of a successful marriage between seismology and tectonics is illustrated in one of the most exciting papers published in the past year. Peter Molnar and co-authors (Science, v. 286, p. 516-519) compiled data on the deformation of a key plate margin, the Pacific-Australian plate boundary in New Zealand, where plates have been sliding past each other over the last 45 million years. Using shear-wave splitting, Molnar’s team showed that New Zealand is underlain by a mantle shear zone a few hundred kilometers wide — a sharp contrast to oceanic transform plate boundaries that are typically narrow.

Shear-wave splitting has established itself as a new technique for determining the structural geology of the mantle. Splitting of shear waves is interpreted to originate from the preferred orientation of olivine, the dominant mineral in the upper mantle, developed during ductile deformation by dislocation creep (Savage, 1999, Reviews of Geophysics, v. 37, p. 65-106).

The work by Molnar and his colleagues has profound implications for understanding the relationship between mantle and crustal deformation, and for understanding the mechanical coupling that exists among the lithosphere’s rheologic layers. This work adds substantially to the theory of plate tectonics by showing that continental plates are not rigid and that the mantle lithosphere deforms over broad regions at plate margins.

Some fundamental questions remain: How far do large faults in the upper crust continue at depth? Is deformation of the continental crust controlled by the imposed displacements in the mantle? What happens to the deforming (thickening) lithosphere over time?

Ultimately, lithospheric coupling must play a significant role in stress loading the upper crust and generating earthquakes along the major faults that dissect continents. It is time for “brittle” and “ductile” geologists to join forces and understand the relationship between continuous deformation and seismic displacements at the crust and lithosphere.
 

Teyssier teaches in the Department of Geology and Geophysics at the University of Minnesota. Visit www.geo.umn.edu/people/profs/TEYSSIER.html
For more on USArray, visit www.iris.iris.edu/background.II.html



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