Title: The formation of cratonic keels: seismic evidence from North America
Supervisors: Ian Bastow and Saskia Goes.
Most tectonic plates are less than 100km thick, but beneath the oldest rocks on Earth (cratons),
they can extend to depths of 250km, where they appear as fast wavespeed anomalies in global
tomographic inversions (Figure 1). How these cratonic ‘keels' formed and why they have
subsequently survived multiple Wilson cycles over billions of years remains poorly understood,
however (see e.g., Darbyshire et al., 2013).
The tectosphere, or lithospheric mantle beneath a craton (Jordan, 1975), has a thermochemical
signature that differs from average lithospheric mantle. These roots are commonly associated with
Archean processes, such as the extraction of komatiitic magmas (e.g., Griffin et al., 1999), to
explain the intrinsic low density of the tectosphere. But why does the Laurentian root persist
beneath both Archean and Proterozoic terranes? Is Archean and Proterozoic lithosphere
compositionally distinct (e.g., Darbyshire & Eaton, 2010), and is there evidence for keel
development occurring in multiple stages? To address these issues, this project will use state of
the art seismological techniques to image mantle structure beneath the Hudson Bay region of
northern Canada.
Cutting edge body-wave tomography (Li et al., 2008) will be used to constrain seismic wavespeeds
beneath the region using data from new seismograph networks that encircle Hudson Bay (Figure
2). These, in turn, will be translated to physical properties of the mantle, including temperature and
composition (e.g., Goes & van der Lee, 2002). The study of depth-dependent anisotropy, and the
use of converted-wave techniques such as receiver function analysis, will allow further
characterization of the lithosphere in a vertical sense, which will subsequently help address the
question of whether keels form in several stages.
The student tackling this project will have the opportunity to participate in fieldwork campaign in
Canada, which will see new broadband seismic stations built in New Brunswick and Nova Scotia
(Figures 2 & 3). They will receive training in the use of seismic imaging techniques, and have the
opportunity to spend a period of time working in North America with project collaborators.
Applicants with a solid background in geophysics, as well as an enthusiasm for understanding
tectonic processes are encouraged to apply for this exciting studentship, which will be cosupervised by Ian Bastow and Saskia Goes at Imperial, and Fiona Darbyshire in Montreal.
References:
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Bastow, I., et al., 2011, The Hudson Bay Lithospheric Experiment, Astronomy & Geophysics, 52(6), 21–
24, doi:10.1111/j.1468- 4004.2011.52621.x.
Darbyshire, F. & Eaton, D., 2010, The lithospheric root beneath Hudson Bay, Canada from Rayleighwave dispersion: No clear seismological distinction between Archean and Proterozoic mantle, Lithos,
120, 144-159.
F. Darbyshire, D. Eaton & I. Bastow, Seismic imaging of the lithosphere beneath Hudson Bay: Episodic
growth of the Laurentian mantle keel. Earth Planet. Sci. Letts., doi:10.1016/j.epsl.2013.05.002, 2013.
Griffin, W., et al., 1999, The composition and origin of sub-continental lithospheric mantle, Mantle
Petrology: Field observations and high-pressure experimentation, Geochem. Soc. Spec. Pubs, 6, 13–45.
Goes, S., & S. van der Lee, 2002, Thermal structure of the North American uppermost mantle inferred
from seismic tomography, J. Geophys. Res., 107 (B3), doi: 10.1029/2000JB000049.
Jordan, T. 1975, The continental tectosphere. Rev. Geophys., 13, 1-12.
Li, C., van der Hilst, R. D., Engdahl, E. R., & Burdick, S. (2008). A new global model for P wave speed
variations in Earth's mantle. Geochemistry, Geophysics, Geosystems, 9(5), doi:10.1029/2007GC001806.
Ritsema, J., et al., 2011, S40RTS: a degree-40 shear-velocity model for the mantle from new Rayleigh
wave dispersion, teleseismic traveltime and normal-mode splitting function measurements, Geophys. J.
Int., doi:10.1111/j.1365-246X.2010.04884.x.
1 Figure 1 (above): Shear velocity at 200km depth from the surface-wave model of Ritsema et al.,
(2011). Note the fast wavespeeds beneath the cratonic core of N. America. Dots are USArray
Transportable Array network stations.
Figure 2 (left): Seismograph stations (triangles) in
northern Hudson Bay, Canada. The black lines
delineate the suture of the Archean and Superior
plates, which collided 1.8Ga during a Himalayan-scale
mountain building event: the Trans Hudson Orogen.
After Bastow et al, (2011).
Figure 3 (right and below): QM-III seismograph stations
currently operating in Quebec after installation in summer
2012. Imperial College will build ten more stations like these.
2