A new hypothesis for the initiation of plate tectonics on Earth: Feedback between subduction and continental crust growth

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This proposal studies how plate tectonics, the concept that Earth's surface layer is broken into rigid plates that move with respect to one another, started and evolved over Earth's history. Plate tectonics is one of the most important processes on Earth, as it is responsible for most earthquakes, volcanoes, and surface landscape features such as mountain ranges and geology. However, when and how plate tectonics begin during Earth's history is not well known. The project will study the role that the formation of continents, chemically distinct regions of Earth's crust that are thicker and sit higher than the corresponding ocean basins, play in initiating surface plate motion. By combining theoretical models with observations of some of Earth's oldest rocks, the proposed work will test a new theory for how plate tectonics started, and thus advance our understanding of this fundamentally important process. The proposal serves the national interest by promoting the progress of science. In particular, the proposal will advance education and diversity in the geosciences, by supporting a graduate student researcher for three years and an undergraduate student intern for one summer. The graduate student will receive valuable training and research experience in geophysics and interdisciplinary Earth systems science, applicable to careers in academia or industry. The undergraduate will be hosted through the Summer Research Opportunities Program at Penn State University, which is geared towards students from groups or backgrounds that are underrepresented in the sciences. As a result, hosting the intern will not only provide an undergraduate student with experience in cutting edge Earth science research, but will also contribute to building diversity in the Earth sciences.The hypothesis to be tested in this proposal is that a feedback between continental crust growth and subduction leads to the progressive initiation of modern-style plate tectonics across the planet. The proposal will use numerical models of mantle convection and thermal evolution models of the Earth incorporating continental crust growth to test this hypothesis. The proposed work will answer three main science questions: 1) Does a continent enhance lithospheric weakening, and thus induce formation of a weak plate boundary and modern-style subduction at its margin, and is the timescale for subduction to begin as a result of continent formation consistent with geologic observations? 2) Is the effect of a continent on lithospheric weakening felt locally or globally, and how does continent size change the length-scale over which this effect is felt? 3) Does the presence of a continent lead to higher rates of felsic magmatism due to enhanced subduction, and does the feedback between crustal growth and subduction cause a global transition to plate tectonics over a ~ 1 Gyr timescale, as indicated by geological observations? For each science question a detailed, thorough set of modeling work will be carried out that constrains the effect of all key model parameters on the results, and allows the robustness of the results to be fully tested. The initiation of plate tectonics was a seminal event in Earth's history, fundamentally reshaping its thermal, chemical, and geologic evolution. Thus the proposed work will significantly advance our understanding of perhaps the most important geophysical event in Earth's history. The results will have direct implications for interpreting Archean geology and geochemistry, by providing a large-scale geodynamic framework that geological observations can be placed in. Plate tectonics also fundamentally alters cycling of volatiles between the surface and interior, and thus plays a role in the long-term evolution of Earth?s atmosphere. The results of this proposal will thus have direct bearing on how Earth's atmosphere, and therefore the surface environment so crucial for life, has evolved over time.