Alaska through time: reconstructing plate tectonics by linking the deep mantle to the shallow surface

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This projects aims to reconstruct the tectonic history and palaeogeography of Alaska by creating a new seismic tomography model and synthesising it with constraints from surface geology and marine geophysics.This is enabled by a wealth of new seismic data recorded by "USArray" in Alaska and by novel analysis methods, such as "tomotectonic analysis" [1], for connecting the surface record of subduction to its deep subsurface record. Turning back time to unravel the palaeogeographic origins of tectonic plates is primarily informed by palaeomagnetic data recovered from the ocean floor and the continents [2]. However, uncertainty in these reconstructions increases further back in time because many older oceanic plates have subducted down trenches into the mantle. These plates may not be accessible to surface sampling techniques, such as boreholes or dredging, but they are not lost; as they slowly sink inside the mantle they remain visible to geophysical imaging methods, such as seismic tomography [3]. Highly resolved images of this subducted lithosphere (`slabs') can be combined with bedrock geology datasets to understand the plate tectonic evolution of a particular area, especially in palaeo-oceanic regions [1].One such region is Alaska, the origins of which are an enigma and often ignored in studies [4]. The bedrock geology of Alaska comprises accreted terranes, many of which must have formed as linearly strung-out island arcs above subduction zones, much like the Izu-Bonin-Mariana Arc in the western Pacific Ocean today. Palaeomagnetic data reconstruct these proto-Alaska islands to some location much further south than their present-day position [5]. How they became accreted onto North America between 85-50 Ma and rotated into the enormous "Z"-shaped geometry of today's central Alaskan mountains is unclear [6, 7] (see Fig. 1a). Recent seismic studies show that the plates that once carried these islands are still suspended in the mantle [1, 7] (see Fig. 1b). Matching the slabs in the middle third of the mantle (~1000-2000 km depth) with the terranes at the surface has been difficult due to the lack of seismic data, as the geometries of these slabs are poorly resolved [8].However, over the last few years many new seismic stations have been positioned in Alaska (see Fig. 1a), most importantly the dense, broadband USArray. These will provide the seismological data needed to more accurately model the mantle beneath Alaska and the rest of North America using computational seismology. With this 3-D model I aim to take a holistic approach to plate reconstruction and link up the subducted slabs with the geological and palaeomagnetic data from the surface to reconstruct the tectonic assembly of Alaska ("tomotectonic analysis" [1]).This project has three key objectives:1.To create a seismic tomographic model of the upper and lower mantle beneath North America and Alaska so that the subducted lithosphere can be identified and characterised, and their geometries and locations can be constrained;2.To integrate this image with surface geology, palaeomagnetism and global plate reconstructions to elucidate Alaskan origins and test the hypothesis that this method is viable;3.To apply this method further in the adjoining areas of the Arctic, Canada or Siberia.