Deep seismic imaging the evolution of continental lithosphere

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Earth's tectonic processes have changed dramatically as the planet has cooled. Soon after its formation 4.5 billion years ago, Earth's surface comprised a thick immobile mafic crust marked by extensive volcanism, perhaps punctuated by major resurfacing events, as plumes transported heat upward in a chaotic mantle convection regime. Approximately 3.0-3.5 billion years ago, a form of embryonic, transient plate tectonics developed, which eventually evolved into the well organised plate tectonic system we observe today. Earth's continents have recorded the effects of over four billion years of this tectonic activity, but the nature of these early tectonic processes remains controversial with vertical subsidence of the dense early crust often invoked as an alternative to horizontal processes tied to seafloor spreading and subduction. Our understanding of the early Earth comes from a wide variety of geoscientific data and increasingly sophisticated numerical geodynamic simulations, which can predict the deformation of the crust and upper mantle in various scenarios. Since deep seismic reflection surveys can reveal in detail structures preserved in the continental lithosphere from earlier episodes of tectonic deformation, these surveys provide an ideal way to verify the predictions of geodynamic modelling, and their depth constraints are a critical complement to geological mapping at the surface. High quality seismic data recently recorded in Australia build upon extensive Canadian datasets from the Lithoprobe program, and provide an unprecedented insight into the structure of ancient continental crust. The objective of the proposed research is to use seismic reflection data, integrated with a broad range of geological and geochronological data, to determine the nature of the changing tectonic processes that operated early in Earth history from creation of the first microcontinents to assembly of the stable cores of today's continents. Using new methods developed to determine 3-D reflector orientations in crooked 2-D surveys, it is now possible to better distinguish multiple episodes of tectonic deformation. With my students, I will study the assembly and subsequent collapse of Archean crust, which probably has no modern equivalent, determine the nature of lower crustal flow, and reveal the setting of Archean greenstone belts that host much of Earth's gold and base metal resources. Seismic surveys indicate that a significant change in the nature of continental assembly occurred during, or just prior to, the Early Proterozoic, with the underthrusting of very thick crustal sections, which may be related to the growth of strong cratonic roots. We will develop methods to distinguish domains in the uppermost mantle by their dominant reflector orientation, aiming to determine if they formed by processes such as magmatism, continental subsidence, or imbrication of subducted slabs.