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Temperatures and tectonic history of the North American continent: Forward and inverse seismic models Supervision: Saskia Goes (Imperial College), Suzan van der Lee (Northwestern University, U.S.A.), Ian Bastow (Imperial College) Background: There are many open questions about the internal structure of the continental plates that we live on, and what this structure tells us about how continents are formed. For example, why are old continental cores impervious to the type of recycling that happens to oceanic lithosphere? Is this due to their different temperatures (due to longer cooling), different composition (e..g, due to different formation processes in the Archean) or a combination of both? Did continental formation processes change in time, and if so was this a gradual or stepwise process? Are continents formed by both lateral and vertical accretion of distinct material? Can continental thermal and compositional structure explain differences in deformability (including the ability to produce earthquakes)? Project aims: To address all these interesting questions, we need to improve on (a) geophysical imaging of the lithosphere, and (b) on linking geophysical structure to temperature and composition. Over the past years, a large-scale seismic experiment (USArray) has substantially increased the resolution of subsurface structure below the North American continent, which comprises an old core in the northeast and many newly accreted terranes in the tectonically active west. This project will use the new constraints from this experiment to work on part (b). Aim is to build a set of synthetic thermal and compositional structures for North America for different hypotheses of continent formation, converting these into synthetic seismic structures and then testing them against the seismic data from USArray and other North American permanent and temporary networks, as well as other constraints including from surface heat flow and mantle xenoliths. This is a purely thermal interpretation of a tomographic S-velocity model for North America [from Goes and Van der Lee, 2002]. It gives a promising first order idea of lithospheric temperatures. However, questions about deep lithospheric heat flow and thickness of the lithosphere cannot be answered well from such a model. 1 The team: The project will be supervised by Saskia Goes (who has ample experience in thermal modelling and converting between seismic and thermo-chemical structure), Suzan van der Lee (who has long worked on tomographic imaging with a broad range of seismic data, including below North America using USarray data) and Ian Bastow (who is involved in a regional experiment to seismically image some of the oldest parts of the Canadian lithosphere with a variety of seismic techniques). Within Imperial’s Earth Science and Engineering Department, the student would form part of the Plates & Mantle group, the core of which consists of 5 academic staff, 5 other PhDs and ~2 postdoctoral researchers that work on seismic imaging of crust and mantle, geochemistry and mantle dynamics. Student Profile: We seek a highly motivated individual with a background in geophysics, physics, or geology with a strong quantitative foundation. Candidates would optimally have a background that includes some seismology and numerical modelling and have a keen interest to do interdisciplinary work on the dynamics of the deep Earth. Application: Application forms and instructions can be found on the departmental web site. For more information on the project please contact Saskia Goes at [email protected]. References and further reading Bastow, I. D., D. Thompson, J. Wookey, J.-M. Kendall, G. Helffrich, D. Snyder, D. Eaton, and F. Darbyshire (2011), Precambrian Plate Tectonics: Seismic Evidence from Northern Hudson Bay., Geology, G31396, doi:31310.31130/G31396.31391. Bedle, H., and S. Van der Lee (2009), S velocity variations beneath North America, J. Geophys. Res., 114, B07308. Darbyshire, F., D. Eaton, and I. D. Bastow (2013), Seismic imaging of the lithosphere beneath Hudson Bay: Episodic growth of the Laurentian mantle keel. Earth Planet Sci. Lett., doi:10.1016/j.epsl.2013.1005.1002. Goes, S., and S. Van der Lee (2002), Thermal structure of the North American uppermost mantle inferred from seismic tomography, J. Geophys. Res., 107(B3), 2000JB000049. Hieronymus, C. F., and S. Goes (2010), Complex cratonic seismic structure from thermal models of the lithosphere: effects of variations in deep radiogenic heating, Geophys. J. Int., 180, 999–1012. Van der Lee, S. (2001), Deep below North America, Science, 294, 1297-1298. Van der Lee, S., and G. Nolet (1997), Upper mantle S-velocity structure of North America, J. Geophys. Res., 102, 22,815-822,838. 2