HuBLE: The Hudson Bay Lithospheric Experiment
Overview

Despite being one of the most striking features of the North America continent, the reason for the existence of Hudson Bay is obscure. It lies in the Precambrian core of North America, which comprises the Canadian Shield and contiguous platform regions (Laurentia - Hoffman, 1988). The region is underlain by the largest continental root on Earth and is the site of one of the largest negative geoid anomalies (Figure 1). When such roots develop, how long they last, and their influence on cratonic basins are unanswered questions which we aim to address.

Hudson Bay           Hudson Bay

Figure 1: Gravity images from the GRACE project. Note the long wavelength anomaly that characterizes the Hudson Bay region.

Gravity images from the GRACE project: www.csr.utexas.edu/grace/gallery/gravity.
These detailed geophysical features are being detected by GRACE with no surface gravity measurements. (July 21, 2003).
[Image credit: Unless otherwise noted, images are provided by University of Texas Center for Space Research and NASA]

The tectosphere, or lithospheric mantle beneath a craton (Jordan, 1975; 1988), has a thermochemical structure that differs from average lithospheric mantle, but is quite variable in depth and size from craton to craton. Cratonic roots are easily recognizable on global tomographic images due to their high shear-wave velocity at 150-300 km depth (Figure 2). Although the Laurentian tectospheric root is relatively cool compared to adjacent parts of the mantle, it has been proposed that approximate equilibrium between thermal and chemical buoyancy will contribute to tectonic stability of cratons over geological time (Forte and Perry, 2001). Based on global tomographic images (e.g., Megnin and Romanowicz, 2000), the thickest part of the Laurentian cratonic root appears to be situated near the western shoreline of Hudson Bay. Cratonic roots such as this are strong, refractory objects that impede asthenospheric flow (Forte and Perry, 2001) and upwelling of deep mantle plumes (Sleep, 1997).

Hudson Bay Seismic Velocity at 200km depth

Figure 2: S-wave velocity perturbations at 200 km depth from global model SAW24B16 (Megnin and Romanowicz, 2000)

Cratonic roots have long been associated strongly with Archaean cratons. This has led to models for root formation that invoke predominantly Archean processes, such as extraction of Komatiitic magmas (e.g., Griffin et al., 1999) to explain the intrinsic low density of the tectosphere. The Laurentian root does not fit easily within this paradigm, since it appears to extend beneath not only parts of Archean Laurentia, but also underlies significant areas of juvenile Proterozoic crust - in particular, the Trans-Hudson Orogen. However, our current picture of the root comes from seismic tomography constrained only by 2-3 permanent seismic stations in the region. Improved station coverage is needed to map in detail the architecture of the tectosphere beneath the region, showing whether or not the root underlies the younger Trans-Hudson.

Seismic Networks in Nunavut

POLARIS (Portable Observatories for Lithospheric Analysis and Research Investigating Seismicity), is a Canadian funded effort to to provide live data for research, education and continuous monitoring of earthquakes throughout Canada. 96 POLARIS observatories are currently operational in five arrays: Ontario, British Columbia, North West Territories, Quebec and Nunavut. Coinciding with POLARIS, HuBLE is a multi-institutional collaboration (please check here for a list of HuBLE participants) that aims to supplement the Nunavut POLARIS seismic network in the region of the Hudson Bay Basin.

HuBLE Scientific Goals

As we collect our data from the field in 2008-9 the HuBLE group will perform a variety of analyses on our data including travel-time tomography, receiver function analyses, SKS shear wave splitting and surface wave studies in an effort to address several unanswered questions in Nunavut including:

  • The structure and evolution of lithospheric roots and their dynamic interaction with mantle flow.
  • The formation of intracratonic basins: (a) postglacial isostatic rebound? (b) Mantle flow (downwelling) beneath Hudson Bay? (c) A phase change from gabbro to denser eclogite in the deep parts of thickened crust (Fowler and Nesbit, 1985; Baird et al., 1995)? (d) What is the role of extension, uplift and denudation?
  • The lithospheric structure of the Trans-Hudson Orogen beneath Hudson Bay and the nature of the Nastapoka Arc.
  • The nature of intraplate seismicity in the region.

    • Publications

  • D.A. Thompson, G. Helffrich, I.D. Bastow, J. Wookey, J-M. Kendall, D. Eaton & D. Snyder. Implications of a simple mantle transition zone beneath cratonic North America. Earth Planet. Sci. Letts., in press, 2011.

  • I.D. Bastow, D. Thompson, J. Wookey, J-M. Kendall, G. Helffrich, D. Snyder, D. Eaton & F. Darbyshire. Precambrian Plate Tectonics: Seismic Evidence from Northern Hudson Bay. Geology, G31396, doi:10.1130/G31396.1, 2011.

  • A. Pawlak, Eaton, D., I.D. Bastow, J-M., Kendall, G. Helffrich, J. Wookey & D. Snyder. Crustal structure beneath Hudson Bay from ambient-noise tomography: Implications for basin formation, Geophys. J. Int., doi:10.1111/j.1365-246X.2010.04828.x, 2011.

  • D. Thompson, I.D. Bastow, J-M. Kendall, G. Helffrich, J. Wookey, D. Snyder & D. Eaton. Precambrian Crustal Evolution: Seismic Constraints from the Canadian Shield. Earth Planet Sci. Letts., doi:10.1016/j.epsl.2010.07.021, 2011.

    • HUBLE POSTERPoster presentation at the Nunavut (CNGO - GSC) geoscience workshop, March 27-28, 2008

    POLARIS and HuBLE Related Publications

  • A.W. Frederiksen, S.-K Miong, F.A. Darbyshire, D.W. Eaton, S. Rondenay, and S. Sol. Lithospheric variations across the superior province, ontario, Canada: Evidence from tomography and shear wave splitting. J. Geophys. Res.,112(B7):20, 2007. doi: 10.1029/2006JB004861.

  • F.A. Darbyshire, D.W. Eaton, A.W. Frederiksen, and L. Ertolahti. New insights into the lithosphere beneath the Superior Province from Rayleigh wave dispersion and receiver function analysis. Geophys. J. Int., 169:173-1067, 2007. doi:10.1111/j.1365-246X.2006.03259.x.

  • F.A. Darbyshire. Upper mantle structure of Arctic Canada from Rayleigh wave dispersion. Tectonophysics, 405(1):1-23, 2005.

  • D.W. Eaton, J. Adams, I. Asudeh, G.M. Atkinson, M.G. Bostock, J.F. Cassidy, I.J. Ferguson, C. Samson, D.B. Snyder, K.F. Tiampo, and M.J. Unsworth. Investigating Canada's Lithosphere and Earthquake Hazards with Portable Arrays. EOS Transactions, 86:169-173, 2005. doi:10.1029/2005EO170001.

  • D.W. Eaton. Shear-wave splitting observations in the lower great lakes region: Evidence for regional anisotropic domains and keel-modified asthenospheric flow. Geophys. Res. Lett., 31(7):4, 2004. doi: 10.1029/2004GL019438.

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    Last updated on 27/10/11