Download Temperatures and tectonic history of the North American continent

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.
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