Collaborative Research: A Study of Deep Subduction Integrating Broadband Seismology and Mineral Physics

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The discovery of plate tectonics has focused attention to the dynamic nature of the Earth's interior. One of the first-order issues is the interaction between subducted plates and the transition zone of the mantle, a key layer at 410-660 km depth that divides the upper and lower mantle. In particular, the amount of plate material that descends into the lower mantle is central to understanding thermal convection and chemical mixing of the Earth's interior. While studies have suggested that subducted plates generally penetrate deep into the lower mantle, a number of recent results from experimental rock mechanics, computer simulations, and focused seismic observations all suggest that cold, rapidly subducting plates may become buoyant in the mantle transition zone due to the presence of important compositional components. A new collaborative study by researchers from 5 different institutions is seeking to understand what is really happening to plates as they travel down into the Earth. To answer these fundamental questions, the researchers are focusing on two key issues affecting the fate of plates: 1) The nature of a key boundary in the Earth's interior (660-km discontinuity), including its topography and density contrast near subduction zones; 2) how changes in rock type and density associated with deep earthquakes appear to mark pieces of plate that stagnated on their downward descent, particularly for plates driven down into the earth quickly. This project is investigating these questions by incorporating data from nature, the experimental laboratory, and computer simulations. The natural laboratories being examined are the deep subduction zones along the western Pacific, where old, cold plate has been rapidly subducting for millions of years. These areas provide an exceptional environment to examine what happens to subducting plates deep within the Earth. Seismologists are studying the subducted plates using the precise distribution of deep earthquakes, their faulting patterns, and the properties of the seismic waves in the rocks that surround them. For the experimental component, the mineral physics laboratory is investigating how seismic observations can be explained by patterns and alignment of crystals within different rock types from great depth in the Earth. Researchers are also examining whether plates become weak as they travel down into the Earth's interior by looking at how rocks behave when they are subjected to intense pressure and heat. To place the natural and experimental results in proper context, advanced computer simulations of plate subduction are investigating the geodynamic effects of changes in properties like composition, temperature, density, and viscosity. This project is providing training for three graduate students per year, and the collaborative nature of the work ensures that their training will be significantly enriched and broadened. In addition to the broad context of the interdisciplinary research, there is a component of education and outreach for the general audience. An animated teaching module is being created with help from the Educational Multimedia Visualization Center in order to illustrate various scenarios of plate subduction, focusing on the plate interaction with the mantle transition zone.