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Results of research.

Donna Jurdy's research uses plate reconstructions and models of mantle convection to investigate the driving forces of the plates, showing that subduction is the main active force, balanced by plate drag. The predictions of these models are being compared to stress measurements.

Seth Stein and students study plate boundary processes and deformation within the lithosphere. One major effort is understanding how global plate motions over geologic time (millions of years) compare with those over a few years measured from space geodesy, and exploring these data`s implications for the evolution of the continents. This is being done using the Global Positioning System (GPS) satellites to study the tectonics of the central U.S. seismic zone and the boundary between the Nazca and South American plates. A second line of research is understanding the evolution of the oceanic lithosphere, the primary manifestation of the earth's thermal evolution, using a new thermal model. The model, which integrates seafloor depth, heat flow, and now satellite gravity data, makes a significant advance in understanding a wide variety of tectonic problems in both the present and over geologic time. Goals include extending tests of the model using satellite altimetry, better understanding the heat transfer between the deep mantle and oceanic lithosphere, and further exploring the model's implications for the volume of ocean water circulating through the crust. A third effort addresses the long-standing question of how earthquakes can occur at depths of 400-700 km where temperatures are too high for the type of fracture seen at shallow depth. The Stein group's approach assumes that these earthquakes reflect phase changes in metastable minerals within subducting lithosphere. A major paper has been published, in which a thermal model is integrated with recent results from mineral physics, and explores the implications for the evolution of subducting slabs and the mantle. Initial results are being derived by application of the model to the subducting Nazca plate near the site of the great 1994 Bolivian deep earthquake. The goals  of this project are to better understand a variety of issues, including the long-term stability (or instability) of kinetic boundaries, the interaction between kinetics, thermal structure, and subduction rate, and the occurrence of unusual deep earthquake geometries.

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